How does a lake retain its water mass?
In: Physics
Most lakes are basins, filling up with water with very little escaping into other rivers. Between these smaller outletting rivers, drainage by humans, or evaporation, the water is removed from the basin.
High up in the mountains, and sometimes from very far away, rain, snow, and ice melts send fresh water down into rivers that flow into the lake, replenish the water levels, and fill the basin.
it lies on a rock bed that is waterproof.
You might have dirt and mug at the bottom, but at some point, it will be waterproof stone.
Dirt and soil alone can also maintain a water body for extended periods of time, but in that case water usually slowly infuse the ground, which requires a more consequent supply of water to maintain it
The lake takes roughly as much water in as goes out.
Some of this you see on the surface with rivers. Under the surface, the sides and the bottom of the lake are probably mud, just wet dirt. If you magically drained the lake dry right this second and started refilling it, a lot of that water would eventually soak into the dirt, but some would just slide over the top at first—dirt can only absorb water so fast. And eventually, that dirt gets thoroughly soaked and can’t absorb anymore until the water goes somewhere else.
Gravity will pull that water further down some. Some of that water will diffuse horizontally. When you get down to layers of rock, their shape will steer water to pool in places, but also limit which directions the water can spread. Eventually, any water that would try to soak into the lake basin can’t really get very far because everything around it is already soaked and there’s not enough force behind the inflowing water to do much pushing. (*Much*, there’s going to be some.) So, most of the water stays in the lake, until it runs out along the surface.
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