Initial Fluvial Response to the Removal of Oregon's Marmot Dam

Abstract

A temporary, 14‐meter‐high earthen cofferdam standing in place of Marmot Dam was breached on 19 October 2007, allowing the 80‐ kilometer‐long Sandy River to flow freely from Mount Hood, Oreg., to the Columbia River for the first time in nearly 100 years. Marmot Dam is one of the largest dams in the western United States (in terms of height and volume of stored sediment) to have been removed in the past 40 years, and its removal exposed approximately 730,000 cubic meters of stored sand and gravel to erosion and transport by the newly energetic mountain river. At the time, its breach represented the greatest release of sediment from any U.S. dam removal. (The March 2008 breaching of Montana's Milltown Dam exposed about 5–10 times as much sediment to potential erosion.)

Rapid Response of a Sand‐Dominated River to Installation and Removal of a Temporary Run‐of‐the‐River Dam

“…Draut et al ., ; Brenkman et al ., ) and post‐dam conditions (e.g. Doyle et al ., 2003b; Wildman and MacBroom, ; Major et al ., ; Pearson et al ., ) of large dam removal projects. However, much less is known about pre‐dam conditions in general or what the effects of removing a RORD might have on a sand‐bed river.…”

Status and trends of dam removal research in the United States

“…The scarcity of longer‐term monitoring (longer than 5 years) may be caused by limited funding for data collection or lack of interest by scientists . In addition, many responses to dam removal can happen relatively quickly (e.g., sediment erosion and deposition), after which there may no longer be motivation to continue monitoring. In contrast, ecological responses to dam removal may take decades to detect …”

Legacy effects on sediments in river corridors

“…At least 121 dams were removed across the United States during 1930-1999 and another 20 to 50 dams were decommissioned each year during the first decade of the 21st century . A common scenario following dam removal is incision of a channel into reservoir sediment, followed by channel widening, aggradation, and eventual stabilization in a manner that reflects the degree of base level lowering following dam removal, as well as particle size and cohesion of reservoir sediment (Stanley et al, 2002;Doyle et al, 2003;Lorang and Aggett, 2005;Cheng and Granata, 2007;Major et al, 2008;Wilcox et al, 2014). The distance over which sediment is dispersed downstream, and associated depositional patterns, depend on sediment supply versus river transport capacity and are likely to be highly variable across time and space as the channel gradually adjusts (Pizzuto, 2002).…”