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| Stream passing through the valley, Tash Rabat, Kyrgyzstan |
Streams are any size body of moving surface fresh water driven towards sea level by gravity (force of attraction between two masses). Water scientists refer to all bodies of flowing surface water as streams regardless of size, yet in common language, streams are considered smaller than rivers.
Stream systems are networks that collect fresh water runoff from the land and carry it to the ocean. Together, tree-shaped systems of small branch streams drain vast areas of the continents into large rivers.
Stream systems of all sizes erode (wear down) sediment (particles of gravel, sand, and silt) along their courses and carve complex patterns into the landscape. They wear down slowrising mountains and fill valleys and lowlands (low and level lands) with layers of sediment. Stream systems change character along their courses.
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Steep mountain streams feed shallow elevated streams that in turn flow into meandering rivers that snake across broad floodplains (flat, low-lying land near a stream that is covered with water when the stream overflows its banks). Deposits of sediment form at river mouths, the area where fresh river water enters the ocean.
If a rubber duck was dropped into a mountain stream on Pike’s Peak in Colorado, it might tumble down the mountain-side in whitewater rapids to Cripple Creek. From there, the duck would rush over gravel beds where Colorado miners once panned for gold, and then float serenely across Kansas, Oklahoma, and Arkansas on the Arkansas River.
It would pause to drift across huge man-made reservoirs, and then plunge through the spillways of dams before entering the swift, muddy waters of the Mississippi River. A few weeks or months later, you might spot the duck heading out to sea amid barges and river boats in New Orleans.
Watersheds and drainage patterns
The land area that drains water into a stream is called a watershed or a drainage basin. A basin is a natural depression in the surface of the land. Watersheds can be as small as a hillside that feeds a wet-weather creek, and as large as a drainage system like the Amazon Basin that carries the runoff from most of a continent.
Large watersheds are composed of many smaller drainage basins. The boundaries between watersheds, called drainage divides, are ridge lines or high points where water flows down and away in all directions.
A divide can be limited, like a ridge between two mountain gullies (deep ditches or channels cut in the earth by running water, usually after a rainstorm), or extensive, like the North American Continental Divide along the spine of the Rocky Mountains.
Water that falls east of the Continental Divide eventually flows into the Atlantic Ocean, and water that falls west of the Rockies ends up in the Pacific Ocean.
Streams are arranged within watersheds in networks that feed water into larger and larger streams. Tree-shaped (dendritic) systems composed of small branch tributaries (small streams that flow into larger streams) that join and flow into large trunk streams are the most common type of stream drainage pattern.
Less common drainage patterns develop where rock layers and geologic features affect the paths of streams. Drainage patterns shaped like cross-hatched garden trellises develop in hilly areas where there are ridges and valleys, and streams flow out from round volcanic mountains in radial patterns like spokes on wheels.
Valley and channels
Streams cut down into the land surface and create valleys. A stream valley includes the entire area between hills on either side of a stream. The water-filled path of the stream at a specific point in time is called a channel. Over time, channels migrate back and forth and fill stream valleys with thick layers of river sediment.
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| snake across wide sediment filled valleys |
Some streams, particularly those in steep, mountainous terrain have narrow, V-shaped valleys and channels that fill most of the valley floor. Others, including most streams in gently-sloping basins and coastal lowlands have narrow channels that snake across wide sediment filled valleys.
For example, the Mississippi River has carved a valley more than 100 miles (161 kilometers) wide and filled it with sediment hundreds of feet (meters) thick over thousands of years.
Channel patterns
Stream channels assume different patterns within their valleys: straight, braided and meandering. While many channels have straight segments between meanders or braids, truly straight channels are quite rare.
They develop in steep, mountainous areas where geologic forces are slowing lifting up the land surface. Water flowing rapidly downhill from mountains saws straight channels down into solid rock.
Braided streams have many intertwined channels and islands of loose gravel that constantly shift across gravel-filled valley floors. They are common in streams that receive large pulses of water and course-grained sediment. The sediment-choked streams that carry water from the toes of melting glaciers are typically braided.
Streams that bend and curve across gently sloping valleys and coastal plains are called meandering streams. (Individual loops and bends are called meanders.)
During normal weather conditions, water flows in a narrow channel that snakes across broad plains of soft sediment. During floods, muddy water overflows the banks of the channel and deposits layers of mud and silt on the surrounding floodplains.
River floodplains are typically fertile farmlands that have been replenished by flood-waters. The coarser grained sediment settles out of flood waters closer to the channel builds natural levees (walls along the banks of a stream channel) along its banks.
The path of a meandering channel changes over time. Meanders grow from slight bends into nearly-circular loops. At a river bend, fast-flowing water erodes the outer channel bank and sediment accumulates on the inside of the curve in a deposit called a point bar.
Eventually, the bends at the neck of the meander grow so close that the water bypasses the loop. This process strands crescent-shaped segments of the former channel and round point bar deposits called oxbows on the floodplain. Oxbow lakes are abandoned meanders that contain water.
Channel patterns change down the course of a stream system between headwater streams and lowland trunk rivers. They also change over time as streams adjust to changing conditions of water flow, land incline, and amounts of sediment. Stream waters continuously erode and deposit sediment over time, and stream channels constantly shift across valley floors.
