|Stream Water Flow|
Water flows downhill due to Earth’s gravity (force of attraction between two masses) pulling it. Streams, like rivers, are gravity-driven bodies of moving surface water that drain water from the continents.
Water scientists, called hydrologists, refer to all bodies of running water as streams, no matter their size so, in one sense, rivers are large, well-established streams). In everyday communication, it is common to refer to streams as smaller than rivers.
Streams transfer water that falls on the land as precipitation (rain, snow, sleet, and hail) to the oceans. Streams, again like rivers, constantly shift their courses and change length. The stream is carried along a defined path, called a channel. Water flowing in stream channels is a powerful sculptor that carves landscapes and molds sediment (particles of rock, sand, and silt).
It wears down mountain ranges and cuts deep canyons through solid rock. Stream waters support vibrant communities of plants and animals, and they have been the lifeblood of human civilization for thousands of years. Streams shape the land and are also integral to the hydrologic cycle (circulation of water on and around Earth).
Erosion and deposition
Streams are the main agent of erosion (wearing away) on land. Water in fast-moving streams is usually turbulent. The flowing water is filled with swirls and small localized whirlpools of swirling water called eddies.
Turbulent water picks up particles of sediment that have weathered from rock and soil and carries them downstream. (Weathering is the breaking up of rocks by physical and chemical processes, such as being exposed to the actions of water, ice, chemicals, and changing temperature.)
Faster-moving water can carry more sediment in the water, and can push larger stones along the bottom of the channel. Some mountain streams move huge boulders, while sluggish lowland (low country and level) streams carry only fine grains of silt and mud.
The sand grains and larger rock fragments that slide and bounce along stream beds wear away solid rock. In a straight stream, the fastest-moving water and area of greatest erosion is generally in the middle of the channel. Where a stream bends, the strongest current (a moving mass of water) is on the outside of the curve.
When water slows down, it drops its sediment load, causing sedimentary deposits to form along stream courses in areas of slow moving water. The slower the current, the finer the sediment it deposits. In straight channels, stream water lays down sediment along the stream banks.
In bending channels, sedimentary deposits called point bars form on the inside of the bends. Individual sediment grains travel downstream like hitchhikers. Sometimes the grains are picked up by a strong current or flood that moves them far downstream, but usually they don’t go very far in a single trip.
The grains of sand on a beach each made a long trip with many stops before they arrived at the ocean. Whether an individual grain of sediment moves depends on the speed of water currents that vary as the amount of water moving through a stream changes. As water currents become faster they can move larger grains.
Stream waters also erode rocks by dissolving its minerals, which causes them to crumble. Chemical weathering, also called dissolution, occurs when the slightly acidic water chemically alters the minerals in rocks, which causes them to break down.
Clear stream water carries the chemical components (parts) of the rocks’ minerals called ions (electrically charged particles). When conditions in the water change (the water slows or cools), the ions recombine into solid mineral crystals. This form of sedimentary deposition is called precipitation.
Limestone, salt, and gypsum form by precipitating from water. Ocean animals like corals and shellfish take in ions and use them to build their shells. Some types of rocks, including chalk and flint (also known as chert) form from the remains of organisms.
Graded streams and base level
All streams strive to reach a constant slope (incline) called a graded profile by eroding and depositing sediment. The profile (side-view) of a graded stream (a stream with a graded profile) is steep near the uphill end and gently sloping near the point at the end where a stream pours its water into a larger body of water.
The position of the downstream end of the profile is determined by the water level at the outlet, called base level. Streams cannot erode below base level. Almost all stream systems run to the sea, so sea level is the ultimate base level for most streams.
Conditions change constantly in all streams, and the process of readjustment by erosion and deposition is ongoing. As conditions change along its course, a stream will readjust its profile by eroding sediment in some places and depositing it in others.
If base level falls, stream waters cut down into the land surface. If it rises, they deposit more sediment. If the movements of the underlying plates of Earth’s crust rise (geologic uplift) to steepen the upper part of a stream, it will erode down to regain its graded profile. Streams also attempt to level out obstructions along their path.
They work to tear down dams, both natural and man-made, by erosion and filling the reservoir behind it with sediment. Lakes are, therefore, only temporary features of stream systems, and dams are interrupting the natural flow of a stream’s water.