Ground-water baseflow to the upper Mississippi River upstream of the Minneapolis-St. Paul area, Minnesota during July 1988

Abstract

Ground-water baseflow to six subreaches of the Upper Mississippi River were estimated for July 1988, a period of drought. Ground-water baseflow to each subreach was estimated on the basis of streamflow gains determined from records of daily discharge at gaging stations. Streamflow gains were adjusted for estimated inflow from tributaries, municipal and industrial discharges, withdrawals, and evaporation. Low-flow frequency characteristics were computed for the Mississippi River near Anoka, Minnesota and the Mississippi River at St. Paul, Minnesota. Introduction The Mississippi River supplies water for the Minneapolis-St. Paul area of Minnesota (fig. 1). A drought in 1988 raised concern about the need for supplemental releases of water from reservoirs in the Mississippi River headwaters. The U.S. Army Corps of Engineers (USCOE) and the Minnesota Environmental Quality Board (EQB) have undertaken a study to develop a methodology that can be used to estimate the timing and volume of releases from the Headwaters Reservoirs necessary to support minimum flows of the Mississippi River in the Minneapolis-St. Paul area. One of the needs identified by the USCOE-EQB is quantification of ground-water contribution to the Upper Mississippi River during periods of baseflow. The U.S. Geological Survey (USGS) was requested to assist the USCOE-EQB study by (1) estimating the ground-water baseflow by subreach to the Mississippi River during July 1988, and (2) computing updated low-flow frequency statistics for the Mississippi River near Anoka and the Mississippi River at St. Paul. Purpose and Scope This report presents the results of estimates of ground-water gains and losses in six subreaches of the Mississippi River from the headwaters reservoirs to Anoka, Minnesota during July 1988 and lists low-flow statistics for the Mississippi River near Anoka for 193393 and Mississippi River at St. Paul, Minnesota for 1895, 1897, 1901-05, and 1907-93. Approach and Methods The scope of work for this investigation identified two primary objectives (1) determination of baseflow discharge to the Mississippi River, and (2) computation of low-flow frequency characteristics for the Mississippi River near the Minneapolis-St. Paul area. An approach and method was developed to address each of the objectives. Ground-water baseflow The study reach, which extends from the Mississippi River headwaters reservoirs downstream to Anoka in the Minneapolis-St. Paul area, was divided into six subreaches (fig. 1). A USGS continuous-record streamflow gaging station is located at the upstream and downstream end of each subreach. Distance in river miles for each subreach was determined from data published in the USCOE users manual for the River Emergency Management Model (U.S. Army Corps of Engineers, 1993). Daily stream discharge records for the gaging stations during July 1988 were used to determine the gain or loss of discharge within each subreach. Inflows from tributary streams and municipal and industrial discharges, both gaged and ungaged, were subtracted from the gain in each reach. Municipal and industrial withdrawals from the Mississippi River and estimated evaporative losses were added to the gain in each reach. The resulting stream discharge was considered to be the ground-water baseflow to the Mississippi River main channel within each subreach. 95' Boundary of Upper Mississippi ' Watershed