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Stretching 395km from its source in Kenya’s Rift Valley to where it flows into Lake Victoria in Tanzania, the Mara River plays an essential role in one of the world’s greatest wildlife spectacles.

Its basin hosts the highest density of large herbivores on the planet, and each year more than a million wildebeest, half a million gazelle and 200,000 zebra migrate from the Serengeti park in Tanzania to the Maasai Mara reserve in Kenya in search of water and grazing. As the only source of water in the dry season, the Mara River makes this vast exodus possible.
 
A recent WWF report provided the first in-depth study of the remarkable freshwater biodiversity in the basin, identifying 473 endemic species, including 4 mammals, 20 amphibians, 40 fishes, 88 waterbirds and 141 vascular plants. Other endemic species remain to be described.
 
But the health of the free flowing Mara is also critical for people, sustaining the livelihoods of 1.1 million people, while contributing between 10-15 per cent of the GDP of both Kenya and Tanzania. Tourists visit from all over the world to see the wildlife, while agricultural land – nourished by the river – is fast increasing in area. The river also helps to sustain the continent’s most productive freshwater fisheries in Lake Victoria, which yield about a million tonnes of fish each year.
 
But the river is at risk - like the other iconic rivers in WWF's 10 Rivers at Risk report.

In recent years, droughts coupled with human activities, including water abstraction for agriculture and tourism, have led to erratic flows, upsetting the delicate natural balance and degrading vital wetlands. With the basin’s human population growing by three per cent each year and the increasing impacts of climate change, these pressures are intensifying all the time.
 
The river’s resilience in the face of these threats depends on it remaining free flowing. For now, the main channel of the Mara remains unblocked by hydropower dams. Several, though, are planned within the Mara basin, including large multipurpose dams at Norera in Kenya and Borenga in Tanzania. These had initially been approached as joint projects, but the two nations are still reviewing the ecological impacts of such major river infrastructure.
 
Tanzania is now calling for the dams to be halted to protect the Serengeti-Mara ecosystem on which so much depends, and with good reason. Disrupt the natural flow of the Mara River - its life-giving flow of water, sediment and nutrients - beyond a certain point and the mass migrations will fade into memories, as will much of its unique biodiversity. Life will grow much tougher too for all the people across the basin who depend on a healthy river for their livelihoods - from fishers and farmers to Maasai pastoralists.

Keep it free flowing and a resilient Mara will be able to sustain people and nature despite the worsening impacts of our warming world.