Streams only form during the runoff period from May through early September. Permafrost causes most of the runoff to take place on the surface, with streams only forming on the largest islands. The longest river is long and forms on George Land, while there are several streams on Alexandra Land, the longest being . There are about one thousand lakes in the archipelago, the majority of which are located on Alexandra Land and George Land. Most lakes are located in depressions caused by glacial erosion, in addition to a smaller number of lagoon lakes. Their sizes vary from to . Most are only deep, with the deepest measured at .