Weaver Bottoms monitoring study: Determining the pros and cons of a rehabilitation project

by Dr. Mary Davis, U.S. Army Engineer Waterways Experiment Station

The banks and marshes of the Mississippi River have always provided healthy environments for many species of wildlife. In the early 1930s, the U.S. Army Corps of Engineers constructed a series of locks and dams in the Upper Mississippi River to improve navigation along the river from Cairo, Ill. to Minneapolis, Minn. Extensive areas of the floodplain were inundated and rapidly became highly productive backwater marshes. Since the early 1960s, however, acreage and vegetation density have fluctuated and gradually declined, lowering the quality of existing wetlands.

Project design

The Great River Environmental Action Team (GREAT) I was organized in 1973 to identify and assess problems associated with multipurpose use of the Upper Mississippi River and to develop recommendations for improved management of its resources. Weaver Bottoms, a 4,000-acre backwater area located between southeastern Minnesota and southwestern Wisconsin, was chosen as a representative site for a study of the marsh problems. An initial assessment of the site attributed the marshes inability to recover to a variety of reasons, including

two major floods in the late 1960s,
plant damage by wind and ice,
the constant change in flow and sedimentation patterns, and
reduced water clarity caused by wind-induced waves due to resuspension of sediments.

As a result, the Weaver Bottoms Rehabilitation Project was designed to do two things: (1) to reduce Mississippi River flows entering the backwater by modifying side channels, and (2) to reduce wind fetch and re-suspension of bottom sediments by creating barrier islands. In addition, the project was to reduce maintenance dredging requirements in the navigation channel and to provide long-term dredged material storage. The Weaver Bottoms project was to be completed in two phases. Phase I construction was completed during fall of 1986 and summer of 1987. Partial or complete closures were constructed across most of the secondary channels leading from the Mississippi River into Weaver Bottoms, and two 16-acre islands were constructed in open water areas. Phase II will be the construction of additional islands and/or implementation of other measures once the effects of Phase I construction are thoroughly evaluated and recommendations for Phase II can be made.

Planning and evaluation

An important part of the project was a comprehensive 10-year Resource Analysis Plan to monitor project effects. An interagency Memorandum of Understanding (1986), assigned leadership for the monitoring project to the U.S. Fish and Wildlife Service with active participation from the Corps and Wisconsin and Minnesota Departments of Natural Resources.

The Resource Analysis Plan called for an assessment of project impacts on hydrodynamics; sedimentation; water quality; emergent and aquatic vegetation; use of aquatic and wildlife habitats by birds, fish, and mammals; and recreational use. Monitoring began two years prior to project construction and continues. The first 5 years of the monitoring program resulted in the following:

Hydrodynamics. Post-project monitoring indicated that secondary channel discharges to Weaver Bottoms were reduced 80 percent, and hydraulic residence time was increased to between 3 and 7.6 days. Current velocities within Weaver Bottoms have been reduced 60 to 90 percent. The two islands have altered flow patterns. However, hydrodynamic impacts of the islands on Weaver Bottoms were small compared to the reduction in inflow due to closure of secondary channels. Wave action continues to be a major factor influencing bottom velocities and sediment resuspension.
Since project construction, the need for dredging has decreased dramatically (by 60 percent) in the Weaver Bottom area. This reduction was probably due to greater channel scouring caused by the increased river flows.

Sedimentation. Bathymetry data from 1986 (pre-project) and 1991 (post-project) were compared with 1935 data. Construction of the project resulted in notable changes in erosion/deposition patterns in Weaver Bottoms. Although the net change in bathymetry from 1986 to 1991 was small, high rates of both deposition and erosion occurred. This fluctuation indicated that internal factors, such as wind generated wave action, have increased their influence on the sedimentation patterns in Weaver Bottoms.
General patterns showed deposition in deep areas and erosion in shallow areas. Three areas in particular showed bathymetry changes since project construction: (1) One area with historically substantial erosion showed significant reduction of erosion since inundation; (2) the delta areas at side-channel openings along the main channel side of Weaver Bottoms showed both deposition and erosion; and (3) an area near the mouth of the Whitewater River showed increased rates of delta expansion as a function of reduced flow velocities into Weaver Bottoms following project construction.

Water Quality. One project objective was to increase water clarity and thus improve vegetation growth. Water quality in Weaver Bottoms did not improve within the first 3 years following construction. Reduction of inflow from the Mississippi River also reduced mixing and flushing rates in Weaver Bottoms. Water quality in downstream portions of the backwater area became more influenced by the Whitewater River, a turbid river which empties directly into Weaver Bottoms. Variation in water quality values increased among areas within Weaver Bottoms after project construction as a result of differences in residence time.

Vegetation. Between 1985 and 1990, a general decline in emergent and submergent aquatic vegetation was recorded in the Weaver Bottoms Rehabilitation Project area as well as in other areas of the Upper Mississippi River. The cause of the vegetation loss is unclear but is apparently related to the 1987 to 1989 drought and not due to project construction.

Birds and Mammals. Aerial waterfowl transect surveys were conducted each fall from 1985 through 1990. Waterfowl use-days increased substantially between 1986 and 1987, but sustained a steady decline during the remaining post-construction period of 1988 to 1990. Annual diving duck (mostly canvasback, Aythya valisineria) use-days were more than double that of puddle ducks during the 1985 to 1987 period, but were below or nearly equal to puddle duck use-days for 1988-90. This post-construction decline in diving duck use probably reflected the drastic losses of American wildcelery (Vallisneria americana), a preferred canvasback food, in Weaver Bottoms during 1989 and 1990. Total use-days of another common migratory waterfowl in the area, tundra swan (Cygnus columbianus), varied but were lowest in 1989 and 1990 when substantial declines in arrowhead (Sagittaria spp.) biomass were detected.
Losses of vegetation and waterfowl use on the Weaver Bottoms project area were not attributable to the rehabilitation project, as evidenced by similar losses in nearby Upper Mississippi River pools. Changes in continental populations, habitat conditions, and weather also influenced migratory bird use of the river. The occurrence of muskrat (Ondatra zibethica), shorebirds, gulls, and terns at Weaver Bottoms was monitored, but no population trends were detected.

Fish. Fish populations were monitored within and outside the Weaver Bottoms project area. Trap nets, experimental gill nets and electroshocking collection methodswere used. Pre- construction sampling yielded 9,323 fish, epresenting 69 species with an average weight of 264 grams per fish. Post-construction sampling yielded 16,992 fish, representing 57 species with a higher average weight of 271 grams per fish. All sample methods showed an increase in catch per unit effort during the post-construction years. The proportion of sport fish, rough fish, and forage fish captured showed little change between pre- and post-construction periods.
Four key species were identified at Weaver Bottoms: northern pike (Esox lucius), carp (Cyprinus carpio), bluegill (Lepomis macrochirus), and black crappie (Pomoxis nigromaculatus). Gill netting data shows a biomass increase for all four species in the post-construction period; carp increased the most. Similar increases in the four species at stations within and outside Weaver Bottoms indicates that population increases may be partially due to factors other than the rehabilitation project.

Short-term monitoring results.

Phase I of the Weaver Bottoms Rehabilitation Project had been constructed to improve habitat quality within Weaver Bottoms and to reduce maintenance requirements of the adjacent navigation channel. Results of the monitoring effort indicate that there has been little immediate influence of the project on habitat quality for vegetation or wildlife in Weaver Bottoms or in adjacent areas of those studied. However, results indicate at least a short term reduction in channel maintenance requirements during the first 4-year period following Phase I construction.

More information about the project is available from Mary Davis at (601)634-2853, e-mail: mdavis@elmsg.wes.army.mil.

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