Duckweed
Getting nutrients is a factor for aquatic plants. Duckweeds tend to be found in fertile, even eutrophic conditions. They can be spread by waterfowl and small mammals on their feet and bodies, and by moving water. In water with constant currents or overflow, the plants are carried down the water channels and do not grow so thickly. In some places, weather causes the plants to multiply greatly when water-flow is low. Then they are carried away as rainy periods follow.