SGER: Dam removal and the response of gravel-bed rivers to sediment pulses

University Of Montana, Missoula MT

Investigators

Abstract

This study will investigate sediment transport and channel evolution in gravel-bed rivers in response to sediment pulses, taking advantage of the field-scale experiment that will be created by the removal of Milltown Dam on the Clark Fork of the Columbia River, MT. Through data collection on pre- and post-dam removal channel conditions and on bedload and suspended load transport out of Milltown reservoir during high flow periods, this research will address fundamental questions about fluvial processes, sediment transport and deposition, and dam removal. Several hypotheses will be investigated about how pulses of fine sediment infiltrate into and affect the mobility of armored river beds, about spatial and temporal patterns of sediment transport and deposition, and about effective discharges for transporting sediment pulses. This research will provide an excellent opportunity to evaluate how previous flume research or field studies of small channels, upon which much of what is known about sediment transport and related aspects of fluvial processes is based, scale up to larger rivers. The data collected here will also be evaluated in the context of the implications of geomorphic responses and sediment geochemistry for habitat and biological processes. Removal of Milltown Dam, which will begin in spring 2008 and continue the following winter, is especially notable because (1) it is the largest dam removal to date in the United States in terms of the volume of reservoir sediment potentially available for downstream transport (approximately 3 million m3; another 2 million m3 are being mechanically removed); (2) the dam is the downstream end of the largest Superfund site in the United States, the Clark Fork Complex, and reservoir sediments are composed largely of contaminated mine tailings; (3) it is occurring on a regionally important river that includes designated Critical Habitat for bull trout, a federally threatened species. Systematic study of this dam removal will therefore have broader impacts on multiple levels, including for understanding the sediment and metals transport associated with a significant dam removal, for the insights it lends to upcoming planned removals of large dams, and for its broader contribution to understanding how gravel-bed rivers respond to increased sediment fluxes. Broader impacts will also derive from the project?s interdisciplinary nature, whereby analyses of geomorphic processes, geochemistry, hydrology, and fish response will be combined to understand downstream effects of dam removal. This project will help a new faculty member build a research program in fluvial geomorphic processes and will directly support one graduate and one undergraduate student. The project will also foster ongoing collaboration efforts between University of Montana and Salish-Kootenai College, a tribal college in Pablo, MT, whose students will assist field data collection and analysis under the guidance of the PI. Further, data collection and analysis related to Milltown Dam removal will be incorporated into graduate and undergraduate geomorphology and geochemistry courses, an integration of teaching and research that will be facilitated by the project?s close proximity to the University of Montana.