The Mara River is a river in Mara Region in Tanzania and Narok County in Kenya, and lies across the migration path of ungulates in the Serengeti/Masai Mara game reserves.
From its sources in the Kenyan highlands, the river flows for about 395 km and originates from the Mau Escarpment.
Eventually the river flows into the world-famous Masai Mara National Reserve, where it merges with three of the four mentioned tributaries. On the Kenyan-Tanzanian border, the river flows into the Serengeti National Park and is joined by the fourth major tributary: the Sand (or Longaianiet) River. In these wildlife parks human activity is restricted to wildlife viewing.
Just after the Mara River flows out of Ikorongo Game Reserve (which borders Serengeti National Park) it meanders sharply northwards. At the location where the river meanders into the southwestern direction again the main channel is lost in different streams, which feed the downstream Mara wetlands. These streams and wetlands continue for about 70 kilometres downstream. In this part of the basin human and livestock densities are high and small-scale subsistence agriculture is the main land use. The Mara River basin is one of the ten drainage basins that feed into Lake Victoria, and is therefore functionally and ecologically related to the socio-economic activities in Lake Victoria and along the River Nile.
The river is a vital source to grazing animals nearby. Although during the dry season it may often appear shallow, it may swell to up to twice its normal size after heavy rainfall. This can create rapids in the river, which may lead to shortage of food for predators that cannot cross the river to hunt.