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. I want you to imagine a landscape with its full complement of beavers.

What do you see this time? No longer is our stream a pellucid, narrow, racing trickle. Instead it’s a sluggish, murky swamp, backed up several acres by a messy concatenation of woody dams. Gnawed stumps ring the marsh like punji sticks; dead and dying trees stand aslant in the chest-deep pond. When you step into the water, you feel not rocks underfoot but sludge. The musty stink of decomposition wafts into your nostrils. If there’s a fisherman here, he’s thrashing angrily in the willows, his fly caught in a tree. Read More from “Eager: The Surprising, Secret Life of Beavers and Why They Matter”