Small depressions in the lava fields contain clay-filled ephemeral lakes, and a drainage network has developed in parts of the field which sometimes carries water during spring. Some craters show evidence of former and ephemeral crater lakes. Beginning in the Messinian, growth of the volcanic field blocked pre-existing drainages, forming a closed basin southwest of Haruj that was filled by Lake Megafezzan, although it is possible that the lake at times overflowed across the volcanic field.