How are some man-made lakes so deep? And where does the water come from and how can that much water be transported? It seems impossible to me that we can fill lakes that are 250+ feet deep.
In: Engineering
Man-made lakes are built on existing rivers and streams. The stream is dammed so that water can’t *leave* but it still comes *in* as the river or stream continues to flow from its source – ultimately, rainwater or snow melt collecting and flowing down towards the ocean.
The river will continue flowing in until it over-tops the lowest level barrier. Artificial lakes are built where there is a valley, gully, canyon, or some other natural depression. The boundaries of that depression form most of the banks of the new lake, with the dam forming the new barrier that blocks the river. The new lake will fill until that depression is full and the water will flow over one of the barriers – almost always through a spillway designed to contain and withstand the often intense flow of water.
Very deep artificial lakes can get so deep because the barriers around the normal river height are themselves very high. For example, the Hoover Dam was built into a canyon that is part of the Colorado River. You can see in [this picture](https://waterandpower.org/2%20Historic%20Photos%202/Hoover_Dam_1934.jpg) how deep the ravine is behind the dam. The dam blocked the flow of the Colorado River so that the river filled up the new reservoir, creating Lake Mead.
Smaller, shallower lakes are built into shallower depressions, like small gullies instead of big, deep canyons, and on smaller streams or rivers. Once the new lake is full, the “river” will continue to flow at the same rate (minus evaporation off the lake, and anything siphoned off for irrigation or drinking water or whatever) as it did before, just through the lake now.
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