A beaver carries a stick that provides both food and structural material for a dam or lodge. Beaver dams provide excellent habitat for young salmon.

A beaver carries a stick that provides both food and structural material for a dam or lodge. Beaver dams provide excellent habitat for young salmon.

Off the Trails: Anadromous fish in Dredge Creek

Dredge Creek begins in a long pond near the Glacier Spur highway, crosses under the road, and flows below the slopes of Thunder Mountain. It crosses the highway again and goes through a series of beaver dams to Dredge Lake in the Mendenhall Glacier Rec Area. It then passes down a channelized ditch, through some more beaver dams, to the Holding Pond. From there, creek water flows out through a concrete structure (formerly used for counting fish as they moved in and out of the creek system) to the Mendenhall River.

Into another side of the Holding Pond flows the outlet of Moose Lake, passing through two beaver dams on the way.

Cutthroat trout and Dolly Varden can sometimes overwinter in the Dredge Creek system and spawn there in the following year. Coho salmon come in the fall to spawn, especially in the upper reaches of the creek. A few coho may spawn near upwellings in Moose Lake, too.

The Dredge Creek system is small (the creek is only about a mile long), but it is a popular spot for local fly-fishers. Bird-watchers can find some unusual species such as redstarts there, in summer, and photographers can get good photos of migrating swans and ringneck ducks, for example. Wildlife-watchers and photographers can sometimes see beavers at work, herons, bears, mink and otters there (to name just a few worth watching).

The beaver dams cause concern for some people, because trails are occasionally flooded by backed-up water and because the dams might block the movements of fish in the system. For over seven years, a group of volunteers, known as the Beaver Patrol (a registered nonprofit organization), has monitored this system to reduce trail flooding and assure fish passage. This group works two mornings a week from spring through fall (usually April into December), opening notches in dams or sometimes removing dams entirely, as needed.

With the cooperation of the Forest Service, the Patrol has also installed several devices that deter beavers from filling culverts. By trial and error, the Patrol has found that different kinds of devices work better in certain situations — one size does not fit all. Unfortunately, some of these devices have been subject to periodic vandalism by persons who do not understand how they are helpful in managing water flow.

Because the area drained by the creek is quite flat and this is “rain country”, it sometimes happens that a big rain event occurs between the patrol’s workdays and trails get flooded for a couple of days. One workday suffices to bring the water levels down again (unless, of course, there is another big rain!), so existing trails are not usually flooded for more than a day or two. Raising some of the trailbeds with additional gravel (an inexpensive process) would reduce the problem more.

Although beaver dams sometimes can be capable of blocking passage of adult fish, opening dams twice a week allows fish passage. In any unmanaged, natural system that has beaver dams, fish often have to wait for days, until high water allows them to pass over or swim around a dam. So if incoming Dredge Creek salmon have to wait a day or two, it is not a problem. Furthermore, if there is a fairly deep pool below a dam, coho are perfectly capable of leaping up and over (they are excellent leapers!). Capable observers have found that many dozens of coho have been able to pass the dams and get to the upper reaches of the little creek to spawn each year.

In the Dredge Creek area, concerned but uninformed people (not part of the Beaver Patrol) sometimes remove a small dam that is actually being helpful to fish passage. For instance, where Moose Lake enters the Holding Pond, two tiny dams help raise the water level sufficiently so that the culvert just upstream is not “perched” above the pool—meaning that the water level is high enough that fish can readily enter the culvert to get into Moose Lake. In this case, the patrol tries to maintain the tiny dams to keep open the passage between the Holding Pond and Moose Lake; sometimes the Patrol has to rebuild the small dams to repair what other people have removed.

In addition to passage of adult fish, management of water flow through the dams allows movements of juvenile fish within the system or out to sea. Young coho stay in fresh water for a year or two after the eggs hatch. They often move about, from pool to pool, up and down the creek. They can weave their way through a beaver dam, or wait for high water, but the patrol’s activity in notching dams surely facilitates the passage of juveniles.

So why not just rip out all the dams and be done? Because the scientific literature shows that beaver ponds provide really good rearing habitat for juvenile salmon. In the Taku system, for instance, both coho and sockeye juveniles grew better and survived better in beaver ponds than in other types of habitat. The literature also reports that the availability of good rearing habitat is often the factor that limits coho populations. This means that if rearing habitat is reduced, as by the removal of beaver dams, the size of the coho run in that system will probably be reduced.

The beavers in the Dredge Creek system are therefore providing a service to the fish populations, and thus to the fly-fishers and wildlife-watchers. The patrol’s volunteer efforts in this system have had a lot of success over the past seven years: fish passage is assured, trail flooding is reduced, and the beavers, juvenile salmon and other wildlife have good habitat.

Educational opportunities abound and school classes sometimes visit. And many trail walkers in the area have greeted the working Patrol with thanks. Is the effort perfect? No, because the weather changes, beavers move around, and because of the activities of a few uninformed folks who may be trying to be helpful but actually are detrimental.

There is no perfect solution to balancing the needs of trail walkers, fish and wildlife. But for the past seven years, because of the volunteer efforts, trail flooding has been reduced, fish passage and rearing habitat have been maintained and wildlife has prospered.

• Mary F. Willson is a retired professor of ecology.

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