Apart from Australia and Southern Africa, it is rare for runoff variability to be high enough for these changes to take place on a useful time scale, and lakes forming in areas where conversions from closed to open or open to closed are likely are very rare. However, Lake Titicaca, the largest lake in South America, has historically been a semi-open lake with very limited outflow, but recent glacial retreat has reduced inflows so much that some believe it could become a completely closed lake in the future. In North Dakota Devils Lake, historically a closed lake, is known to have been an open lake during the Medieval Warm Period.