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Season's wildlife shows itself at Mendenhall Lake Mary Willson On the Trails

Posted: Sunday, December 16, 2007

A walk along the shores of Mendenhall Lake can be rewarding for a curious naturalist, now that the water levels are down. One might see a northern shrike in the shrub thickets; this small predator nests in the Interior but comes here sometimes in winter. The resident black wolf might sing or trot along the beach. Trumpeter swans were sometimes seen in open water near the outlet of the lake or in the sloughs left by old river channels.

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The low water levels expose broad reaches of sand and mud, which are good substrates for registering the tracks of passing critters when the ground is not frozen. Not long ago, we found track of otter, a small bear cub, and herons.

There also have been many long, thin, wiggly "worm trails" left in the mud, some of them several yards long. Most of these are probably made by wandering caddisfly larvae, which often can be seen if one looks closely. These inch-long larvae make protective cases for themselves out of hemlock needles or tiny pebbles, but they also appear to burrow into the sediments. It is not clear why they wander around on the wet mud. Usually they live in streams, and they may have been washed out of the sloughs by high rains. One kind of caddisfly is reported to migrate to dry land to pupate and transform into the adult, but the wanderers on these beaches mostly travel in irregular paths that seem unlikely to reach dry land, and they have not yet been identified specifically. So the mystery remains.

Steep Creek was a happening place until a few weeks ago. The sockeye run was over, but coho were straggling in. American Dippers still forage and sing and chase each other all along the stream. Earlier this fall I saw the male dipper that nested near the west edge of the glacier in summer - he wears a distinctive combination of colored plastic leg bands that permits me to know precisely who this bird is. Dippers in winter often leave their nesting streams and visit other locations.

Human habitués of Steep Creek may have noticed some changes where the paved road crosses the creek. The big culverts beneath the road had been filled completely by a family of very busy beavers. A few sockeye were able to pass over the dam in summer, but by the time the coho arrived, the culverts were totally blocked off. So coho were milling around in the big pond below the culverts, unable to reach their spawning areas upstream.

However, the culvert dams were breached, at least temporarily. The minute that the water flowed freely through the main culvert, a dozen coho shot through and headed upstream, along with several Dolly Varden. Two dippers zoomed through the opening and foraged in the shallow, flowing waters above the road.

An hour later, coho had arrived at the new footbridge. By the next day, a bear had captured at least one coho and left the remains in the woods, where eagles and magpies soon feasted.

The unfortunate beavers work hard at repairing their dams. They tow whole trees down into the culverts and pack smaller branches and short logs around them. It seems a shame to cause them all this extra work, when perhaps they would be storing winter food near their lodge on an adjacent pond. If there were no culverts constricting waterflow, the beavers would probably build a long, low dam across the little valley, and the salmon could pass over it like they do the downstream dams on this creek. The culverts are there so the road can pass over the stream, and they are good, big ones that normally allow easy fish passage. But they inevitably constrict the flow of water, and the beavers can readily clog them up, blocking the passage of fish.

Discommoding the industrious beavers is the price paid to allow adult salmon to get to the spawning area, juvenile salmon to move downstream toward the sea, Dolly Varden to swim up to spawn and down to the lake where they overwinter, bears to spread out their fishing activities and perhaps encounter each other less often, and dippers to avoid disastrous encounters with moving traffic on the road. The beaver lodge is not in the drained pond, so the cost to them was chiefly in wasted work.

If the beavers are sufficiently persistent, next year they may once again fill the culverts and back up the water into a deep pond. But, for this year, at least some coho and Dolly Varden could spawn above the road and some bears may have been better fed, than if the dams had not been breached.

• Mary F. Willson is a retired professor of ecology and a Trail Mix board member.



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