HABITAT QUALITY FOLLOWING A CHANNEL RECONFIGURATION PROJECT: STEWART RIVER, MINNESOTA, USA

Along the North Shore of Lake Superior in Minnesota, discharge and water temperature are primarily moderated by precipitation and air temperatures, as streams lack abundant groundwater inputs. Coldwater species in these streams are threatened by low summer discharges and associated high water temperatures. In 2015, a major channel reconfiguration project near Two Harbors, Minnesota, regraded the channel and floodplain, decreased width-to-depth ratios within the channel, created in-channel habitat, and constructed seven off-channel floodplain ponds. The ponds were constructed in part to help increase baseflow and decrease stream temperatures. To evaluate the influence of these off-channel ponds on baseflows and temperature, we installed water level and temperature sensors in ponds, shallow wells, and the channel. To assess habitat suitability, we also measured dissolved oxygen and surveyed thermal refuge locations between 2016 and 2018.

In general, ponds did not provide suitable low flow habitat for fish. Half of the ponds were warmer than the channel and six out of seven ponds exhibited low dissolved oxygen levels (≤ 7 mg/L). Habitat quality in ponds as measured by temperature and dissolved oxygen is related to connectivity to the channel, shading, and width-to-depth ratios within the ponds. Ponds with little shading became significantly warmer than shaded ponds or the channel (R² = 0.6, p-value < 0.0001). Pond width-to-depth ratios are positively associated with dissolved oxygen levels (R² = 0.7, p-value = 0.02). Summer precipitation events that generate high flows recharge the ponds. Pond water elevations recede slower from storm peaks than the channel, but not slow enough to provide significant contributions to baseflows over weeks to months. This study illustrates that care should be taken when constructing off-channel ponds in terms of geometry and cover, so that they provide positive habitat enhancements to the channel system.