Eugene Weekly : Natural Resistance : 7.3.08

Dam-happy Friends
In praise of reclusive beavers
by Mary O’Brien

This month’s column is for all you beaver fans in Corvallis. As for we Eugene duck fans, we need to let go of the hard feelings long enough to appreciate what beavers do for ducks and some other things we love in the West.

Of course, ducks do have their advantages over beaver. For instance, some of them will walk right up to you, if you have some bread to hand out. (This is a particularly wondrous process for 2- and 3-year olds.) They’re good eating if you’re a duck hunter or friend of a duck hunter. It’s both easy and fun to watch ducks. And they have the astounding ability to fly, unaided.

Beavers just don’t measure up by these standards. First, unless you spend hours sitting quietly by the same beaver pond for many evenings with apples or corn or are a very quiet angler, beavers are not likely to venture near you. Since out of experience they fear you, their flat tail slapping the water as they dive away from you is what you usually see. They aren’t a favorite meal, at least for Oregonians, and no beaver has successfully flown on its own.

Nevertheless, two supremely wonderful beaver features completely lacking in ducks are their desire and ability to build dams across streams that aren’t too steep, narrowly confined, or big. These dams, often (but not always) built out of aspen or cottonwood branches and logs, change more things than you might guess. The most obvious is that a pond forms behind the dam. The next most obvious may be that ducks like beaver ponds (there you go, duck fans). Perhaps the third most obvious is that beavers build their domed lodges in some of the ponds, with underwater entrances to discourage predators and above-water platforms in which to rest and spend winter. (A friend of mine once heard beavers talking to each other inside their winter lodge.)

Beaver ponds drown trees, providing welcome habitat for cavity-nesting birds such as woodpeckers. The ponds provide still, warmish edges with vegetation, perfect for particular frogs. They contain still, deep water in which young fish can grow and hide. Beaver dams trap sediment that would otherwise be lost from the area. The trapped sediment raises the stream bed, which is especially beneficial for streams that have become incisions, ditches in the ground due to too much cattle or elk browsing of deep-rooted, bank-holding plants such as cottonwood, willows, and sedges.

One thing you might not have known is that some of the water passing below the dam will flow underground, subirrigating and increasing the productivity of a downstream valley’s meadows and riparian areas. The wetted banks provide sites for new willows, cottonwood, and nonwoody plants to replace, if not increase, production of plants the beaver has eaten and/or cut down for dam-building and winter food stores.

You also might not have thought about the role dams play in buffering some climate change impacts. With global warming, for instance, winter snows melt and leave the landscape earlier in the season, resulting in smaller or nonexistent stream flows later in the season. As air temperature rises, the water warms, stressing fish and other aquatic organisms who count on cool water. When storms come, at least some are expected to become deluges, which tear at stream banks and gouge streams ever deeper into the earth, effectively isolating the surrounding floodplains from life-giving water.

Beaver dams can reduce all these impacts. They slow the movement of water through the landscape, extending stream flows later into the season. The dams store some water underground, away from the sun’s heat. A series of dams can at least partially resist and therefore reduce the erosive power of floods.

There’s a catch, though. As with elephants, wolves, prairie dogs and other “keystone” species that support myriad ecosystem functions and species, beavers need room. They need room for their ponds and for biodiversity-rich riparian areas to spread outwards from streams and creeks. They need wild places where irrigation ditches won’t be blocked and stream-hugging roads won’t be drowned. They need places where too many cattle, elk, deer and/or sheep won’t munch the willow, cottonwood and aspen sprouts to nubbins, effectively eating beaver out of house and home.

As with all life, in other words, we receive in proportion to what we share. 

Mary O’Brien of Eugene has worked as a public interest scientist since 1981. She can be reached at





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