UNRAVELING MULTIPLE DIURNAL AND STORM EVENT WATER-QUALITY FLUCTUATIONS IN A SMALL WISCONSIN TROUT STREAM: SUMMER 2009

During the dry summer of 2009, complex diurnal and storm event fluctuations of multiple water quality parameters occurred in a small trout stream in western Wisconsin. Water-quality data were monitored from late June to late August at 15-minute intervals from two stations in the South Fork of the Kinnickinnic River (South Fork) at River Falls, Wisconsin. The South Fork is a gaining stream that drains a mix of agricultural fields, upland bluffs, wetlands, and urbanized areas. The materials in its valley are unconsolidated heterogeneous alluvium and glacial deposits over fractured dolostone.

During dry spells, large diurnal cycles were observed in temperature, D.O., and pH, while smaller diurnal cycles were noted in water level and specific conductance. In the early morning water level and specific conductance are at their maximum, whereas pH and, slightly later, temperature are at their minimum. In contrast, maxima in temperature and pH, and minima in water level and specific conductance occur over a longer time span in the afternoon. These variations probably result from a combination of cycles in solar radiation, respiration, and input of groundwater affected by evapotranspiration from floodplain plants. D.O. does not follow this pattern. Its maximum occurs midday at times of higher solar radiation and its minimum occurs at night, presumably due to cycles in photosynthesis.

Rain events cause significant inputs from stormwater outfalls and lead to rapid increases in stream depth, turbidity and temperature, and similar quick decreases in specific conductance and pH. One especially large storm on a hot day in August caused temperature to rise to 20 °C, representing a significant thermal impact. Decreases in specific conductance and pH are indicative of rainwater. High turbidity levels indicate upstream erosion from stream banks, farmland, and disturbed areas.

Multiple processes, both natural and human-caused, affect the South Fork. Groundwater, stream ecosystem, meteorological, and human systems combine to create complicated diurnal water-quality fluctuations.