Channel Migration and Bank Erosion of the Clark Fork River at Grant-Kohrs Ranch N.H.S., Western Montana

by Parmar, Nisha Pravin

Abstract (Summary)
Large-scale mining, concentration and smelting in Butte and Anaconda, Montana over the last c.a. 150 years led to contaminated tailings being distributed along the entire upper Clark Fork River System. The extent of hazardous material distributed along the Clark Fork River is a major concern because the floodplain is a large reservoir of contaminated sediment that is being eroded into the river, and is a main source of river contamination and metal toxicity in aquatic ecosystems This study examined the geomorphic changes within the river system that have occurred in a 4-km long stretch of the Clark Fork River near Grant-Kohrs Ranch National Historic Site. Aerial photographs of the reach from 2001, 2004 and 2007 were analyzed using ArcGIS 9.2. Areas of erosion and areas of deposition were calculated for the main meander bends, and results show that these areas are unbalanced throughout the study reach. The average current erosion rate for the time interval between 2001 and 2007 was determined to be 0.170m²/m/yr, while the rate of deposition was determined to be 0.222m²/m/yr. An average of about 680m² of the contaminated floodplain are reworked every year, and at this rate it will approximately 1000 years to rework the entire floodplain within Grant-Kohrs Ranch. Eight cross sections of the channel within the study reach were measured to determine the change in channel area and elevation to determine if the bed is aggrading or degrading. Slope and thalweg measurements, along with a Wolman pebble count were conducted along each segment. An Incipient Motion Method was used to determine at which discharge motion would begin. From the record of flow, and cross section analyses, results indicate that it is the moderate flow events which are responsible for the sediment transport regime within the site.
Bibliographical Information:

Advisor:Johnnie Moore; Donald Potts; Andrew Wilcox

School:The University of Montana

School Location:USA - Montana

Source Type:Master's Thesis



Date of Publication:01/15/2009

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