The base level of a river or stream is the lowest point to which it can flow, often referred to as the 'mouth of the river'. For large rivers, sea level is usually the base level, but a large river or lake is likewise the base level for tributary streams. All rivers and streams erode toward sea level, which is also known as the "ultimate base level." A rather rare exception can be seen in the Jordan River, for which the base level is the Dead Sea, 417 m below modern sea level. If a river is dammed, a new base level (the level of the reservoir) replaces the ultimate base level. As a result, the stream’s base level is raised. Consequently, this reduces the stream’s velocity, leads to deposition, and a reduction of the gradient upstream from the reservoir.