One of the deeper bends of the Kéran river, signposted (and reachable on foot from the riverside track) as the "hippo pool", holds a permanent small group of hippopotamus year-round. Their grunts and splashing carry across the savanna and the pool is one of the few reliable wildlife sights in the park.
Stand well back — a hippo will defend itself with surprising speed if it feels cornered — and approach the pool only with a ranger. Late afternoon, when the heat fades and the hippos start to move around, is the most active period. Bring binoculars for the best view of the surface activity.