A lake is a body of still water separated from the sea. Most lakes were formed at the end of the Ice Age.
Lakes are not formed by rivers, although they are commonly found in river valleys. Rivers are steadily filling in lakes throughout the world because the material they carry settles out in the still waters of the lake.
Most long, narrow lakes fill deep trenches in the landscape scoured out by ancient glaciers. Because of their long, thin shape, they are called finger lakes.