A Water Drainage Basin or Watershed is the morphology and geology shaped by the influence of water.
The surrounding area of land where water from rain and melting snow or ice along with groundwater converges to a single point, usually the exit of the watershed drainage basin. This point is where the waters join another water body, such as a river, lake, reservoir, estuary, wetland, sea, or ocean.
We all live in a watershed.
The morphology and geology of basins and channels influence, if not determine, the movements and condition of water and ales that aquatic communities can exist in it. Conversely, water to a great extent and aquatic communities to a lesser extent, determine the morphology and substrate of watershed basins and channels. Conditions in the aquatic environment cannot be understood apart from this reciprocal relationship.
Crustal movements of the earth have formed some of the largest, deepest and oldest of the earth’s lakes. Other lakes are found in craters of extinct volcanoes; however, the greatest number of lakes formed by the action of ice or water itself. The principle forces involved are shaping river channel cutting and erosion, glaciers excavating basins or depositing debris and damming streams, and water dissolving underlying rocks from formations like limestone sinks.
Other processes alter the morphology and substrate of lakes once they are formed. Around lake margins, the littoral areas begin to form as a result of mineral debris deposited by tributaries and eroded from banks. In these areas, plants begin to grow and add their organic material to the sediments. Also, throughout the lake in deep and shallow water alike, organic debris from expired aquatic organisms rains to the bottom adding to the sediments. These processes of sedimentation are so important to the biological community of the lake that they may lead ultimately to the lakes extinction.
Streams and rivers generally flow in channels narrower than the basins of lakes. Although the waters are not deep, the channels they cut may become great canyons because water moving with the force of gravity carries tremendous loads of mineral and organic debris. In this way, streams and rivers deepen, widen, and even lengthen their channels.
The substrates of streams and rivers are primarily of native imported mineral materials, but the organic materials that accumulate are important to the metabolism of stream communities. Thus, the substrate of the stream channel is an important determinant of the kinds of organisms that can live there.
Beaches and stream bottoms of sand moving and grinding with the force of water are not very hospitable environments for most aquatic organisms, but where water flow is less, and silts and organic materials accumulate, or where streams are turbulent, and rocky bottoms are stable, the kinds and numbers of organisms adapted to live in these places are many.
We see then that the morphology of basins and channels, their shape, depth and extent determines movements of their water. These water movements coupled with the nature of the substrates, in turn, determines which aquatic ecosystem exists.