Ah.
It's important to keep in mind that beavers aren't dwarves. Not only can they all swim, they *enjoy* it. So instead of building bridges across streams, all you need to do (and all you should do) is build a stairs down into the water, a path along the riverbed, and a stairs on the other side.
This gives your citizens a mood bonus from having wet fur. Which is not, you might think, a penalty.





