Others are nearly flat. The boundaries of a watershed are marked out on a map by drawing a line connecting the highest topographic points of two adjacent areas. These topographic highs can be envisioned as the lip of a funnel. The collection area or water body would be the spout of the funnel. Uplift is the physical force resulting in mountain building.
Degradation is the erosion of landforms by water, wind, and gravity.
More than any other factor, geology controls the size, length, shape, and slope of watersheds. The water that does not evaporate, percolates to deep aquifers, or become diverted to another watershed by humans ultimately flows to the river, stream, lake, or ocean that drains the watershed. These surface water systems receive their waters from a combination of overland surface flow and groundwater flow. In many complex geologic structures, surface-water systems may receive groundwater from aquifers that has originated from a different watershed. For the purpose of this minimonograph, discussions will be limited to surface watersheds.