What is the name of this excellent wildlife documentary?

I think it's an old one from the 70s or early 80s but could be mistaken. The narrator's voice is very British.

The documentary revolves around a river or lake in Africa (Serengeti?) and shows the interaction between wildlife. The habitat is alive at first, then the draught sets in and the river/pool begins to dry up and the wildlife struggles to cope. When all seems lost, the rain comes back in full force and the cycle begins anew. The scene I recall vividly is how gators have to somehow immerse themselves in only mud to keep cool.