Figure 12-4 Long-term trends of summer (July-September) mean, tenth, and ninetieth percentile streamflow at St. Paul, Minnesota (USGS Gage 05331000). Source: USGS, 1999a.
data recorded for water years 1892-1998, monthly flow ranges from a maximum of 26,060 cfs in April to a winter minimum of 4,544 cfs during February and a summer low of 8,060 cfs during September. Over the period of record from 1892 to 1998, annual average discharge of the Upper Mississippi River at St. Paul has been 11,630 cfs, with the lowest daily mean flow of 632 cfs recorded on August 26, 1934, and the highest daily mean flow of 171,000 cfs observed on April 16, 1965 (USGS, 1999a). Using historical records from 1936-1979 to represent streamflow variability after the series of locks and dams were constructed on the Upper Mississippi River, the 7-day, 10-year flow (7Q10) for summer conditions (June-September) at St. Paul is reported as 1,633 cfs (MPCA, 1981).
The long-term (1940-1995) interannual variation of mean, tenth, and ninetieth percentile summer (July-September) streamflow is shown in Figure 12-4. The historical record exhibits pronounced year-to-year variability of summer streamflow. Based on data from 1951-1980, the long-term mean summer (July-September) flow of 10,659 cfs is used to compute a normalized streamflow ratio for each summer from 1940 to 1995 as dry (< 0.75), normal (0.75-1.50) or wet (> 1.50). For example, the summers of 1962, 1972, 1978-1979, 1983, 1985-1986, 1991, and 1993-1995 were all characterized by wet conditions, where the flow was greater than 150 percent of the long-term summer mean. The summers of 1960-1961, 1964, 1967, 1970-1971, 1973-1974, 1976-1977, 1980, and 1987-1989, in contrast, were characterized by dry conditions, where the flow was less than 75 percent of the long-term summer mean. The extreme droughts of 1976 (1,725 cfs, 16 percent of summer mean) and 1988 (2,334 cfs, 22 percent of summer mean) and the Great Flood of 1993 (47,789 cfs, 450 percent of summer mean) are particularly noticeable in the 55 years of historical records for the Upper Mississippi River at St. Paul.
POPULATION, WATER, AND LAND USE TRENDS
Beginning in 1838, when the Twin Cities area was first opened for settlement, the abundant land and water resources attracted homesteaders. The confluence of the Minnesota River and the Upper Mississippi River served as an important transportation link between military and trading posts and the growing towns and cities along the Mississippi River. St. Anthony's Falls provided a natural source of power for lumber and grist mills. The fertile soil supported an agricultural economy, and the vast forests provided resources for a growing wood products industry. Uses for the Upper Mississippi River have included municipal and industrial water supply, commercial navigation, log transport, commercial fishing, hydropower, and water-based recreational activities. Beginning with the construction of an urban sewer system in 1871, the Upper Mississippi River has also been used for wastewater disposal.
As a component of the Lake Pepin Phosphorus Study, conducted from 1994 to 1998, historical records of land uses, agricultural practices (e.g., manure applications and commercial fertilizer uses), and wastewater discharges, compiled beginning ca. 1860, were used to correlate long-term changes in land uses in the watersheds of the Upper Mississippi River, the Minnesota River, and the St. Croix River with long-term changes in sediment and phosphorus loading to Lake Pepin (Mulla et al., 1999). As of the mid-1990s, the USGS (1999b) had classified about 60 percent of the watershed (Upper Mississippi River, Minnesota River, and St. Croix River) as agriculture and 23 percent as forest. The remaining 17 percent of the drainage basin was classified as urban and suburban (5 percent), water (5 percent), and wetlands (7 percent) (USGS, 1999b).
The Upper Mississippi River case study area includes a number of counties identified by OMB (1999) as a Metropolitan Statistical Area (MSA) (Table 12-1). Long-term trends in the population of the 13-county Minneapolis-St. Paul MSA are shown in Figure 12-5. Resident population in this MSA increased by 150 percent, from 1.1 million
TABLE 12-1 Metropolitan Statistical Area (MSA) Counties in the Upper Mississippi River Case Study
Anoka County, MN Carver County, MN Chicago County, MN Dakota County, MN Hennepin County, MN Isanti County, MN Ramsey County, MN Scott County, MN Sherburne County, MN Washington County, MN Wright County, MN Pierce County, WI St. Croix County, WI
Source: OMB, 1999.
in 1940 to 2.76 million in 1996. After a small increase of population from 1940 to 1950, the greatest rate of growth occurred during the 1950s and 1960s, when population increased by 23 to 27 percent. The rate of population growth then declined during the 1970s to 8.5 percent, with an increase to 15 percent during the 1980s. During the period from 1990 to 1996, population increased by 9 percent (Forstall, 1995; USDOC, 1998). Reflecting population growth in the Twin Cities, the population served by the Metro wastewater plant increased from 1.04 million in 1962 to 1.68 million in 1997. By 2020, the plant is expected to provide service to 1.94 million people (Larson, 1999).
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