Streams are any surface water where flow is confined in a channel. Over the long term, streams are the most important shapers of the land surface. Streams contain little volume of water compared to the rest of the hydrologic cycle but they are the most important in shaping the land because their water flows from the mountains to the sea and returns in a short time.
Streams reduce the elevation and also level the land. The long term result of stream erosion is a broad flat plain. The mechanisms of stream erosion also apply to the flow of density currents under the ocean where they form submarine canyons. Mars, where water once flowed, shows the effects of stream erosion. Liquid methane flowing on the frozen water on Saturn's moon, Titan, shows the same features.
Streams, rivers, creeks, rills, brooks, and more are all streams. They may be large or very small but the rules are the same. The small streams merge with larger streams until they finally flow into their lowest level, called the baselevel. , Baselevel for a stream might be a lake or a flat plain, but the ultimate baselevel for all streams is sea level.
The source area where the stream begins is the head of the stream and the location where the stream enters its lowest pointl is called the mouth of the stream. Smaller streams entering larger streams are tributaries and the stream they form may itself be a tributary to a still larger stream.
Streams can both erode and deposit material. Whether a stream erodes or deposits depends on several factors.
1. the slope or gradient. The steeper the slope the greater the ability to erode
2. the discharge or volume of water. The greater amount of water flow the greater the ability to erode
3. the friction the stream has with the channel or within itself. With greater friction, the ability to erode is less
The slope or gradient of a stream is the distance the stream drops vertically compared to the horizontal distance it travels. Sometimes it is referred to as "rise over run".
At right is a map of 6 mile creek from a region near Normal, Illinois.(click on the map for a larger size). You can get the complete quadrangle (4 Mb) here.
In this map, a tributary of 6 mile creek starts below section 3 at an elevation of 8000 feet. The tributary flows into a small lake on the north end of the map at an elevation of 760 feet. The distance along the path of the stream is 2.2 miles. We can find the gradient to be
(800-760) feet divided by 2.2 miles = 18 feet per mile.
The White River link shows an area between Petersburg, IL and Mt. Carmel, IL. showing the White River . Use the data on the map to determine the gradient for this river.
Although the towns are only about 20 miles apart the path of the stream is 44 miles. You should get about 1 foot per mile gradient.
Streams with a high gradient, more than about 10 ft / mile, are downcutting," youthful" streams that have not yet lowered the slope. . Streams with a low gradient, less than about 5 ft / mile are" mature" and are usually done downcutting and have begun side cutting.
As a stream cuts downward, material falls into the channel from the sides and is carried away by the stream. This creates a V-shaped valley.
In a photograph, the V shape of the valley is obvious. On the map, the V shape looks like this stream valley.
A V- shaped valley means a stream is cutting down, reducing the elevation and indicating a youthful stream.
A stream must have some slope in order to flow. When the gradient has been reduced so much that the stream can no lomger cut down, the stream begins to erode the sides of the valley. The stream meanders back and forth across the valley floor eroding the sides of the valley making it wider and leveling the surface.
Side-cutting produces a flat area next to the channel. When the stream floods this area may be covered with water so is called a floodplain.
Stream erosion depends on the speed of the water. The water speed may be greater at some times of year than others. Spring, for instance, is a time of flooding and much erosion. Summer is often a time of lower flow resulting in deposition.
The speed of water may also be different in different parts of the channel. Meanders have faster water and erosion on the outside of a bend. The slower water on the inside of the bend allows deposition.
As they erode and deposit meanders move across and enlarge the flood plain.
Streams cut the outside of the bend increasing the curve until finally the stream makes a shortcut through the neck of the bend. This leaves the old meander to silt up and form a U-shaped lake called an oxbow lake. Oxbow lakes are former channels. Eventually they fill in with sediment and become part of the flood plain.
A meandering stream means the stream is side cutting, leveling the landscape and indicating a mature stream
The volume of water that passes through the stream channel in a given time period is called the discharge. If a stream channel has to carry more water, it must either move through the channel faster, or the channel must become wider. A river usually gets wider as it goes downstream because tributaries flow into the main stream increasing the volume of water. The downstream channel also tends to become deeper.
Streams that drain a region will initially cut downward in a "youthful" stage. As we go toward the mouth of the stream the stream should become progressively more developed and attain a "mature" stage with meanders and broad floodplains.A stream that has created its own valley will have a smoothly decreasing gradient.
A dam or waterfall is a sudden change in the gradient and means the stream has not yet made the valley its own.A sudden change in the gradient will cause both erosion and deposition until the gradient has been made smooth again.
Tributary streams flow into the main stream. If the stream has created the drainage in the region, the tributaries will be younger than the main stream.
If the stream's degree of development is different from its valley or its tributary, another process has changed the conditions.
For instance, if a stream has meanders indicating mature, but has a V shaped channel indicating down cutting, there is a difference in stage of development between the meanders and the down-cutting. Below is the Colorado river in the Grand Canyon. The photograph shows the meander in the topographic map at right. You can see the river has meanders but it has a V shaped channel indicating it is down-cutting. This is a process called "rejuvenation" in which the land surface has risen in elevation after the stream had made the land flat.
When the land rose, the stream started down-cutting instead of side-cutting and so was trapped in its meanders. These are called entrenched or incised meanders.
Sometimes a stream may run in a region made flat by some other agent of erosion. In this case the stream may meander back and forth but won’t have a well defined valley because it did not create it.
This map of an area in Wisconsin (lat 42.833 , long. -88.719) shows a meandering river but also marshes and lakes. A mature stream would have drained the area so the marshes and lakes ( not oxbows) would not be there. This area was glaciated and the topography is uneven so the stream has little gradient for flow.
In this map of the Princeton Indiana area (lat. 38.438, long. -87.667), the White River meanders across its flood plain. The White river has a very wide floodplain, oxbow lakes, and other signs making it an "old age" stream that did create it's valley.
The Google map terrain view also shows the floodplain and meanders of the same area.
In section 3 in Knox county (just below the “plain” in floodplain) you can see some meander scars on the other wise flat floodplain. These and the oxbow lakes are the result of stream erosion and deposition.
In the area marked "down-cutting" you can see small tributaries, which are down-cutting into the higher hilly region. These hills are here simply because the river has not yet eroded them flat. A region long eroded by streams will be flat and the stream will have meanders.
Drainage PatternsThe way the tributaries connect may be influenced by the land they flow over. If streams start at the top of a cone shaped mountain. such as a volcano, the stream flows downhill making a radial pattern. If there are ridges or joints they may connect only where they have cut through the ridges forming a rectangular pattern.
If a stream is eroding an even slope with more or less uniform composition, the drainage pattern will be denrditic, like a tree and its branches, as in the satellite picture near Muscoda, Illinois shown below. As the stream flows toward the Blue River (towards the bottom in this photo) the river becomes more mature with a wider valley and more meanders.
The map and satellite photo below cover the Illinois, Kentucky, Indiana area. (Water appears dark in this image.)The Tennessee River on the right side of the photograph flows north from Tennessee through Kentucky. The dark lines of the river (Water appears dark in this image.) seem to stop at the north end. The odd pattern appears because dams have blocked the water forming a large lake that has flooded the valley upstream from the dam.
You may notice there are quite a few dams in this photo. Part of the reason for these dams is an attempt to control flooding along the river.
In the picture below you can see the same Mississippi system during the 1993 flood.
Notice the large increase in water in the Mississippi downstream from St. Louis. Flooding and urbanization are related and are a continuing controversy in Illinois. You can learn more about rivers and floods at the web site given here.
virtual courseware: Rivers and Flooding
You have heard of the continental divide. It is where rain falling on one side of the mountains drains to the Atlantc Ocean and water falling on the other side drains to the Pacific Ocean. Actually there are many divides between stream systems. The area drained by a stream is called its drainage basin. This map shows the drainage basins of the United States. The key to the numbers is below the map. The boundaries between these basins are all divides.
You can see that Chicago is on the divide between the Great Lakes basin and the upper Mississippi basin. Diversion of water from the Great Lakes drainage basin is severely restricted so the location of these boundaries is very significant.
The key rivers and drainage basins above are: 1 the lower Mississippi River, 2 the upper Mississippi River, 3 the Missouri River, 4 the Ohio River, 5 the Arkansas River, 6 the Columbia River, and 7 the Rio Grande River.