The songs of the robin and the varied thrush are sure signs of spring for many folks in Juneau. But for beavers, the sound of running water is the song of spring.
Swedish biologist Lars Wilsson spent years studying captive and wild beavers, and he gained remarkable insights into their behavior. He raised beavers in an outdoor enclosure and in a large indoor terrarium. The glass-walled terrarium allowed him to film beavers' life and activities in their lodge, to observe their swimming movements underwater and document their responses to a number of different tests.
Wilsson initially captured four adult beavers and later he raised a number of beavers from infancy, some in small colonies with their parents and some completely isolated from adult beavers. He isolated the young beavers to see what beavers learn from their parents and what behaviors are instinctive.
He found that young beavers - who had never even seen a beaver dam - were able to build almost-perfect dams at the first opportunity.
The foundation of sticks and logs anchored to the stream bottom, the interwoven lattice of trimmed branches, the mud chinking, every aspect of dam building was hard-wired. Beavers do get more skilled at dam building as they gain experience, but the building behavior is instinctive.
Wilsson learned that the sound of running water is the cue for dam building and dam repair. In one experiment, he played a recording of running water, and the young beavers built a dam in a tank of still water in the terrarium. In another peculiar experiment, his captive beavers built a "dam" on a concrete floor against a loudspeaker that played the sound of running water.
Wilsson made a number of modifications to his beavers' dams to see their reactions. He installed siphons that silently drained the water from behind the dams, and found that beavers were unable to identify the problem and make repairs. He also installed a clear plastic hood and vent that allowed beavers to see the water escaping through a deep furrow in the top of their dam, but since the outflow was silent they couldn't identify the source of the sizable leak.
This trait has since been used to the advantage of people who need to thwart beavers' dam building activity. Perforated pipe can be inserted through beaver dams, with both ends under the water surface. Water drains out through the pipe without making noise, and beavers will not block or remove the pipe. These devices are often referred to as water levelers.
The coming of spring has rekindled beavers' dam building activity in the Mendenhall Glacier Recreation Area. Their relentless endeavors are blocking culverts and flooding trails, denying access to people and migrating salmon. A number of areas are affected, including Steep Creek right next to the road near the parking lot. Volunteers are dismantling problem dams, trying to keep the industrious rodents from getting into trouble. The Forest Service has trapped problem beavers in the past, and volunteers are hoping that nonlethal measures can address the flooding issues.
Biologist Mary Willson is one of the volunteers. She said water levelers - and similar devices known as beaver bafflers and beaver deceivers - have been used successfully in parts of the Dredge Lakes area, and new devices are being considered for some other areas. But there are many situations where they don't work. Most designs do not accommodate movements of fish, an important issue here. The drains have to be designed so that adult salmon can pass through going upstream, she said, as well as letting the outmigrating young salmon move downstream.
Forest Service biologist Dennis Chester said his agency has tried a variety of commercial and homemade bafflers/water levelers over the past 15 years. He said they do work in some situations but they do require regular maintenance. In some cases, the streams are too shallow, or the grade and landscape are unsuitable. They are ineffective if the beavers can just go up or downstream a bit and build another dam, which is often the case in the flat terrain of the Dredge Lakes area, he said.
Probably the most convenient place to see a beaver dam is on Steep Creek. The creek goes under the Glacier Spur Road just before the Mendenhall Glacier Visitors Center parking lot, and the dam is next to the road. Beavers are also active in the ponds near the Visitors Center.
Riley Woodford is a writer for the Alaska Department of Fish and Game. He is the editor ofAlaska Fish and Wildlife News and produces the Sounds Wild radio program.