A beaver dam underneath the "Avondale Road -Powerline Bear Creek bridge" was removed by the city a few weeks ago, with permitted approval from Fisheries and Wildlife. The County has jurisdiction over the creek, but the bridge is owned by the City.
Teresa Kluver, city parks employee says:
Due to the proximity of the dam structure to the bridge and the anticipated movement of Salmon it was decided early removal would be beneficial. We are currently monitoring the location for renewed beaver activity.
Lindsey Walimaki of PSE said salmon can usually swim through beaver dams. Could it be that some of the vegetative-woody debris floating from the PSE power line project contributed "building materials" for the beavers? The salmon are expected under the bridge anytime now. In this picture, the dam is "under construction" by beavers. The entire creek was eventually damed up by some very busy beavers.
Special thanks to a
Water Tender for the tip.
Reported by Bob Yoder
Photo by Yoder
I'd be interested to know what it cost the city to remove the dam, and whether doing so was truly necessary to enable the salmon to migrate upstream.
ReplyDeletePrior to the arrival of trappers a few hundred years ago, beavers and salmon co-existed in the Pacific Northwest. Salmon did just fine without the Fish and Wildlife Department removing beaver dams to help the salmon move upstream. Near eradication of beaver left us thinking that beaver and their dams were not part of the ecosystem. In fact, studies in Canada have shown that beaver dams raise water tables in valleys, allowing wetlands to continue to discharge into streams during dry summers. Ponds behind beaver dams are often too deep for herons to stand in while searching for juvenile salmon. Similar to the way adult salmon will wait in the Pacific Ocean along the coast of Washington and Oregon until a high tide raises the water level at the mouth of such rivers as the Umpqua and Siletz so that the fish can get over the sand bars and into the river, the salmon in Bear Creek will wait below a dam until a heavy rain causes a surge in the flow of the creek and a rise in the water level above and below the dam. Beaver dams always have weak points that water flows through or a waterfall point that the water flows over. The salmon will locate this point and use it to cross the dam. An essential feature that we can’t see from the surface is that waterfalls over beaver dams (and waterfalls in general) create deep kettles underwater on the down-stream side. The salmon use the kettle by dropping their tails downward and then launching themselves vertically over the waterfall. (Watch for this behaviour the next time you visit the Issaquah Hatchery.) Removing a dam will obviously make fish passage at the point in the creek easier, but it may make long term survival for the eggs and offspring more precarious.
ReplyDeleteBeaver dams don’t co-exist well with people whose homes may be flooded by the ponds that fill behind the dam, but most homes along Bear Creek are set far back from the creek or above the typical flood level so that they won’t be affected. Beaver dams occasionally fail catastrophically, suddenly releasing thousands of gallons of water and creating a flashflood. Debris released during the flashflood can build up at bridges and cause small bridges to fail so the economic impact of sudden failure also needs to be considered when removing a dam.