Elmer and the Hippos by David McKee
Elmer, the patchwork elephant, has been a favorite of children around the world since the first book debuted in 1989.
In Elmer and the Hippos, the elephants are not happy. The hippos' river has dried up, and they have moved into the elephants' river. Some of the elephants don't like sharing. Elmer decides to investigate. He discovers the hippos' dry river could be fixed—but only if the hippos and the elephants drop their prejudices and work together.