This was originally an aquarium for native fish species of the Sacramento River watershed. The addition of the beaver is relatively recent, but it looks like it works well.
This document describes the history of Turtle Bay and explains the development of the wildlife area. The aquarium is being redeveloped to exhibit beavers and diving ducks.
There are still a number of fish species from the Sacramento River watershed in the tank but the addition of beaver(s) has made this an even better exhibit. With the glare through the window being the only flaw, this enclosure is easily the best one in the entire park. The idea of the tree trunk coming through the glass is great!