Maasai Mara National Reserve covers some 1,510 km2 in south-western Kenya. It is the northern-most section of the Mara-Serengeti ecosystem, which covers some 25,000 km2 in Tanzania and Kenya. It is bounded by the Serengeti Park to the south, the Siria escarpment to the west, and Maasai pastoral ranches to the north, east and west. Rainfall in the ecosystem increases markedly along a southeast–northwest gradient, varies in space and time, and is markedly bimodal. The Sand, Talek River and Mara River are the major rivers draining the reserve. Shrubs and trees fringe most drainage lines and cover hillslopes and hilltops.