STREAM TEMPERATURE AS AN INDICATOR OF NATURAL AND HUMAN-INDUCED MODIFICATIONS

Stream temperature is an important indicator of stream health and can be altered by natural processes and human modification of the stream system. In summer 2014, we deployed four HOBO temperature loggers recording every 15 min. in Spring Brook, a 1^st to 2^ndorder stream in south-central Rock County, Wisconsin. We have tested the effects of the absence of shading riparian vegetation and of point-source discharges from waste-water-treatment and food-processing facilities. We also documented the effect of groundwater influx on stream temperature. Each of these has a unique temperature signature.

Stream temperature varies daily and variation is particularly obvious on warm, sunny summer days. Warmest temperature occurs in late afternoon and slowly cools throughout the evening hours reaching a minimum at dawn when a slow warming begins. In southern Wisconsin, the daily range could vary from 5-7°C. In an unshaded 400-m stretch of the stream, we documented a 2-3°C increase relative to the upstream section during the day. In contrast, the shaded 400-m section directly downstream showed no change.

The waste-water-treatment facility is at the “headwaters” of a 1^st order tributary and typically discharges 750 m³/day at a fairly constant rate. Its signature is a mid-afternoon maximum and near-dawn minimum, a range of about 3°C, and a less smooth curve, presumably indicating slight pulses in discharge. This outflow joins the main, groundwater-fed, but very exposed stream where temperature maximums are similar, occurring in late afternoon, but minimums are much cooler, yielding a larger temperature range of 5-7°C. A food-processing plant releases holding-pond water in the late afternoon, with the effect of producing a slightly delayed warmer maximum andminimum of 1-2°C.

We identified one anomalous temperature pattern on the stream that shows a 2-3°C higher maximum delayed to 10 pm and a slightly delayed 2°C higher minimum. We can only explain this pattern by a large, non-permitted, early evening release to the stream; we are currently investigating this explanation.