Physical Setting And Hydrology

The Upper Mississippi River basin (Hydrologic Region 7) covers a drainage area of 171,500 square miles over a reach of 1,170 miles from the headwaters in Lake Itasca to the confluence of the Missouri River with the Mississippi River at Alton, Illinois, just upstream of St. Louis, Missouri (Iseri and Langbein, 1974) (Figure 12-1). The water quality of the Upper Mississippi River has historically been dominated by wastewater loading from the Twin Cities, as well as sediments, nutrients, pesticides, oxidizable materials, and other pollutants from the Minnesota River basin. The watershed of the Upper Mississippi River basin (Catalog Unit 07010206) described in this case study includes a drainage area of 8,520 square miles, extending 83 miles from the confluence of the Crow River (UM milepoint 894) in Morrison County upstream of Anoka, Minnesota (UM milepoint 871) to the confluence of the St. Croix River downstream of Lock and Dam No. 2 at Prescott, Wisconsin (UM milepoint 811) (Figure 12-2).

Characterized by rolling hills and plains with numerous lakes, the basin topography reflects the effects of successive glacial advances over the region. Upstream of the Twin Cities, the major tributaries to the Upper Mississippi are the Minnesota River, the Rum River at Anoka, and the Crow River. Within the portion of the watershed influenced by wastewater loading from the Twin Cities, five locks and dams have been constructed for flood control, navigation, and hydropower purposes. Because of the flow-regulating nature of the series of locks and dams, the river essentially flows as a series of controlled backwater pools with relatively constant surface elevations. Over the 69-mile reach from Coon Rapids Dam upstream of Minneapolis

Figure 12-2 Location map of Upper Mississippi River (Accounting Unit 070102) near Minneapolis-St. Paul, Minnesota. (River miles shown are distances from confluence of Mississippi River with Ohio River at Cairo, Illinois.)

(UM river mile 866) to Lock and Dam No. 3 at Red Wing, Minnesota (UM river mile 797), the river drops from an elevation of 830 feet to 661 feet above mean sea level (Hydroscience, 1979).

The series of locks and dams, supplemented by dredging, maintain a 9-foot-deep navigation channel for commercial barge traffic. The navigation channel was authorized by the U.S. Congress in 1928, and the locks and dams were authorized in 1930. The U.S. Army Corps of Engineers is conducting a controversial environmental study assessing the impact of the lock and dam system on the ecological integrity of the Upper Mississippi River. In addition to the ecological effects of the flow control structures, the Great Flood of 1993 (Wahl et al., 1993) has generated investigations of the role that artificial drainage and flood-control structures might have played in actually increasing the extent of severe flooding in some areas of the watershed.

On a seasonal basis, streamflow of the Upper Mississippi River reflects peak precipitation during late spring snowmelt and early summer with severe subfreezing winter conditions (Figure 12-3). Although the minimum flow occurs during winter due to a reduction in watershed runoff as precipitation changes from rain to snow and ice, the critical period for water quality problems is during the low-flow, summer months. On the river itself, winter ice cover is intermittent, varying considerably both spatially and temporally. Ponded areas of the Upper Mississippi River, such as Lower Pool 2 and Lake Pepin (Pool 4), have permanent ice cover for about 3 months during the winter; the more riverine reaches freeze over only during extended periods of severe cold. DO levels are generally high during winter because of very low water temperature and open water conditions that allow oxygen exchange across the air-water interface. Reliable streamflow records from a USGS gage 300 feet upstream of the Roberts Street Bridge in St. Paul (UM milepoint 839.3) are available from 1892 to the present to characterize long-term monthly, annual, and extreme flow statistics over a drainage area of 36,800 square miles (USGS, 1999a). Based on the historical

Figure 12-3 Monthly mean, tenth, and ninetieth percentile streamflow (1951-1980) at St. Paul, Minnesota (USGS Gage 05331000). Source: USGS, 1999a.

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