Add swimming and bathing facilities for our beavers
We know, that river water is polluted so beavers can't drink and swim in it without filtering it in pumps. Though it sounds fishy, lets put it aside for now, and stress out that beavers are still semi-aquatic animals and water procedures should be an import demand.
1. Add beaver's demand for water contact. Not filling it should give penalties. It's appears beavers mate in the water, so probably it should affect reproduction.
2. Add buildings or other structures that can fulfill this demand.
Starting showers\barrel baths filled from pump or tanks for partial filling need in early game.
Bigger bath houses, pools, saunas for later games.
3. Consider ways to purify water bodies, e.g. pond or backwater cleaned by advanced filters or aquatic plants, algae and molluscs.