What's Happening at MSRF?
Ice Dams and the Floodplain
Photo by Steven Foreman
Warming temperatures in the Methow Valley recently resulted in a dramatic ice flow on the Twisp River just upstream from the town of Twisp. This is not an uncommon thing to happen in winter, but the ice flow raised concerns of ice dams and flooding in nearby residential areas.
Riverbanks constrained by dikes and levees can form choke points where ice and debris can lodge and block up the river. Because the dikes and levees prevent water from simply going around a blockage, the water backs up and often causes flooding upstream. When the dam blows out, the water and debris can flood or wash out areas downstream and threaten bridges. In constrained sections of the river, small blockages can cause potentially dangerous flooding in much the same way that a narrow roof gutter overflows when blocked by a clump of leaves.
More-natural floodplain sites like MSRF's Twisp Ponds site serve as a sort of safety valve for the river, a place where the river can spread out beyond its normal banks without hurting anything. Over the past decade, MSRF has removed several small levees at the Twisp Ponds (just upstream of the town of Twisp) to restore its function as floodplain. If a dam forms downstream from the site, backed up water can run out onto the land here without threatening other properties. This also reduces the volume of water that could rush downstream when the dam fails. If a dam fails upstream, floodplain sites like the Twisp Ponds can absorb some of that surge, again reducing its destructive effect on sites downstream. The sloping banks of an unrestrained river floodplain are also less likely to form dams in the first place.
The recent ice flow passed without human intervention or damage to structures. While maintaining levees and dikes is necessary in some places for the protection of existing homes, roads, and utilities, flood control also relies on the preservation and restoration of floodplain where flooding can be managed.
Floodplain habitat is also home to many species of plants and animals, and is very important to the life cycle of salmon and other local fish.
Photo of Chinook Salmon by John Crandall
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