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Overview of Lake Pepin-Upper Mississippi River TMDL

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The Total Maximum Daily Load (TMDL) process, extending from study to implementation plan, provides a framework for addressing several inter-related water quality impairments on the Upper Mississippi River between the confluences of the Minnesota River and Lock and Dam 4, together with downstream concerns, in a coordinated fashion. Problems addressed include: turbidity impairment and the related issue of suppressed growth of submersed aquatic vegetation (SAV) along the main channel border, in side-channels and backwaters; eutrophication; and accelerated sedimentation of Lake Pepin. Though not addressed explicitly in the TMDL, the implementation plan will address problems of excess nutrients downstream of Lake Pepin ranging from eutrophication of the Mississippi and associated backwaters to hypoxia in the northern Gulf of Mexico.

Turbidity impairment extends from the Minnesota River to its confluence with the Mississippi River in Navigation Pool 2, through upper Lake Pepin. Two TMDL end points apply to this impairment: a turbidity standard of 25 nephelometric turbidity units, and a site-specific standard for SAV being developed by a team of biologists from Minnesota and Wisconsin state agencies and the U.S. Geological Survey. A site-specific standard for eutrophication in Lake Pepin is being developed by the Minnesota Pollution Control Agency in consultation with the Wisconsin Department of Natural Resources with targets for Total Phosphorus, Chlorophyll-a, and Secchi transparency. Nutrient reduction goals developed by the Mississippi River Gulf of Mexico Watershed Nutrient Task Force will be integrated into implementation planning.

A three-dimensional hydrodynamic water quality model has been developed for the Mississippi River from Lock and Dam 1 to Lock and Dam 4 to evaluate how these inter-related targets may be achieved by varying inputs of sediment and nutrients to the system. Sediment source identification in the Minnesota River by means of radionuclide tracers, GIS mapping of erosive landscape features, and construction of a sediment mass balance for a sub-watershed of the Minnesota River Basin are being combined with watershed modeling using Hydrologic Simulation Program Fortran to route major sources of sediment and nutrients to the river model domain. A U.S. Army Corps of Engineers model is used to estimate how reduced solids loads and consequent improvements in transparency are likely to affect the growth of SAV.

A committee of residents living near the river from lower Pool 2 through Upper Lake Pepin has been convened to select a set of indicators to guide the process of adaptive management as local, state and federal agencies coordinate efforts to restore the Mississippi River. A technical committee with links to the new Navigation and Ecosystem Sustainability Program administered by the U.S. Army Corps of Engineers will develop metrics for each indicator. It is hoped that this will allow close integration of pollutant load reductions from upstream tributaries with river management measures such as island-building and water level drawdowns to stimulate the growth of SAV in parts of the river where high levels of turbidity pose significant challenges.
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Keywords: Lake Pepin; TMDL; eutrophication; turbidity

Document Type: Research Article

Publication date: 2009-01-01

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