Formation
Ponds can result from a wide range of natural processes. Any depression in the ground which collects and retains a sufficient amount of precipitation can be considered a pond, and such depressions can be formed by a variety of geological and ecological events. Rivers often leave behind ponds in natural flood plains after spring flooding, and these can be very important to breeding fish, particularly in large river systems like the Amazon. Retreating glaciers can leave behind landscapes filled with small depressions, each developing its own pond; an example is the Prairie Pothole Region of North America. Many areas of landscape contain small depressions which form temporary ponds after spring snow melt, or during rainy seasons; these are called vernal ponds, and may be important sites for amphibian breeding. Some ponds are created by animals. Beaver ponds are the best known example, but alligators also excavate ponds as well. In landscapes with organic soils, fires can also create depressions during periods of drought; these become open water when normal water levels return.
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