The Watershed Wisdom of the Beaver

You know what a stream looks like. It has a pair of steep banks that have been scoured by shifting currents, exposing streaks and lenses of rock and old sediment. At the bottom of this gully—ten to fifty feet down—the water rushes past, and you can hear the click of tumbling rocks as they are jostled downstream. The swift waters etch soil from first one bank, then the other as the stream twists restlessly in its bed. In flood season, the water runs fast and brown with a burden of soil carried ceaselessly from headwaters to the sea. At flood, instead of the soft click of rocks, you can hear the crack and thump of great boulders being hauled oceanward. In the dryness of late summer, however, a stream is an algae-choked trickle, skirted by a few tepid puddles among the exposed cobbles and sand of its bed. These are the sights and sounds of a contemporary stream.

 You don’t know what a stream looks like. A natural North American stream is not a single, deeply eroded gully, but a series of broad pools, as many as fifteen per mile, stitched together by short stretches of shallow, braided channels. The banks drop no more than a foot or two to water, and often there are no true banks, only a soft gradation from lush meadow to marsh to slow open water. If soil washes down from the steep headwaters in flood season, it is stopped and gathered in the chain of ponds, where it spreads a fertile layer over the earth. In spring the marshes edging the ponds enlarge to hold floodwaters. In late summer they shrink slightly, leaving at their margins a meadow that offers tender browse to wildlife. An untouched river valley usually holds more water than land, spanned by a series of large ponds that step downhill in a shimmering chain. The ponds are ringed by broad expanses of wetland and meadow that swarm with wildlife.

As Bill Mollison has observed, everything gardens. The beaver, however, goes far beyond simple gardening to feats of complex ecosystem transformation. Beaver don’t merely build dams that create ponds. They control the flow of vast amounts of energy and material. With tough incisors and instinct, beavers create a shifting mosaic of moist and dry meadows, wet forests, marshes, bogs, streams, and open water that change the climate, nutrient flow, vegetation, wildlife, hydrology, and even geology of entire watershed

There aren’t very many articles that I want to copy and post on this website from the first declarative sentence to the last dizzying conclusion. This one by Toby Hemenway breaks all prior records. Go read the whole thing and then think very seriously about what America did to it’s streams and pastures by eliminating the beaver.

Beaver choose the gently sloping lower reaches of valleys for their work. A small dam on flat land impounds more water behind it than one on a steep slope, doing the least work to create a large pond. The water that backs up behind the dam saturates the soil beneath it, creating a blend of anaerobic and aerobic pockets, varying with water depth, vegetation, soil type, and distance from the pond edge. Decomposition at the anaerobic sites is slow, preserving organic matter. Dead trees and snags left by the beaver or killed by flooding become home to a wide array of animals and microbes. The structural, biological, and chemical complexity of the region increases.

Vegetation drowned by the pond rots, releasing vast flows of nutrients into the water. The pond bubbles methane into the atmosphere. Erosion caused by the lapping of the expanding upstream shoreline pulls more nutrients into the water. In the pond and downstream from the dam, biomass now surges because of the water’s increased fertility. The growing plants and animals trap these nutrients and begin to cycle them.

 Ecosystems that retain nutrients recover more easily from disturbance than nutrient-losing ones. This means the pond communities and those around it are likely to persist for a long time.

But just as significant are the varied habitats that ring beaver ponds. Upstream and down are open stretches of flowing water, home to stream species. At the pond edges the beaver have created bogs, marshes, wet meadows, and riparian forests. The new wetlands and meadows contain more nutrients than the older uplands, and so support more types and numbers of living beings. Edging the wetlands are dry meadows and woodlands. And beaver meadows are very persistent, because their previous flooding has acidified the soil, helping them resist invasion by shrubs and trees.

Toby Hemenway is the author of Gaia’s Garden: A Guide to Home-Scale Permaculture, which was awarded the Nautilus Gold Medal in 2011, was named by the Washington Post as one of the ten best gardening books of 2010, and for the last eight years has been the best-selling permaculture book in the world.

Go. Read. It. Seriously. Then pass it on.