How does a glacier create lakes?

Answer 1

Unlike a river, a glacier of ice may be many thousands of feet thick. This creates a significant pressure on the bed of the glacier - the rock on wich it rests.

At the base of a glacier is a quantity of rock rubble, which is thus ground along the bed under the weight of the ice above. This glacier bed is able to deepen a portion of its course, much deeper that its natural outlet.

Lake Manapouri for example, has a surface level of about 177m above sea level, but its deepest point is 444m. My local lake Wanaka, is 150km from the (drainage) coast has a surface level of just under 300m, but the deepest point is a number of metres below that. The glacier that created this lake was over 1000m thick - easily read from scouring of the mountains around.

Eventually in a warming period, the glacier retreats and in that phase retreats quite fast, much faster than the river inflows can fill it with gravel and silt.

But in the long run, it will be filled. All lakes are temporary features (land warping excepted).

The same is true of Fiords, they are quite deep a few kilometres in from the coast, but at the point where they enter the ocean, their bed rises sharply as any ice beyond that is broken off by buoyancy processes.

Also but minor, the glacier creating the fiord dumps its residual moraine debris at that point, as a mound at the mouth. So a fiord is a lake without a confined outlet.