Streams in the Aleutian Islands are short and swift. Many plunge into the sea over waterfalls. Volcanoes of porous rock have stream courses that are filled with water only during exceptionally heavy rains. Many lakes occupy irregular ice-carved basins in rolling topography on glaciated islands. Numerous ponds were enlarged when ice, expanded by freezing, shoved the banks aside to form ramparts of soils and turf.
Lakes fill a few volcanic craters and calderas.
Wetlands occupy 11 percent of the area. These islands are frequently affected by intense storms, spawning high winds, and heavy rains. Settlements are sparse and restricted to the coastline. The region is used primarily for subsistence and recreational hunting and fishing. Native plant communities have been negatively affected by the introduction of exotic species.