Recall from Lecture 5 that water in rivers and streams on earth makes up only 0.003% of all fresh water on earth. Yet, this very small fraction of the hydrosphere has a huge influence on the shape of the lithosphere. Water is a primary instrument in weathering processes (Lecture 9) with its unique ability to expand in its solid form, and in its liquid form dissolve nearly every rock-forming mineral. The process by which water wears away at the lithosphere is called erosion. The mechanism by which the eroded material is moved by the stream is called transport. When the material is deposited by the water in a new location, it is called deposition.
Stream related processes are called fluvial processes. The term river is applied to large flows of runoff, while stream applies to smaller flows. When water initially falls on the ground through precipitation, if it is not absorbed by the ground it runs off through sheet flow or overland flow. The water follows the pull of gravity and runs to lower areas, eventually collecting in the stream basins. A drainage basin or watershed is the terrain surrounding a stream valley that contributes to the water and sediments carried by the stream. Drainage basins are determined by topography and gravity. Water always runs down with gravity, so watersheds are always bowl or trough shaped. Watersheds can be divided from large to smaller and smaller units. The largest watershed unit in America is the Continental Divide which runs through the Rocky Mountains. Water that falls on the west of the continental divide eventually drains into the Pacific Ocean, water that falls east of the continental divide eventually drains into the Atlantic Ocean.
You can find the watershed that you live in through the EPA's Surf Your Watershed site.
Watersheds are open systems with inputs of precipitation from the atmosphere and material weathered from the lithosphere. Energy and materials are redistributed as the water and lithosphere material are carried to new locations. The topography of a region determines the drainage pattern of the area.
K. Allison Lenkeit-Meezan (Foothill College)