Surface Water and Groundwater Exchanges in Fine and Coarse Grained River Bed Systems and Responses to Initial Stages of Dam Removal, Milltown, Montana
Year of Award
Master of Science (MS)
Department or School/College
Department of Geosciences
William W. Woessner
Johnnie Moore, Donald Potts
University of Montana
More than 620,000 miles of rivers are affected by over 79,000 dams in the United States. The dams are a valuable asset, however dams block fish passage, and disrupt the physical and biological systems, and alter hyporheic exchange. In efforts to remediate rivers impacted by dams, dam removal is being proposed. Changes in the location and timing of groundwater and river water exchanges in post dam remediated river systems are typically not identified. The goal of this study was to examine the factors controlling surface water and groundwater exchange rates and locations in river systems during dam removal. Specific tasks included establishing a river elevation and groundwater elevation monitoring network, defining riverbed properties and exchange rates using in-riverbed measurements, developing a site conceptual model for different river reaches, and constructing and calibrating a finite difference numerical groundwater model representing site conditions. Results of monitoring indicate that the Clark Fork River above the dam and reservoir is connected to the aquifer system, containing losing and gaining and parallel flow reaches. The Clark Fork River below the dam appears to be gaining approximately 600 ft below the dam and then becomes a perched losing river that at times throughout the year may become linked to the underlying water table. The reservoir pool and its Blackfoot River arm are losing systems with pool elevations well above the local water table. In-river measurements indicate the river and groundwater exchange water at a rate of 0.02 to 13 ft3/(dayft2). A calibrated numerical groundwater model was used to examine how pre-dam removal groundwater, river and reservoir exchanges were likely to change once the dam is removed. Simulations suggest that historically (with the dam) the reservoir contributed approximately 21% of the recharge to the local Milltown aquifer system. Once the dam is removed this recharge contribution may be reduced to less than 1%. As a result water table elevations will decrease locally and exchange locations and rates above the dam location will be altered. Reducing uncertainty of the modeling results and predictions could be improved by further data collection and evaluation of various conceptual models.
Farinacci, Anthony Joseph, "Surface Water and Groundwater Exchanges in Fine and Coarse Grained River Bed Systems and Responses to Initial Stages of Dam Removal, Milltown, Montana" (2009). Graduate Student Theses, Dissertations, & Professional Papers. 10828.
© Copyright 2009 Anthony Joseph Farinacci