Rivers are the primary shapers of the earth’s surface.

HOW RIVERS WORK
On the Moon, Mercury and Mars, craters dominate the landscape. But on the continent of Earth, stream valleys are the most abundant landforms. All of these mountains, hills and valleys were shaped by flowing water. Rivers are the primary shapers of the earth’s surface. In fact, most of the earth’s surface is some type of drainage system.

1. A River System. We’ve all witnessed how one creek flows into another creek to form a larger stream, then observed how that stream flows downhill to join the main channel of the river. But what about the magnified view you see here, the tiny network of water channels that look like the veins in a leaf?

The next time it rains, walk out into your back yard. Watch how the raindrops gather in tiny rivulets even on flat ground. One trickle flows into another and that flows into another until they form a small stream. The new stream merges with another and that with another until they eventually flow into a creek and from there into a river.

A river system is a network of connecting water channels. Water from rain, snow and other sources collects into the channels and flows to the ocean. A river system has three parts: a collecting system, a transporting system and a dispersing system.

Collection. A river’s collection system consists of a network of tributaries in the headwaters region. This network collects and funnels water and sediment to the main stream. One the most surprising characteristics of a collection system is its intricate network of innumerable tributaries. Look at the illustration above. This is not the entire collection system. It’s a very tiny portion of it. Even the smallest tributary has its own system of smaller and smaller tributaries until the total number becomes astronomical. In fact, most of the earth’s surface is some type of drainage system.

Transporting. The main task of a river’s transporting system is to carry water and sediment toward the ocean. But it does other important work as well. More water and sediment are “collected” into the system. The river meanders back and forth eroding its banks. On the inside of the meanders, sediment drops out forming point bars. More sediment drops out of the river when it overflows its banks during floods, creating flood plains.

Dispersing. A river’s dispersing system is a network of channels at a river’s mouth where coarse sediment, fine-grained sand and water are dispersed into the ocean.

2. Watershed Defined. A ridge of high ground borders every river system. This ridge encloses what is called a watershed – the region draining into a river system. Beyond the ridge, all water flows into another river system. Just as water in a bowl flows downward to a common destination, all rivers, creeks, streams, ponds, lakes, wetlands and other types of water bodies in a watershed drain into the river system. A watershed creates a natural community where every living thing has something important in common – the source and final disposition of their water.

3. Where Do Rivers Come From. Water is the main difference between Earth and other planets in the Solar System. Earth’s just the right distance from the sun to have water. Any closer and it would be too hot. Water would evaporate. Any farther away and it would be too cold. Everything would freeze. Approximately 71 percent of the earth is covered with water. It’s for good reason that Earth has been called the “water planet.”

If you look into outer space, you can see that there’s no magic pipeline supplying water to earth from somewhere else. The water that is here is all the water that has ever been here – for billions of years – and all the water that will ever be here.

All of the water on earth is recycled again and again through what is called the hydrologic system, or simply, the water cycle.

The water that falls on land comes from about 10 percent of the water evaporated from the oceans. This water looses its salt content during the evaporation process. The other 90 percent returns directly back to the oceans in the form of rainfall.

Evaporation is when water goes into the atmosphere as a vapor. Transpiration is the water given off by plants. The combination is called, “evapotranspiration.”

The 10 percent of the water evaporated from the oceans combines with water taken up from the land in the evapotranspiration process and returns to the earth as rain, snow, sleet or hail to form streams and rivers.

The United States averages 30 inches of rain annually, but this rain is unevenly distributed across the country. Western Washington averages about 35 inches annually; Nevada, 7 inches; east of the Mississippi 30-to 50 inches; some regions of the west, less than 10 inches. Georgia receives approximately 50 inches of rainfall each year, primarily in the winter and early spring. Due to the varied topography of the state, the mean annual precipitation varies from place to place and ranges from a maximum of 68 inches to a minimum of 40 inches. Of the state’s annual average annual rainfall of 50 inches, only 9 to 24 inches (depending on the part of the state) becomes a part of the surface or ground water system. The remaining inches return to the atmosphere by evaporation or transpiration.

4. Where on Earth is the Water? 0.0001 percent is in the atmosphere. 0.01 percent is soil moisture. 0.02 percent is in rivers, lakes and inland seas. 0.5 percent is ground water. 1.9 percent is in ice caps and glaciers. 97.6 percent is in the oceans.

5. River Change is Constant. One of the most important things about a river system – maybe the most important thing – is this: Rivers are always changing and change on one part of a river affects all other parts.

A river is always trying to reach a kind of perfect balance called “equilibrium,” where it doesn’t erode the banks or deposit any sediment. Of course the river never reaches that balance, but it is always trying.

Here are the most important variables in stream flow. These variables constantly adjust toward a state of equilibrium.

Gradient. The gradient of a stream is its slope. The gradient is usually expressed in the numbers of feet or meters the stream descends for each mile or kilometer of flow. The headwater streams, which drain the Rocky Mountains, can have gradients of over 50m/km; the lower reaches of the Mississippi have a gradient of only 1 or 2 cm/km. One way a river changes its gradient is by lengthening itself through the creation of meanders (see below).

Velocity. The speed of flow, which varies depending on gradient, volume of water and the location of water in a stream channel.

Sediment Load. The combination of dissolved material, fine particles and coarse material moving in the stream channel.

Discharge. The amount of water passing a given point during a given interval of time. Usually measured in cubic feet per second (cfs).

These variables in a river constantly adjust toward a state of balance or equilibrium.

Knowing this helps to understand how Earth’s landscape has evolved over the years. And it has a practical side, too. If we change rivers to suit our needs with dams and dredging, we should know how the rivers will react.

6. Equilibrium. To get a better understanding of equilibrium in a river system, let’s look a at hypothetical stream in which equilibrium has been established.

1. The variables in the stream system (discharge, velocity, gradient, base level and load) are all in balance so neither erosion nor sedimentation occurs along the stream’s profile. There is just enough water to transport the available sediment down the existing slope. Such a stream is in equilibrium and is known as a “graded” stream.

2. The stream’s profile is displaced by a disruption that creates a waterfall.

3. The increased gradient across the falls greatly increases the stream’s velocity at that point so rapid erosion occurs and the waterfall, or rapid, begins to migrate upstream. The eroded sediment added to the stream segment on the dropped fault block is more than the stream can transport because the system was already at equilibrium before the faulting occurred.

4. The river, therefore, deposits part of its load at that point, thus, building up the channel gradient. The rapid is removed and a new profile or “equilibrium” is established.

5. Erosion has erased the waterfall at the cliff formed by the fault, and only a small rapid is left.

7. Drainage Patterns. Tributaries of a river usually form one of three types of drainage patterns.

Dendritic. The dendritic pattern is the simplest drainage pattern. This forms when the tributaries flow at random so that a map of the river and its tributaries look like a tree with the river itself forming the trunk and the tributaries forming the branches.

Trellis. Geography has an effect on the drainage. If the landscape consists of a number of parallel ridges, which happens frequently where there are beds of hard rock alternating with beds of soft rock, the result is a trellis pattern. The rivers and tributaries either follow the parallel valleys formed in the soft rock or they cut directly through the ridges of hard rock. As a result, the tributaries and rivers meet each other at right angles in a square, blockshaped arrangement.

Radial. When a new hill is pushed up in the form of a dome, the rivers will flow down the sides of the dome away from the high center. The result is a radial drainage pattern with all the rivers flowing outward.

8. Springs. Rain and melted snow seep into the ground. Ground water emerges from the surface of the earth as a spring. It may be a trickle or a gusher.

9. Waterfalls and Rapids. Tumbling water does not wear away the river bed rock at an even rate. Some rocks are less resistant to erosion than others. When the river reaches a layer of harder, more resistant rock, it runs across it and drops off the end as a waterfall. The softer, less resistant rock beyond the hard rock is eroded quickly by the force of water and the stones dropped upon it. A deep plunge pool forms at the foot of the waterfall. The swirling water in the plunge pool undercuts the hard rock and the rock face of the waterfall. A new waterfall then forms slightly upstream on the newly exposed rock face. This erosion gradually moves the position of the waterfall upstream and eventually the vertical drop disappears altogether.  (See Establishing equilibrium, above).

10. Transporting Sediment. Running water is such a powerful force in shaping the earth, not only because it can abrade and erode its channel, but also because of its enormous power to transport loose sediment produced by weathering.

The capacity of a stream to transport sediment increases to a third or fourth power if its velocity. That is, if the velocity is doubled, the stream can move 8 to 16 times as much sediment.

Within a stream system, sediment is transported four ways.

Dissolved Load. The dissolved load is matter transported in the form of chemical ions and is essentially invisible. All streams carry some dissolved material, which is derived principally from the ground water that emerges from seeps and springs along the riverbanks. The most abundant material in solution are calcium and bicarbonate ions; but sodium, magnesium, chloride, ferric and sulfate ions are also common. Various amounts of organic matter are present, and some streams are brown with organic acids from the decay of plant material

Suspended Load. The suspended load is the most obvious, and generally the largest, fraction of material moved by a river. In most major streams, silt and clay-sized particles remain in suspension most of the time and move downstream at the same velocity as the flowing water to be deposited in a lake or on a flood plain.

Traction Load. Particles of sediment too large to remain in suspension collect on the stream bottom and form the bed load or traction load. These particles move by sliding, rolling and saltation (short leaps).

Bed Load. The bed load moves only if there is sufficient velocity to move the large particles. Thus, it differs fundamentally from the suspended load, which moves constantly. There is not always a sharp distinction between the largest particles of the suspended load and the smallest particles of the bed load because the velocity of a stream fluctuates constantly. Part of the bed load can suddenly move in suspension or part of the suspended load can settle. The bed load can constitute 50 percent of the total load in some rivers, but it usually ranges from seven to ten percent of the total sediment load. The movement of the bed load is one of the major tools of stream abrasion because as the sand and gravel move, they wear away the sides and bottom of the stream channel. In some rivers, such as the Colorado and Little River in Alabama, the grinding action of the bed load can be heard as large boulders are moved along the river’s bottom

Running water is the major agent of erosion. Rivers in the United States transport an average of 1.3 million tons of sediment per day to the oceans. The rivers of the world combine to unload about 10 billion tons of sediment into the sea every year, including the weathered remains of the mountains where some of them were born.

11. Regolith. One of the most important processes of erosion is the removal and transport of rock debris, or “regolith,” produced by weathering. The process is simple but important. Loose rock debris is washed downslope into the drainage system and is transported as sediment load in streams and rivers. In addition, soluble material is carried in solution. The net result is that the blanket of regolith created by weathering is continually being removed and transported to the sea by stream action. As it is being removed, however, the weathering of fresh bedrock is also continually regenerating it.

Measurements of the amount of sediment carried by rivers indicate that the surface of the land is being lowered by on the average of 6cm (2.4 in), every 1000 years.

Stones carried along by a river tend to have their corners knocked off. After being carried long distances, they are worn to a round shape and polished. If we look at a river sediment that consists of jagged lumps of rock, we can be sure that this material has not been carried very far. River gravel formed of spherical pebbles has been brought great distances.

If the river suddenly slows down, perhaps because it is coming out of a mountain gorge and onto a plain, most of the material it carries along is dropped suddenly. If we find a river sediment in which course stones and muds are jumbled together, we can say that the current suddenly slowed here. On the other hand, distinct layers of particles that are all the same size means that the current has a steady flow.

12. Pot Holes. Pot holes are erosional features usually found in the rocks of fast moving streams. These circular depressions in rocks are formed by the grinding action of trapped sand and gravel being swirled around by strong currents within the river. Starting points for potholes are usually niches or breaks in the rocks, such as poorly developed joints along foliation planes, where water flow becomes disturbed and begins to swirl and erode.

13. Flood Plain. A flood plain is a relatively flat area adjacent to a stream creek or river that is frequently covered by water during flooding. A flooded stream has the capacity to carry more and larger sediments in its channel due to the high velocity. As the stream overflows its banks, the velocity of the overflow water decreases and some of the stream’s sediment load is deposited adjacent to the stream in the flood plain.

Natural Levees. Natural levees are natural dikes, or flood barriers, that build up on the banks of a river each time it overflows onto the flood plain and deposits sediment. Of all the sediment suspended in floodwaters, the finest sediment is carried by the flood waters farther out into the flood plain, The coarsest sediment drops out first closest to the river channel where it builds up a high embankment. With each flooding, the river dumps more and more material on the edges, and the natural levees continue to grow higher. At the same time, the river channel is also rising due to the deposit of sediment. In time, the river can actually be higher than the surrounding floodplain. The natural levees of the Hun Huaang River in China are so high that the riverbed is 15 feet above the flood plain.

Backswamp. As a result of the growth and development of natural levees, much of the flood plain may be lower than the river flowing across it. This area, known as a backswamp, is poorly drained and commonly the site of marshes and swamps.

Yazoo Streams. Tributary streams in the backswamp are unable to flow up the slope of the natural levees so they are forced either to empty into the backswamp or to flow as Yazoo streams that run parallel to the main stream sometimes for many miles. Yazoo Streams are named after the river of that name in Mississippi.

Meanders and Point Bars. As the stream rounds a curve, the water on the outside of the curve has to flow faster than the water on the inside. This causes the bank on the outside of the river bank to erode. Conversely, on the inside of the curve, the stream is going very slow and part of the sediment it is carrying drops out, creating a point bar. More on meanders and point bars below.

Oxbow Lakes. An oxbow lake is an abandoned meander that has been cut off from the rivers main channel. The river shortens its course by cutting across the neck of a meander instead of flowing around it. Oxbow lakes soon become filled with mud and silt, which aid the growth of reeds, and eventually disappear. Under normal conditions, it may take hundreds, or even thousands, of years for a river to create an oxbow lake but; at the height of a flood, it can create one in less than an hour.

Crevasse Splay. At times the levee may be breached and the water will flow out over the flood plain forming a Crevasse Splay across the plain from the breach.

14. Stream Flow. The location of a river’s channel depends on the configuration of the stream. Where the stream is straight, the channel is in the middle. As the stream turns, the channel shifts to the outside bend or curve. The flow in a channel follows a corkscrew pattern. Water on the outside of the curve is forced to flow faster than on the inside of the curve. This difference in velocity, together with normal frictional drag on the channel walls, produces a corkscrew flow pattern. As a result, erosion occurs on the outer bank and deposition occurs on the inside of the bend. These processes produce an asymmetrical channel, which slowly migrates laterally.

15. Meanders. Meanders begin where the stream’s flow is deflected by an irregularity and moves to the opposite bank where erosion begins. The meander enlarges and migrates laterally. At the same time, a point bar is growing on the opposite side of the river. There is a general downslope migration of meanders as they grow larger and, ultimately, cut themselves off to form oxbow lakes. The new, straighter channel will not remain straight for long. Any curve in it will begin to erode on the outside and build up sediment on the inside, and the process will start again.

16. Stream Terraces. Stream terraces develop along the sides of flood plain valleys when the geological forces raise the riverbed. The river cuts downward and gradually forms a new flood plain. If the riverbed is raised again, the process is repeated. A river may erode stream terraces as it meanders across the valley, or a river can bury a stream terrace under sediment.

1. A stream cuts through a valley by normal down cutting and headward erosion.

2. Changes in climate, base level or other factors that reduce energy flow cause the stream to partially fill its valley with sediments forming a broad flat floor.

3. An increase in flow energy causes the stream to erode through the previously deposited alluvium. A pair of terraces is left as a remnant of the former flood plain.

4. The stream shifts laterally and forms lower terraces as subsequent changes cause it to erode the older valley fill.

Go to the GA 128 Bridge west of Roberta and look upstream to see this on the Flint River.

17. Eddy. Confronted with a boulder, water, which can’t be compressed like air can, piles up on the upstream side. When it flows around the boulder, it creates an eddy. Since the water in the eddy downstream is lower than the water piled up in front, the water flows back upstream toward the boulder to fill the resulting depression. We usually think of everything in a river flowing downstream, but even in the swiftest whitewater river, there are strong upstream currents.

18. Braided Streams. Braided streams have channels that run in various ways. They split up and come together, creating little islands called bars. These intricate patterns can be the result of melted water from glaciers or because a river is so choked with sediment it can’t maintain a single channel.

19. Stream Piracy. All streams have a tendency to erode upstream at the headwaters. When this occurs, the tributaries of one stream can extend upslope and intersect the middle course of another stream, thus, diverting the headwaters on one stream to another. This process is known as “Stream Piracy.” Stream Piracy is most likely to occur if one stream has a steeper gradient or more easily eroded rocks than another.

Here is an example of Double Stream Piracy that occurred on the Flint, Chattahoochee and Apalachicola rivers millions of years ago.

Geological evidence suggests that the Apalachicola River was once only a small tributary of the Chattahoochee. The Apalachicola actually captured the Flint. Then the Flint captured the Chattahoochee. Originally, the Flint and Chattahoochee flowed west around what is known as the Tallahassee Highlands. At this point in time, the Apalachicola was a small tributary of the Chattahoochee just beginning to cut into the Highlands.

Still a tributary of the Chattahoochee, the Apalachicola cut through the Highlands, capturing the Flint and diverting its waters southward along the newer channel. Meanwhile, the beheaded section of the Flint became an inverted stream, flowing back into the Apalachicola while slowly cutting headwaters to the west.

The beheaded portion of the Flint itself became a pirate and captured the Chattahoochee. and rapidly enlarging the Apalachicola River valley.

20. Wetlands. Wetlands are natural areas that hold water. There are many different kinds of wetlands: swamps, marshes, bogs and mangrove swamps are some of them. Generally found in low-lying areas, wetlands may be as small as a wading pool or as big as a lake. Water is an essential ingredient of a wetland but it isn’t always there. Some wetlands stay wet all year while others dry out for months at a time. Wetlands on the coast fill up and drain twice a day because of the ocean’s tides.

• Wetlands benefit people by providing free services that are worth billions of dollars each year.
• Wetlands help clean our water. They can be a great help in cleaning up polluted water. Because water moves slowly in wetlands, silt and sediments settle out. Wetland plants absorb certain nutrients and chemicals that pollute rivers, ponds and lakes.
• Wetlands help control floods. When rivers overflow, wetlands help control flood waters. They hold excess waste and slow the fast currents of overflowing rivers.
• Wetlands provide recreation. They provide opportunities for fishing, canoeing, hiking and bird watching.
• Wetlands provide homes for wildlife. On a per acre basis, more plants and animals live in wetlands than anywhere else. About 35 percent of all plant and animals listed as threatened or endangered in the United States either live in wetlands or rely on them in some way. In wetlands, you see birds, ducks, turtles, fish, muskrats, beavers, deer, raccoon and a host of other animals.
• Wetlands, particularly coastal wetlands, are breeding grounds for many fish. Most of the fish and shellfish we eat live in wetlands when they are young, where they find food and protection from larger fish. Wetlands along the gulf and Atlantic coasts are especially important as fish nurseries. Wetlands support a multi-billion dollar a year commercial and recreational fishing industry.

Over the past 200 years, expansion of human development has caused the destruction of valuable wetlands. Of the approximately 215 million acres of wetlands originally existing in the lower 48 states, fewer than half remain.

21. Lakes. Base Level. The base level of a stream is the lowest level to which the stream can erode its channel. It is a key feature in the study of stream activity. A temporary base level may be a lake. The ultimate base level is where the stream enters the ocean.

When a river reaches a hollow in the landscape, the water may build up to form a lake. The river current slows down and the rocky debris is deposited on the lake bottom. When the river leaves the lake, the water is clearer than when it went in.

Lakes tend to be comparatively temporary features. A river deposits sediments on a lake bed, which builds up and eventually fills in. At the same time, reeds and other water plants grow around the edges, and when they die, the vegetable matter builds up trapping even more sediment. The lake will fill up completely and become first a marsh and then dry land. The area then becomes a valley floor or a floodplain with the river flowing across it.

Dam construction greatly effects the equilibrium in a river system. In the reservoir behind a dam, the gradient is reduced to zero. So where the river enters the reservoir, its sediment load is deposited as a delta and layers of silt and mud build up on the reservoir floor. Because most sediment is trapped in the reservoir, the water released downstream of the dam doesn’t have much sediment and is, therefore, capable of much more erosion than an undammed river, which carried a sediment load, adjusted to its gradient. As a result, extensive scour and erosion commonly occur downstream from a new dam.

22. Marshes. The coastal marshes are up to seven times as productive as cultivated wheat fields, complete with irrigation and fertilizer. Economic evaluation of the marshes shows that from a production value they are worth $89,000 per acre, a renewable resource with no overhead.

23. Deltas. When a river is full of rocky debris and sediment and enters a sea that is quiet and has no strong currents, the sediment is deposited at the river mouth. Layers of silt and sand are built up and the river breaks into several channels winding around the sandbanks and entering the sea by a number of mouths. The result is a delta.There are several types of deltas:

Bird’s Foot. Sometimes, as at the mouth of the Mississippi, the delta consists of levees that extend out into the sea. The individual channels are lined by long narrow banks, producing a levee that resembles a bird’s foot.

Arcuate. When the sediment is dumped immediately, the channels are continually being blocked and the water finds new outlets. The constantly changing patterns of channels produces a rounded front, an arcuate delta like that of the Nile.

Cuspate. When a delta builds out into the sea gradually, there may be only a single mouth to the river with the delta sands curving back to the land on each side. This is a Cuspate Delta like that of the Tiber River and the delta found at Apalachicola Bay.

24. Beach Sand. A beach is made up of whatever a stream carries to the shore. Each beach has its own special material. The Flint and Chattahoochee rivers form the Apalachicola, which drains straight out into Apalachicola Bay forming a delta. If you trace those rivers back to where they came from, you’ll see that they came from the Piedmont and Blue Ridge. The Piedmont and Blue Ridge are made out of schists and gneiss and things like that. If you get rain falling on granite, which is made of mica, feldspar and quartz, the mica and feldspar start to deteriorate and turn into clay and so they wash away. Then all you have left is little grains of quartz. Quartz is very stable and tends not to break down chemically, partly because it is such a simple composition – the silicon oxygen bond that holds quartz is one of the strongest in nature. Now these little grains are loose. They are no longer part of a rock. So, the stream transports them as sand, and they’ll end up on a beach at St. George or St. Vincent Island or somewhere else.

If you get far enough down into Florida and look at the beaches down there, they are not made of stuff transported from the Piedmont and Blue Ridge because they are being fed by streams coming off the Florida Peninsula, which is covered with limestone. So, the Florida beaches are just eroded limestone. They have particles of limestone and little shell fragments and things like than.

The beaches are made of whatever the rivers carry to the shore.

25. Estuary. A constantly changing mix of fresh and salt water at the junction of rivers and oceans, estuaries are home to a tremendous abundance of life. River currents send freshwater downstream and ocean currents push saltwater upstream. When the two meet, they form a wedge; the lighter freshwater continues flowing on top toward the ocean, and the heavier salt water is driven downward and up the channel. If the freshwater is clear, the wedge can easily be seen. Clear water flows downstream on the surface, but underneath, dark muddy water flows in the opposite direction. The location of the wedge within an estuary zone changes often – sometimes within a matter of minutes – depending on tides, seasons and weather conditions. Strong incoming tides and storms can push it far up the channel. So can low river flows during summer and winter. Conversely, outgoing tides or spring river floods send it far into the bay. Temperatures in the wedge vary as well. In summer and autumn, incoming freshwater is warmer than the ocean. In the winter and spring, the opposite is true. Life is extremely difficult in the estuary because of rapidly changing water levels, currents, salinity content and temperatures. Despite the harsh conditions, there is plenty of food for estuary life. Much of the nutrient load that comes with rivers is slowed or stopped by the incoming ocean current. The settling organic matter feeds marsh grasses, algae and microscopic plants and continues up the food chain. Estuaries support more life than any other natural environment. They serve as a spawning ground for over 75 percent of all commercially harvested shrimp, crabs, oysters and fishes.

The Flint and Chattahoochee rivers meet to form the Apalachicola River, which flows 106 miles through Florida to Apalachicola Bay. The estuary where the Apalachicola River meets the ocean is not only beautiful, it is one of the most productive estuaries on a per acre basis in all of North America and one of the most important estuaries in the world. Shrimp, oysters and many types of fish are either born here or spend part of their early life in this estuary. More than 90 percent of Florida’s oysters are harvested in the bay.

26. Harnessing the Power of Rivers. In building first their mills and then their cities along rivers, the pioneers of America were following a pattern of settlement as old as civilization.

• Early settlers harnessed the power of rivers to operate their gristmills. The first textile mills were water powered.
• They built small dams on rivers that held the water back and created reservoirs.
• The water in a reservoir was diverted into a millrace that sent it into bucket-like paddles that turned a waterwheel.
• The rotary motion of the waterwheel is transferred to the millstone through a series of gears.
• Water is returned to the river through the tailrace.

27. How Hydroelectricity Works. The principles used in a modern hydroelectric plant are the same as those used centuries ago. Back then, the natural energy of falling water, or flowing water, was harnessed and changed into mechanical energy by paddle wheels like those seen on old gristmills. The water turned the wheels and the wheels turned the machinery.

In the late 1800’s, hydro energy was first changed into electric energy by allowing water to spin turbines connected to electric generators instead of paddle wheels. To meet the ever-growing demand for energy, dams were built to hold and store the enormous amounts of water in reservoirs. Water was periodically released from the reservoirs to produce thousands of kilowatts of electricity. In this illustration, you can see how a hydroelectric power plant generates electricity.

Falling water from the reservoir (1) passes through the penstock (2) to enter the powerhouse. The flowing water turns the propeller-like water wheel, or turbine (3), which is connected by a shaft to the generator (4), which spins and produces electricity.

Turbine. The same water that flowed through the turbine (3) is then discharged through the draft tube (5) where it enters the tailrace (6) and returns unaltered to the river below the dam.

The electricity produced by the spinning generator (4) is conducted to the power transformer where the voltage is increased. The high-voltage electricity is then fed into transmission lines for distribution to electricity customers.

Hydroelectric power is one of the most important benefits of civilization’s modification of rivers. Building dams across river valleys means that running water can be contained so it can be used how and where it is most needed, whether it is to prevent flooding, divert water into irrigation channels for agriculture, provide reservoirs for water storage, or to provide a source of power for electricity. At the same time, the construction of dams and the generation of hydroelectric power changes the normal dynamics of river flows.

28. Water for Cities. Rivers and streams are the primary sources of water for cities and the way cities dispose of their wastes.

The water system of a modern city is designed to provide 130 gallons of water per person per day. That’s used for bathing, washing dishes, flushing toilets, doing laundry, watering the grass and drinking. In most water systems, up to 30 percent of the water pumped from the river is lost because of leaks.

• In the treatment plant, impurities are removed and germs destroyed to make the water safe for drinking. Sedimentation, filtering and the addition of chemicals are the main methods of treatment.
• Water is pumped from a river into the treatment system.
• Waste from the sedimentation process is collected into a basin.
• In the sedimentation tank, chlorine and slaked lime are added.
• Filtration and sterilization take place.
• Sulfur dioxide is added to eliminate chlorine.
• Water is stored in a reservoir. The height of the reservoir provides pressure for water distribution. The water is distributed from the reservoir to the buildings in the city.
• Pressure in the system sends water to the top of the buildings.
• What comes in must go out. Drainpipes send water to the sewer system.
• These sewer pipes are oval, an efficient shape for carrying away waste.
• Solid waste settles to the bottom of the tank.
• Sewage and sludge are separated.
• Sludge is treated.
• Several stages of sedimentation and aeration take place during the treatment  process. Some communities have created “wetlands” where natural processes do much of the work of a sewage treatment plant.
• After treatment, water is returned to the river.

Cities can draw fresh water from only two sources: Rivers and lakes and the ground. Most U.S. cities, especially those with fewer than 5,000 persons, get their water from underground supplies. Most larger cities get theirs from rivers and lakes. Nationwide, most Americans are served by rivers and lakes.

29. How Rivers Work is Changing. The earth is about 4,500 million years old. And rivers have been around just about that long. Humans have been here only a tiny, tiny fraction of that time… Maybe a million years? For most of that time, they had little effect on the way rivers worked. But just in the last 200 or so years, humans have had a major impact on the natural river system. Since the industrial revolution, the growth of tons and tons of discharges from factories has added to the problem and has changed rivers.

It’s harder and harder for Earth’s finite water supply to purify itself. Nature now has to cope with new chemical compounds that won’t break down, such as plastics, many different kinds of chemical wastes produced by industrial plants, and loads of sewage from growing cities. An increasing percentage of the world’s limited water supply is being rendered unfit for human use.

• Urbanization changes the surface of the earth and affects the way the water runs off the land. Roads and sidewalks repel water. They don’t take in water. Water is channeled through gutters, storm drains and sewers. As a result, flooding increases in intensity and frequency.
• When river water is used to cool a nuclear reactor or coal-fired electric power plant, it returns to the river warmer than when it was taken out. This causes changes to plant and animal life.
• It’s possible that some of the chemical wastes that are released into rivers by modern industry may poison the water.
• Bacteria in the water breaks down organic wastes dumped in rivers. This process uses oxygen. If too much waste is dumped, the oxygen in the water may be used up before the river has purified itself. Fish thrive in water that is rich in organic matter; but without enough oxygen, they very soon die.
• Industries and vehicle exhausts give off gasses (including sulfur dioxide and nitrogen oxides), which combine with water vapor in the air to form acid rain. This rain, which is diluted sulfuric acid, is fatal to vegetation. Acid rain has killed many of the trees in the mountains along the East Coast in the United States.
• Pesticides sprayed from the air are dangerous to humans and other animals.
• Pesticides and fertilizers used in agriculture filter through the soil and may eventually poison aquifers and the springs that those feed.

Under natural conditions from 80 to 100 percent of the surface water filters into the subsurface and from 0 to 20 percent flows through the drainage system.

In urban areas, from 0 to 10 percent filters into the subsurface and from 90 to 100 percent moves as surface runoff, increasing the water volume and temperature.

Many cities are not near rivers or lakes large enough to meet their needs. These cities use water that is stored underground. This water comes from rain that soaks into the ground. As it trickles downward, it fills spaces between grains of sand and cracks and pores in rocks. In time, the water reaches a layer of rock or other material that is watertight. The water collects above that watertight layer and the ground becomes saturated. This saturated zone is called an aquifer. The top of the zone is called the water table. Cities obtain underground water by drilling wells below the water table and pumping up the water.

The most influential factor in water quality is land use. Storm water runoff from various land uses transports pollutants to streams.

30. River Fact and Quotes
Running Water Shapes the Planet. The most vigorous mover and shaper on terrestrial Earth is running water. Stream valleys are the most abundant and widespread landforms on the continents.

How Much does Water Weigh? A gallon of water weighs 8.33 pounds. A cubic foot of water weighs 62.4 pounds.

Measuring River Flow. Cubic Feet per Second or CFS is a way of measuring water flow. It is the volume of water moving past a specified point at a specified time. A small river might flow 600 cubic feet per second, meaning that a slice of water containing 600 cubic feet of water weighting 37,440 pounds, flows past a given point in one second. A large river might run over 100,000 cfs. The Colorado River in the Grand Canyon averages 30,000 to 40,000 cfs. From 1977 through 1992, the discharge of the Flint River based on mean daily flow at Newton, Georgia was 4,030 cfs.

Damned Rivers. Nationwide, 600,000 miles of rivers are dammed by 68,000 large dams (over two stories high). The Yellowstone River in Wyoming and Montana  (more than 600 miles long) is the only major American river that remains undammed. There are nearly 2 million small dams (dams under two stories) in the United states.

Water Power. Water can dissolve almost any substance. It dissolves the hardest rocks as it runs over the land and seeps through the ground. In time, it carries the dissolved minerals to the oceans. Water also dissolves the nutrients that all living things need. It dissolves and carries the nutrients in soils to plants and to the cells within plants. Water also dissolves the food that humans and animals eat and carries it to the cells.

Where Does River Fog Come From? Ever wonder why mists often shroud rivers, creeks and valleys during cloudless nights in the summer and fall? During these times of year, the sun warms up the water causing it to evaporate a lot of moisture into the air next to it.Throughout the night, however, the water surface and adjacent humid air lose heat rapidly to the colder atmosphere above.  During calm and clear nights, the temperature of the humid surface air often falls below the dew point. When this change happens, the air cannot hold as much moisture as before and condenses to form innumerable water droplets, which form a fog. If the air is windy, the fog tends to blow over the water and dissipate. If the sky is overcast, there is usually enough heat radiating from the clouds back to the earth to prevent cooling of the surface air to the dew point and consequent formation of fog.

Land Use and Rivers. Land use is the most important factor in a river’s water quality. Although urban and suburban land use accounts for only 5 percent of the Apalachicola Chattahoochee and Flint Basin, it has the most important effect on stream-water quality. The intensity of the land use effect on water quality varies in proportion to various measures of urbanization, such as impervious area, population density and percent of industrial and transportation land use. As the percentage of urban land use incases within a watershed, nutrients, pesticides, trace elements and organic compounds are more prevalent and occur at higher concentrations in streams. Source: U. S Geological Survey Circular 1164.

Longest Free-flowing River. The Yellowstone is the longest free-flowing river in the lower 48 states.

Georgia’s Free-flowing Rivers. With 153 miles of free-flowing water, the Flint River is one of only 42 free-flowing river reaches longer than 125 miles, remaining in the contiguous 48 states. Other Georgia rivers with 125 miles or more of free-flowing water are: the Alapaha (125), Altamaha (128) Ocmulgee (184), Oconee (134) and  Savannah (170).

Don’t Mess with Rivers. In the world there is nothing more submissive and weak than water. Yet for attacking that which is hard and strong nothing can surpass it. Loa-Tzu (Chinese philosopher of the 6th century BC)

31. River Glossary
Aquifer. Underground rock layer, which collects and holds water. Aquifers may consist of sedimentary and porous rocks, fractured and cracked rocks, and loose deposits of sand and gravel.

Bed Load. Material transported by currents along the bottom of a stream or river by rolling or sliding, in contrast to material carried in suspension or solution.

Base Level. The level below which a stream cannot effectively erode. Sea level is the ultimate base level, but lakes form temporary base levels for inland drainage systems.

Buffer Zones. Buffer zones are the protective margins mandated by most states and federal resource agencies to protect stream banks from logging and development. Opinions differ as to how wide minimum buffer zones should be. Georgia buffer zones are 25 feet for warm water streams and 50 feet for trout streams.

Discharge. Rate of flow. The volume of water moving through a given cross section of a stream in a given unit of time.

Drainage Basin. Area of land in which all water falling as rain or snow runs via ditches, streams or other water courses into a single river or lake or into the sea.

Drainage System. An integrated system of tributaries and a trunk system which collect and funnel surface water to the sea, a lake or some other body of water. The Drainage Basin is the total area that contributes water to a single drainage system.

Erosion. The process that loosens sediment and moves it from one place to another on the Earth’s surface. Agents of erosion include water, ice, wind and gravity. In the case of water, it breaks off fragments of rock on sloping ground, carries them away, and eventually breaks them down into smaller pieces.

Estuary. A bay at the mouth of a river formed by deposition of the sand or by a rise in sea level. Fresh water from the river mixes with and dilutes seawater in an estuary.

Flood Plain. The land adjacent to streams that is subject to periodic flooding is called flood plain land.  Flood plains function as emergency storage space, seepage areas and passageways for storm water during floods. Flood plains hold the excess flows until the streams normalize again.
Flood plains are often more diverse than the adjacent upland areas. This unique environment represents a gradient in vegetation, moisture and soils, which create a number of habitats. Twigs, branches and leaves, falling from the floodplain vegetation, provide important instream habitat for insects and fish. Additionally, this vegetation provides a food or energy source that is important to the entire aquatic food web. Land is often referred to as being in the 100-year flood plain or the 500-year flood plain.  These terms may be misleading. The term does not mean that a 100-year flood will occur only once over a 100-year period. The 100-year flood has a 1% chance occurring in any given year. Likewise the 500-year flood has a 1-in-500 chance of occurring in any given year.

Groundwater. Water below the Earth’s surface. It generally occurs in pore spaces of rocks and soil.

Headwater Erosion. Extension of a stream headward, up the regional slope of erosion.

Karst Topography. A landscape characterized by sinks, solution valleys, and other features produced by groundwater activity.

Load. The total amount of sediment carried at any given time by a stream.

Marsh. Wet, low-lying ground that is temporarily or permanently covered with water, characterized by aquatic, grasslike vegetation.

Runoff. The process by which water that has fallen on land runs from higher ground to lower and, eventually, makes its way to the sea.

Sand. Sedimentary material composed of fragments, ranging in diameter from 0.0625 to 2 mm. Sand particles are larger than silt particles but smaller than pebbles. Much sand is composed of quartz grains because quartz is abundant and resists chemical and mechanical disintegration, but other materials, such as shell fragments and rock fragments, can also form sand.

Sediment. Material, such as gravel, sand, mud, and lime, that is transported and deposited by wind and water.

Sedimentation. A natural phenomenon whereby a river as it reaches lower ground and its speed lessens, deposits the small particles of sand, mud and rocks it was carrying.

Shoal. A bank of sand or rocks just below the surface of a water body. In general, an area of shallow water.

Silt. Sedimentary material composed of fragments, ranging in diameter from 1/265 to 1/16 mm. Silt particles are larger than clay particles but smaller than sand particles.

Slough. Old river channel that now resembles a lake, pond or canal

Solution Chamber. A hollow in a water-soluble rock layer underlying surface soil, caused by the dissolving action of water percolating from the surface.

Sorting. The natural separation of particles, according to size, shape or weight. It occurs during transportation by running water.

Stream Load. The total amount of sediment carried by a stream at a given time.

Suspended Load. The part of a stream’s load that is carried in suspension for a considerable length of time without contact with the streambed. It consists mainly of mud, silt and sand. Contrast with bed load and dissolved load.

Transpiration. Plants draw water from the ground through their roots. Then, the plants passe the water out through their leaves as vapor in a process called “transpiration. A birch tree gives off about 70 gallons of water a day. An acre of corn gives off about 4,000 gallons of water per day.

Tributary. A stream flowing into or joining a larger stream.

Ultimate base level. The lowest possible level to which a stream can erode the earth’s surface: sea level.

Upland. High ground, not wetland.

Water Table. The upper surface of the zone of saturation. Only a certain, sometimes small, percentage of water runs off in stream channels. Usually, water percolates downward through the soil until it reaches a point of complete saturation. Above this level, known as the water table, is filled with air. Water tables fluctuate considerably, according to the amount of water received and absorbed. Some soils hold water for a long time, but others lose it rather quickly.

Watershed. The ridge or crest line separating two drainage basins. Rain falling on either side of the watershed will run off towards one river system or the other.

Wetland. Any area that is more or less regularly wet or flooded, where the water table stands at or above the land surface for at least part of the year. Dr Eugene Odum, the legendary ecologist at the University of Georgia, quips that the plot should pass the squish test. “If you put a foot in and you hear a squish, it’s a wetland.” Wetlands are areas with enough surface or ground water to support vegetation adapted to life in saturated soil conditions. Some wetlands are wetter than others. They may hold water permanently or only a few days each year. Marsh, swamp, floodplain forest, bottomland, bog, fen, slough, wet meadow, prairie pothole - each of these names may apply to a wetland, depending on where the wetland is located, what grows in it, or how it gets its water.

Zone of Saturation. The zone in the subsurface in which all pore spaces are filled with water.