How do man-made lakes get filled with water?

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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

Rainwater. Lakes are just low-lying areas that water builds up in. All the humans need to do is prevent an already low area from draining, usually with a dam of some sort, and the water cycle will do the rest of the filling.

The principles of lakes, man-made or not, are quite simple: Water flows downhill. Normally this would form a small stream, then combine together to form larger streams and rivers until it all eventually reaches an ocean.

Lakes are formed by localized areas of lower elevation, such that there is no unobstructed downward path. At that point water can do nothing but pool, forming puddles which eventually grow to become ponds and lakes. They will grow until the water rises to the point it can flow over whatever barrier prevented it from flowing, like a cup becoming overfull.

A man-made lake then is formed by simply blocking a river or stream. The height of the barrier and the surrounding terrain will determine how large the resulting pond or lake will become. Given the right circumstances a relatively small barrier or “dam” can create a very deep lake!

All of this relies on the natural water cycle to provide the appropriate inflow; nobody is digging a hole and trucking in water to fill it in order to make lakes.

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.

The key is the [drainage basin](https://en.wikipedia.org/wiki/Drainage_basin), or catchment area. The water comes from the rain but it’s not just the rain that falls into the lake itself but the water that falls onto the land around that is sloped towards the lake. So all the rain that falls on the valley sides will flow in to form the lake.

Sometime when the mine for minerals and leave giant holes in the ground and they are not actively using a sump pump to keep the water out it fills up and you got a lake