THE EFFECT OF THE ST. CROIX FALLS HYDRODAM ON RIVERBED TEMPERATURE

Alese Colehour, Department of Biology, Macalester College, St. Paul MN
Daniel J. Hornbach, Departments of Biology and Environmental Studies, Macalester College, St. Paul MN
Mark C. Hove, Department of Biology, Macalester College, St. Paul MN
Kelly R. MacGregor, Department of Geology, Macalester College, St. Paul MN

Recent changes in the operation of the St. Croix Falls dam from peaking to run-of-river may have an effect on the thermal environment of endangered mussel populations at Interstate Park, a site just downstream of the dam. We hypothesize that a change in dam operation to run-of-river has led to an improvement in the thermal habitat for winged mapleleaf by reducing hourly water temperature variability due to holding back constant flow of water. As part of a research collaboration with the National Park Service, riverbed water temperatures were collected at Interstate and compared to two sites: Wild River (a control site upstream of the dam) and Franconia (~10 km downstream of Interstate). Data collected at Interstate were also compared with data from this location collected in 1998 when the dam was operated as a peaking facility. Other factors such as water depth, depth in the substrate, water discharge, and air temperature were also analyzed. In June 2009, IBTag temperature sensors were placed at three locations (Interstate, Franconia, and Wild River). Four sensors were embedded in stakes (at 0, 5, 10, and 20 cm) and hammered into the riverbed at two shallow (< 1 m) and two deep (>1 m in most cases) sites at each location. A total of 48 sensors were programmed to log once per hour from June 6 until July 7 to the nearest 0.0625 °C. Discharge data were downloaded from the USGS waterdata.usgs.gov website. After accounting for water and substrate depth, the magnitude of hourly thermal variation is lower at Interstate than at Franconia or Wild River. This suggests the reservoir is acting as a temperature buffer for downstream mussel populations. A comparison of temperature data from 1998 and 2009 demonstrates that higher water discharges significantly reduces thermal variation despite pre-run-of-river operation; this suggests that water discharge is at least as important in controlling temperature variability as dam operation. Additional data is being collected during summer and fall 2009, and future analysis will look more closely at water discharge and at air and water temperatures over several seasons.