THE IMPACT OF STORM ON THERMAL TRANSPORT WITHIN THE HYPORHEIC ZONE OF A LOW GRADIENT THIRD-ORDER SAND AND GRAVEL BEDDED STREAM

The importance of temperature in stream and hyporheic exchange is far reaching, and one event that affects temperature in the hyporheic zone (HZ) is a storm (high flow) event. The goal of this study was to evaluate the impacts of storm events on thermal transport in the HZ and stream of a low gradient sand and gravel bedded stream. Six (6) wells were installed along a 25 m straight stretch along the Little Kickapoo Creek. Each well contained 4 temperature loggers positioned at 30 cm, 60 cm, 90 cm and 150 cm to measure the streambed temperature signals. The stream temperature and stream stage were also measured. The HZ temperature profile revealed a seasonal thermal reverse in the after-storm substrate temperature response. There were spikes in the after-storm substrate temperature response in the summer and a drop in the winter. The distributions of the two seasons were entirely different and should therefore be studied separately. An increase in the amplitude of a storm event led to an increase in the expected seasonal after-storm streambed thermal pulse only 25-30% of the times. This means heavy storms will not necessarily lead to an increase in after-storm substrate temperature response. The amplitude of after-storm substrate thermal response rather depends on factors such as antecedent moisture conditions, the wetting front, and the temperature difference between the incoming runoff and the prestorm substrate temperature. There were dampening troughs and dampening spikes with increasing substrate depth during cold and warm periods respectively. However, these damping responses from the 30 cm to the 150 cm depths were not significantly different at the 0.05 level of significance (t-test, p = 0.45 for cold periods and p = 0.31 for warm periods). This is an indication of high percolation of the stream water into the streambed during high flow, making the reach a losing stream during high flows.