Rapidan Dam and Reservoir are located along the Blue Earth River south of Mankato, Minnesota. The dam was constructed in 1911 as a source of hydroelectric power to supplement the surroundings communities. Currently, the reservoir is heavily silted and provides little hydroelectric benefit while proving costly to maintain. This study (1) defines the sedimentary, geomorphic, stream flow and suspended load characteristics of the reservoir for 2008-2009 and (2) compares these parameters with those available from 1985 (23 years prior). Stream gauging and sediment sampling took place in 2008 and 2009 at three monitoring locations (two upstream and one downstream of the dam) to assess the mass balance through the reservoir. Fifty reservoir bottom sediment samples were collected for sieve and settling tube particle size analyses of grain size distributions. Multiple years of aerial photographs were also obtained to evaluate the surface area lost to siltation since 1939. Results indicate that the trap efficiency is altered. Currently Rapidan Reservoir cannot retain the silt and clay fraction of the Blue Earth River's suspended load. In the 23 year period, the average grain size within the reservoir increased from silt to medium sand. The average maximum velocity required to deposit that sediment has also increased by more than a factor of ten, (i.e., from 0.27 to 3.20 cm/sec). The increase in velocity corresponds to the accumulation of numerous sandbars that decrease the area for water to spread out in the reservoir. This association of cause and effect is supported by the analyses of eight aerial photographs dating from 1939 to 2006 that show that the overall surface area in the reservoir has decreased by 56% since the late 1930's. Increased velocities also provide the mechanism to incise and channelize the Blue Earth River through the reservoir and remobilize previously deposited sediment. Stream flow and loading results from 2008 and 2009 show that the reservoir serves as a source for suspended sediment to downstream reaches of the Blue Earth and Minnesota Rivers. A study of this nature paired with follow up studies could inform decision making processes for either removal or further rehabilitation. Removal would provide an excellent opportunity for researchers to study a large scale experiment in river restoration, both the positive and negative effects from reopening a waterway that has been segmented for over a century.
Date of Degree
Master of Science (MS)
Social and Behavioral Sciences
Rassmussen, Katherine Brosch, "Trap Efficiency of a Silted Prairie Reservoir: Rapidan Reservoir, Blue Earth County, Minnesota" (2012). All Theses, Dissertations, and Other Capstone Projects. 138.
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