Mara River

The Mara River is the artery of the Maasai Mara National Reserve. It runs north to south through the middle of the reserve, divides the Mara Triangle from the rest of it, and carries water into the Serengeti and on to Lake Victoria. For a few weeks each year it is the most photographed stretch of water in Africa. For the other eleven months it is something more interesting: a river holding more than four thousand hippo, a large crocodile population, and an ecology that scientists have spent two decades taking apart.

The wildebeest crossings are covered on the Great Migration in Kenya page. This page is about the river itself.

Where the Mara River Starts and Where It Goes

The river rises in the Mau Forest Complex in western Kenya, at roughly 2,900 metres above sea level, and falls through farmland and rangeland before entering the reserve at around 1,400 metres. It crosses into Tanzania, runs through the northern Serengeti, and empties into Lake Victoria through the Mara wetland. Published figures give the river a length of about 395 kilometres and a basin of roughly 13,500 square kilometres shared between the two countries.

Inside the reserve the Mara is joined from the east by the Talek River and runs south towards the Tanzanian boundary. Its western bank is the edge of the Mara Triangle, the third of the reserve that sits between the river and the Oloololo Escarpment. That geography is worth holding onto, because it explains why the river appears on almost every game drive in the reserve regardless of what season it is: nearly every route either follows it or has to reckon with it.

Flow is strongly seasonal. The river runs high during the long rains from March to May and again, less dramatically, during the short rains from October to December. In a dry year the channel narrows to a chain of pools linked by shallow riffles, and it is that low-water state, rather than the flood, that shapes most of what follows.

The Mau Forest Catchment and the Pressure on It

Almost everything that happens to the Mara River happens first in the Mau. The forest complex is the largest closed-canopy montane forest in East Africa and the catchment that feeds the river. Clearing in the Mau has been linked by hydrological work to lower dry-season baseflows, higher flood peaks and a greater sediment load reaching the reserve. A river that used to release water slowly through forest soils now delivers it in surges and then runs short.

The state of the river in any given year is therefore set several hundred kilometres upstream, in a forest most Mara visitors will never see. Abstraction adds to it. Water is drawn from the upper and middle basin for irrigated agriculture and for domestic supply, and enforcement of abstraction permits has been repeatedly described as weak. Demand across the basin is projected to keep rising. Kenya and Tanzania signed a memorandum of understanding on the joint management of the Mara River Basin in 2015, and Kenya has a restoration programme in the Mau that involves reclaiming encroached land and replanting indigenous species.

Hippo, and What Four Thousand of Them Do to a River

The Kenyan reach of the Mara holds more than 4,000 hippo. They spend the day in pools and the night grazing on land, which makes them a conveyor belt moving grass into water. Research published in 2018 put the daily load at roughly 8,500 kilograms of dung entering the river.

What that does is counterintuitive and it is the most interesting thing about the Mara River. During low flows, the pools stratify and microbes breaking down the organic matter strip the oxygen out of the water, producing ammonium, hydrogen sulphide and methane. When rain arrives and the river doubles its flow, that deoxygenated water is flushed downstream in a slug. Over three years of monitoring, researchers recorded 55 flushing flows, saw dissolved oxygen fall during 49 of them, and recorded nine that killed fish in numbers.

So a mass fish die-off on the Mara is not necessarily pollution and not necessarily a disaster. It is a natural consequence of a very large hippo population in a river whose dry-season flow has fallen. It also means the smell along some stretches in the dry months is entirely authentic.

Nile Crocodile on the Mara River

The Mara holds Nile crocodile, including very large individuals, and they are resident year round rather than seasonal visitors. They concentrate at the pools and the crossing points, and outside the migration they take whatever comes to the water to drink.

The crossings have given crocodiles a reputation the research does not entirely support. In the study of wildebeest that drown during crossings, Nile crocodiles were estimated to take only around two per cent of the total carcass input. The rest goes to fish, to scavenging birds and to the river itself.

What the Drowned Wildebeest Do for the River

This is the part of the migration story that belongs on a page about the river rather than a page about the crossings. Work published in 2017 estimated that an average of about 6,200 wildebeest drown in the Mara each year, amounting to roughly 1,100 tonnes of carcass, and that mass drownings occurred in 13 of the 15 years studied.

That input is enormous and it is absorbed in stages:

Soft tissue decomposes over two to ten weeks and, while it is present, made up between a third and a half of the diet of several fish species.
Terrestrial scavengers, including marabou storks and three vulture species, took an estimated six to nine per cent of the soft tissue.
The bones remain for more than seven years, releasing phosphorus slowly, and the biofilms growing on them were found to make up a substantial share of some fish diets months after the drownings.

The river, in other words, is still digesting migrations that happened years ago. A skull or a rib cage on a gravel bar in March is not a sign of a recent crossing.

Seeing the Mara River Outside the Migration

Between January and June the river is quiet, and it is the best time to look at it as a river. Hippo pods are visible at close range from the vehicle, crocodiles bask on the bars, and the riverine forest along the banks is where the reserve’s leopard spend a good deal of their time. Black-and-white colobus, baboon and vervet use the same forest strip.

Birds are the other reason to stop at the water. African fish eagle, hamerkop, goliath heron, saddle-billed stork and a heavy concentration of kingfishers all work the channel, and the riverine strip carries species that the open plains do not. The country-wide picture sits on the birdwatching in Kenya page.

Practical Notes for Visiting the Mara River

Access is by vehicle on a normal game drive. No separate permit or charge applies to the river; it is inside the reserve and covered by reserve entry.
Walking and night driving are not permitted inside the national reserve, so riverbank access on foot is not an option. Conservancies outside the reserve boundary have different rules.
Off-road driving to reach the bank is prohibited. Vehicles stop at established viewpoints and crossing points.
The river is the boundary of the Mara Triangle, and the bridges across it are limited. Ask which crossing your route uses, because a closed bridge after heavy rain can add a long detour.
Banks are undercut and unstable in places. Standing up through a roof hatch is the normal way to see over them.

Mara River Questions

Can I see the Mara River if I am not visiting during the migration?

Yes, and it is arguably a better river visit. The hippo and crocodile are permanent residents, the riverine forest is at its most productive for leopard and birds in the green months, and there is no competition for space at the viewpoints.

Is the Mara River safe to approach on foot?

Foot access is not permitted inside the reserve, which settles the question. Even where walking is allowed on community land outside the boundary, riverbanks with resident hippo are among the more dangerous places in the ecosystem and are walked only with an armed guide.

Why does the river sometimes smell and why are there dead fish?

Both are usually the same phenomenon. Oxygen-poor water builds up in hippo pools during low flows and is flushed downstream when rain raises the river, which can kill fish over a short stretch. It is a documented natural process on this river rather than a sign of pollution.

Does the Mara River ever dry up?

The channel has not been recorded as drying completely through the reserve, but it falls to a chain of pools in dry years, and reduced dry-season flow is the trend that hydrologists working the basin are most concerned about. The cause sits upstream in the Mau catchment rather than in the reserve.

Which side of the river should I stay on?

The Mara Triangle, west of the river, has fewer vehicles and is reached through Oloololo Gate. The eastern and northern sectors have more camps, more road access from Nairobi and more traffic at sightings. Both put you on the river. The trade-offs are set out on the Maasai Mara guide.

Where the Mara River Fits

The river is the reason the reserve holds the wildlife it does, and the reason the migration takes the route it takes. It is also the one feature of the Maasai Mara whose condition depends on decisions made outside the ecosystem entirely. A visit that stops at the crossing points misses that. The Oloololo Escarpment gives the elevated view of the whole floodplain, and the full index of named sights across the country sits on the Kenya attractions hub.

1 Place to Visit in This Area

Other sites in Maasai Mara National Reserve, each with its own guide.

Oloololo Escarpment

Safaris That Visit Maasai Mara National Reserve

3 of the 30 safaris listed here that visit Maasai Mara National Reserve, from different operators where the listings allow it. Each is sold and run by an independent tour company. This site lists them and passes on enquiries; it takes no booking and charges you nothing.

Great Migration