TEMPERATURE CONTROLS IN A SOUTHEASTERN MINNESOTA TROUT STREAM

Many public and private agencies in the Driftless Area of Minnesota, Wisconsin, and Iowa are increasingly focused on the restoration of trout streams. Successful stream restoration in trout streams must address the issue of thermal regulation of the stream waters. Previous to European settlement, many stream reaches in the Driftless Area had prairie vegetation within the riparian corridor. In recent years there has been a growing move toward restoring prairie habitats along Driftless Area trout streams. Prairie-based stream restoration approaches have been recognized as effective and beneficial solutions in terms of bank stability and ecosystem dynamics. However, the mechanisms by which prairie-banked riparian zones regulate stream temperature are poorly understood. This study evaluates the environmental factors contributing most significantly to the thermal regulation of Pleasant Valley Creek, a designated trout stream in southeastern Minnesota.

Thirteen temperature data logging sampling sites were established in a 175 meter reach of Pleasant Valley Creek to collect water and air temperature every minute for 24 hours. The sampling sites were chosen based on differing canopy densities and sediment substrate size distributions. The resulting temperature data were analyzed to identify periods of sustained high incoming solar radiation at each station. The corresponding water temperature response was compared to substrate, width-to-depth ratios, and over story density percentages.

It was originally thought that riparian vegetation, and subsequently the amount of solar radiation that reaches the water surface, was the most significant factor contributing to the stream’s temperature. We found that although solar radiation is the overall driving factor, the over story density did not matter as much as the size of the stream substrate in controlling the warming of the water. Finer, well-sorted substrates warm the stream water more rapidly than coarser, poorly sorted substrates. We hypothesize that coarse substrates present a smaller surface area perpendicular to incoming solar radiation and, as a result, convert light to heat less efficiently than finer, more homogenous, and less “macro-topographically diverse” substrates.