Beavers are ecological Swiss Army knives, capable of tackling water quality, flooding, and climate change.

Close your eyes. Picture, if you will, a healthy stream.

What comes to mind? Perhaps you’ve conjured a crystalline, fast-moving creek, bounding merrily over rocks, its course narrow and shallow enough that you could leap or wade across the channel. If, like me, you are a fly fisherman, you might add a cheerful, knee-deep angler, casting for trout in a limpid riffle.

It’s a lovely picture, fit for an Orvis catalog. It’s also wrong.

Let’s try again. This time, I want you to perform a more difficult imaginative feat. Instead of envisioning a present-day stream, I want you to reach into the past—before the mountain men, before the Pilgrims, before Hudson and Champlain and the other horsemen of the furpocalypse, all the way back to the 1500s. I want you to imagine the streams that existed before global capitalism purged a continent of its dam-building, water-storing, wetland-creating engineers.