I have been studying about how the lakes are formed, and would share what I have learned. Most of the inland lakes have been formed by Glacial erosion. Yes Glacial erosion has been the cause of formation of most of the lakes and the biggest ones. So how does it happen?
Glaciers covered most of the earth 12,000 years ago. Glaciers are not still they keep on moving. While moving they make deep impressions in loose soil and soft bedrock. These are called basins.The crumbled ice may fill the hole which later melts to form a lake. Generally such phenomena occurs near a stream bed. The Glacial lakes are actually formed when the glaciers were draining out into the larger water bodies – The Sea and Oceans. So most of the lakes formed are on a slope. The glaciers cannot erode the hard rocks and hence there are high landforms near the lakes. The softer rocks and soil get eroded forming large water holes called basins. The rainwater later collected in these basins giving rise to lakes. The salt content of the land decides whether it will be a freshwater or salt water lake. Once these lakes are formed various plants and animals thrive in them depending upon their surroundings. The lakes keep on evolving and changing shapes just like rivers. We cannot notice because it is very slow. These water sources are actually history telling us about what happened and is happening with the land plates then and now. Whether you are able to read these books of nature depends on the skills and interest.