Summary

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Berg, J.A., T.A. Petersen, Y. Anderson, and R. Baker. 2004. Hydrogeology of the Rock River watershed, Minnesota and associated off-channel habitats of the Topeka shiner. Final report submitted by the Minnesota Department of Natural Resources. 13+ pp.

Summary:

The Topeka shiner (Notropis topeka) is a native species minnow that was once common in headwater streams of the Midwest and western prairie. The species is estimated to have had a wide range across several states but is now restricted to portions of these areas. The species is in decline in Kansas, Missouri, Nebraska, and Iowa. The Topeka shiner now exists in less than 10 percent of its historic geographic range in highly fragmented populations. The U.S. Fish and Wildlife Service (USFWS) listed the species as endangered on January 14, 1999. Recent studies in Minnesota have shown that relatively abundant populations appear to be surviving across much of the southwestern portion of the state in the Big Sioux watershed.

Research has shown that off-channel habitats (OCHs - ponds and meander cut-offs) may be particularly important to the species' survival by acting as sanctuaries or critical habitats in the species' life cycle. We suspected these habitats were fed mostly by ground water from the surficial alluvial aquifer associated with the river system. Therefore, a better understanding of the shallow ground-water system appeared important to protecting the species in this area. In an area where aquifers typically have limited capacity, large ground-water appropriations from shallow aquifers, near OCHs, could dewater them. Poorly planned ground-water appropriations could, in this manner, result in the loss of habitat.

The project had three main phases. The first phase was an assessment of the species? distribution. In spring 2002, populations of Topeka shiners were documented at 29 OCHs by capturing all the swimming aquatic organisms with a one- or two- person seine and visually identifying the Topeka shiner individuals. Previous assessments showed that the in-stream occurrences of the Topeka Shiner are fairly evenly distributed within the river system. Similarly, the species was found in most of the OCHs. Together, these data underscore the generally favorable conditions that appear to exist in this area for the species.

The second phase was an assessment of ground-water and surface-water interactions at the OCHs. Ground water maintains a steady temperature that is close to the mean annual air temperature. We measured sediment temperatures with a temperature probe beneath the OCHs during late summer and found moderate to strong ground- water connections at most of the OCHs.

The third phase of the project was the creation of a series of maps describing the regional boundaries, base elevation, water-table elevation, and saturated thickness of the Rock River valley alluvial aquifer. The base of the surficial aquifer elevation map was created from a combination of existing well and soil boring information from the County Well Index and surface resistivity image data collected by the Minnesota Department of Natural Resources (DNR) for this project at 60 locations. The water-table elevation was created by interpolating shallow water-table soil information, elevations of surface-water features, and historical water-level measurements from water-table wells. The main map product, the saturated thickness map of the aquifer, was derived by subtracting the gridded elevations of the aquifer base from the gridded elevations of the water-table elevation.

The saturated thickness map shows a fairly regular pattern of aquifer thickness laterally across the aquifer with the thicker portions existing in the center of the Rock River valley. The northern portion of the Rock River valley aquifer, especially around Edgerton, has a greater maximum thickness range (approximately 60 feet to 80 feet) than the maximum thickness range (approximately 40 feet to 50 feet) of the southern portion of the aquifer.

The Rock River portion of the aquifer also appears to be significantly thicker than the aquifer beneath the major tributaries. Therefore, the OCHs in the tributary areas would be more vulnerable than most of the OCHs in the Rock River valley. Any of the identified OCHs, and others that have not yet been identified, could be affected by adjacent, large-capacity pumping activities.

Acquisition of a DNR ground-water appropriation permit requires completion of an interference pumping test to determine whether the requested volume of water will affect water levels in nearby wells or aquatic resources. We recommend monitoring water levels in these and other unidentified OCHs that are within the possible critical radius during any pumping tests conducted for a permit application.

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