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The smallest dams on Putah Creek
Woodland Daily Democrat -
February
08,
2009
By Amy Boyer
The smallest dams on Putah Creek
The Woodland Daily Democrat – 2/09/09
By AMY BOYER
Beavers are one of the first mammals in the U.S. to have been restored to healthy populations after near-extinction. When Europeans discovered America, they also discovered beavers occupying "every river, brook and rill," according to Samuel de Champlain.
Some of the earliest European explorers of the Central Valley were Hudson Bay Company trappers looking for beaver pelts.
By the time of the Gold Rush, beavers were rare enough to have California trappers turning to other animals, and they were nearly extirpated from much of the East Coast by 1895. By searching out remaining beavers and moving them to protected habitat, early conservationists successfully reintroduced them; they are now fairly common throughout the U.S.-and along Putah Creek, where there are "many, many beaver," says Dirk Van Vuren, professor at UC Davis.
Historically, California beavers tended not to build as extensively as others. According to Van Vuren, beavers will dam streams in order to raise water levels high enough to protect their dens, which may be lodges or bank-side burrows, but always have underwater entrances.
The dams serve other animals as stream crossings, and you can sometimes see their scat on the dams.
Putah Creek in summer is a dam-worthy creek, but river beaver will simply hole up in banks, piling up sticks over the air hole of the den. In all cases, beavers are highly territorial and mark their areas by piling up mud and marking the mud pies with their scent.
Within their dens are close-knit families. Two adults mate for life. Both male and female build dams and lodges, and both take care of the kits, raising one to six kits per year.
The youngsters stay with the parents for two years, with the yearlings helping with dam maintenance and kit-care, in between playing with each other.
Hope Ryden's delightful book Lily Pond: Four Years with a Family of Beavers describes beavers nuzzling each other, grooming each other, giving kits rides on their backs, and seemingly talking to each other in the beaver lodge during long dark iced-in winter days. When mature, the beavers take off along their stream or even cross-country, searching for a suitable site with plentiful forage-and for a mate.
Beavers' architectural tendencies are innate, with beavers raised in captivity competently building dams, lodges, and even canals (to ease transporting sticks) without parental training, but they likely learn finesse during their two-year apprenticeship. Ryden says they are leisurely workers, but little by little they can build big.
I have seen a structure in North Carolina transforming an easily jumped stream meandering through bottomlands into a pond many yards across, with the dam being about five feet high at its highest and narrowing to a long berm of sticks and mud only a few inches high, arcing through the trees as far as I could see.
Ecologically, beavers are major players. A typical pattern on many streams is that beavers dam a stream, creating a pond where sediments settle; water-loving animals and invertebrates follow; beavers use up forage and move on; dam breaks, pond dries up, leaving richer bottom land for plants to colonize; forage grows back, beavers return.
According to Van Vuren, this pattern doesn't seem to hold for Putah Creek. However, beavers' choice of plants can have a strong influence on forest structure-and on restoration efforts. Beavers will eat a wide range of non-woody plants, but they require woody plants as well.
Local beavers have a strong preference for cottonwood. "They love them. It's like a buffet," says Andrew Fulks, UC Davis Putah Creek Riparian Reserve Manager. They also like willow, and the easiest way to see beaver evidence is to walk along near the creek's summer channel looking for willow branches that have been neatly sheared off near the ground, as if pruned with loppers.
According to Fulks, "Other riparian trees, like ash, box elder, valley oak, and shrubs like elderberry may get some incidental browsing, but generally are left alone." The answer for restorationists is to cage favored plant species.
Fulks notes that beaver pruning may actually be beneficial: "The willows in areas further from the creek have less access to water in the ground, and I've found that those that have been browsed and re-sprout have better survival. ... I suspect the reason they survive better after cutting down is that by removing the top growth you reduce the amount of water use, which works to their favor in a drier environment." He stresses that he hasn't actually researched willow survival rates.
Beavers and humans often have different ideas about what should be dammed and where.
A beaver damming an irrigation canal or denning in a levee bank can be a nuisance or even a danger. But beavers in streams and rivers increase wetland areas and can improve groundwater recharge, according to an article in Science News.
Fulks agrees: "Beaver are a natural part of the ecosystem, and I'm happy they are out on our creek!"#
http://www.dailydemocrat.com/news/ci_11662503
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