Base level is the lowest elevation to which a stream can erode its bed. Ultimate base level is sea level (or, for endorheic basins, the terminal lake); temporary (local) base levels are any obstruction — a lake, a resistant rock layer, or a dam — that locally halts downcutting upstream of it. A tributary is not itself a base level; rather, it is controlled by the base level of the trunk stream it joins.
Sea level (ultimate base level for streams draining to the ocean), dams (artificial temporary base levels that pond water and halt erosion upstream), and lakes...
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