Synoptic sampling was used to determine chemical and biological characteristics of the St. Croix River within a study reach that extended from near Danbury, Wisconsin to the confluence with the Mississippi River at Prescott, Wisconsin. The study was conducted August 7- September 25, 2000 during summer low flow.
Dissolved-residue concentrations were found to increase gradually as the river
flows downstream, with an abrupt increase downstream of the confluence with
the Sunrise River that was primarily attributed to an increase in calcium and
magnesium. Dissolved residue concentrations were further augmented by increased
yields of chloride and sulfate in the part of the St. Croix Basin between Nevers
Dam near St. Croix Falls, Wisconsin and Marine on St. Croix, Minnesota.
Nearly all of the nitrogen in transport was in the form of nitrate and organic
nitrogen. Organic nitrogen, mainly in particulate form, accounted for most of
the gain in total nitrogen load within the study reach. Nitrogen loading to
the mainstem indicated relatively uniform nitrogen inputs from the Clam River,
Kettle River, and Snake River watersheds. The rate of nitrogen load accrual
increased downstream of the confluence with the Sunrise River and in the subreach
extending from Nevers Dam through St. Croix Falls to Franconia, Minnesota. Nitrogen
load also increased, primarily because of nitrate input, in the part of Lake
St. Croix downstream of the confluence with the Kinnickinnic River. Total phosphorus
concentrations and loads reflected variations in the amount of particulate phosphorus
in transport. Phosphorus loading increased in the part of the St. Croix River
that includes Danbury and the confluences of the Yellow River and Clam River.
Phosphorus loading also increased downstream of the confluence with the Sunrise
River, but the greatest load increase occurred between Nevers Dam and Franconia.
Phosphorus load decreased substantially as the river flowed through the pooled
reach of Lake St. Croix downstream of Stillwater.
Suspended-sediment concentrations were low, ranging from 4.0 to 36 milligrams per liter. The small amount of sediment in transport was reflected in turbidity measurements that ranged from 0.5 to 3.6 Nephelometry Turbidity Units, and transparency tube measurements that were greater than 60 centimeters at all sites.
Biological measures of resource quality change in the St. Croix River along its course from Danbury to Prescott. Changes in the biological indicators of resource quality (fish and invertebrate community composition) are most notable just upstream and downstream of the dam at St. Croix Falls, Wisconsin. Aquatic communities in the upper St. Croix River, from near Danbury to near Rush City, Minnesota, indicate minimal physical and chemical disturbance as evidenced by relatively high taxa richness and greater proportions of taxa intolerant to physical and chemical disturbance. In contrast, aquatic communities downstream of the Sunrise River to Marine on St. Croix indicate both physical and chemical disturbance.
Resource monitoring, consisting of short-term diagnostic studies, may be needed in parts of the St. Croix River mainstem and tributaries where results from this study indicate constituent loading is greatest and where the aquatic community composition indicates disturbance. Longer-term trend monitoring may be needed to detect physical, chemical and biological responses to natural processes and human activities in the St. Croix River Basin.
Abstract
Introduction
Environmental setting
Approach and methods
Water sample collection
Quality-assurance results
Streamflow determination
Physical habitat measurements and analyses
Fish collection and analyses
Aquatic macroinvertebrate collection and analyses
Streamflow
Water quality
Constituent concentrations
Dissolved oxygen and major ion concentrations
Nutrient concentrations
Suspended solids, sediment, turbidity, and transparency
Constituent loads
Nutrient loads
Suspended-sediment loads
Constituent yields
Major-ion yields
Nutrient yields
Suspended-sediment yields
Physical habitat
Aquatic community characterization
Fish community
Invertebrate Community
Factors influencing aquatic resource quality
Water-quality monitoring in the St. Croix River
Summary
References
Supplemental information
¹U.S. Geological Survey
²Minnesota Department of Natural Resources
Mound View, Minnesota, 2002
For additional information write to:
U.S. Geological Survey
District Chief
2280 Woodale Drive
Mounds View, MN 55112
Copies of this report can be purchased from:
U.S. Geological Survey
Branch of Information Services
Box 25286, MS 517
Federal Center
Denver, CO 80225
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