Explain howa small lake can become a forest?

Ponds and lakes typically become forests by silting up. The sediments of a river can become trapped in a shallow lake and eventually becomes higher than the water level or water table. This forst forms a meadow, and later the larger trees grow in. This is now occurring at Mirror Lake in Yosemite National Forest.

Lakes that dry up will usually not become forests because they lack groundwater, and there may be a concentration of alkali salts in the lake bed.