The Condit Dam was a hydroelectric facility constructed in 1912–1913 on the White Salmon River, Washington. Over its nearly 100-year existence, the dam trapped sand, silt, and clay that washed down from the southern Washington Cascade Range, including Mount Adams. The dam was breached in late 2011 and removed to restore fish passage to upstream spawning grounds.
Cameras were set-up to capture erosion of reservoir sediment following breaching and measurements of suspended sediments were made at stations downstream for a 15-week period. Upon opening the outlet tunnel at the base of the dam, the impounded Northwestern Lake drained within 90 minutes. As reservoir erosion proceeded, sediment concentration increased and flow became hyperconcentrated, at first noisy and turbulent but then quiet and viscous. Within 24 hours, 20% of the sediment stored behind the dam washed downstream. In the days and weeks after the breach, erosion and transport of sediments slowed. The channel evolved, lowering to near its original elevation at a gauge site within 15 days of breaching.
The study of the geomorphic response to dam removal was an opportunity to observe erosion processes and the downstream behavior of released sediment. New insights may be applied to the understanding of other disturbances that inject substantial sediment volumes into rivers such as volcanic eruptions, landslides, or when floods entrain great volumes of channel sediment. Read Rapid reservoir erosion, hyperconcentrated flow, and downstream deposition triggered by breaching of 38-m-tall Condit Dam, White Salmon River, Washington.
USGS and its partners invite educators to join us for a fun and informative teacher workshop at Mount Rainier National Park. This five-day classroom and field-based workshop offers science information, hands-on classroom activities, and resources that will enrich your understanding of Cascade volcanoes and aid in your ability to teach about volcanoes in your classroom and on class field trips. Credits and clock hours are available. Visit the Mount Rainier Teacher Professional Development webpage to register; download the Mount Rainier Teacher Training flyer to share with colleagues.