Analysis of streamflow and water-quality data at two long-term monitoring sites on the St. Croix River, Wisconsin and Minnesota

Streamflow of the St. Croix River has been monitored on a long-term basis by the U.S. Geological Survey (USGS) at streamflow-gaging stations on the St. Croix River at Wisconsin State Highway 35 near Danbury, Wisconsin and below the dam at St. Croix Falls, Wisconsin. Trend analysis indicates that annual streamflow peaks, annual mean flows, and annual 7-day low flows have remained relatively stable at the Danbury (upstream) station from 1914 to 2001 but have increased by 0.45 to 0.55 percent per year at the St. Croix Falls (downstream) station from 1902 to 2001. Water-quality sampling conducted by the USGS at two associated water-quality monitoring sites from 1964 to 2001 have included analysis for nutrients, suspended sediments, metals, bacteria, and pesticides. The water-quality data at the two sites had considerable temporal variation and was often limited in extent by the data-collection needs of previous projects. The absence of consistent long-term water-quality data made detection of statistically significant trends in the water-quality data difficult. Nutrient, suspended-sediment, and major ion concentrations and field parameters were generally higher and more variable at the St. Croix Falls site than at the Danbury site. However, disparate sampling periods and strategies could cause possible biases in the comparison of water quality between sites. Introduction The U.S. Geological Survey (USGS) has performed long-term continuous streamflow measurements of the St. Croix River at two streamflow-gaging stations: at Wisconsin State Highway 35 near Danbury, Wisconsin and below the dam at St. Croix Falls, Wisconsin (fig. 1). Water-quality measurements of a variety of constituents such as nutrients, suspended sediments, metals, bacteria, and pesticides have been collected at two water-quality monitoring sites associated with these streamflow-gaging stations. This report will refer to both the streamflow-gaging stations and associated water-quality monitoring sites as sites. This study, which was initiated in 2001, was a cooperative effort between the USGS, National Park Service (NPS), and the Joint Water Quality Commission of Danbury and St. Croix Chippewa Indians of Wisconsin to analyze the long-term USGS flow data for temporal trends and provide summaries and analysis of the USGS water-quality data collected at these sites. This report presents trends in flow and differences in water quality among sites, studies, and over time. Streamflow data from 1902 to 2001 and water-quality data from 1964 to 2001 are included in this study. Differences between the long-term data sets can be used to determine the part of the St. Croix River Basin where change has occurred. Trends at the Danbury site reflect changes in the upper basin (1,580 square miles), trends in the difference between the St. Croix Falls and Danbury sites reflect changes that occurred in the part of the drainage basin between those two sites (4,660 square miles), and trends at the St. Croix Falls EXPLANATION M I N N E S O T A