Author(s): Andrew W. Tranmer, Peter Goodwin, Matthew G. Tiedemann, Daniele Tonina, Robert Ettema
Linked Author(s): Andy Trammer
Keywords: Ice dynamics, stream temperature, dam operations, managed rivers, winter streamflow.
Abstract: In the regulated rivers of the Pacific Northwest, winter flow standards are frequently established, that set a constant minimum instream flow. The magnitude of discharge, consistent with observed winter flows for the pre-dam condition, is often based on very short-term records or sporadic point measurements. Detailed winter monitoring conducted in a remote and largely undisturbed watershed in Idaho has identified several unanticipated processes in both the regulated river and unregulated tributaries that have potential impacts on the winter survival of fish and offers new insights on potential management strategies. Areas of marginal ice formation exist at mid-latitude and mid-elevation ranges where destabilized ice cover can open leads in the river for substantial distances. Loss of ice cover can result in detrimental effects such as increased predation or formation of frazil ice that is harmful to many aquatic species. In unimpaired tributary streams, significant short-term variabilities were detected by stream gages and confirmed by observation with remote cameras. These abrupt flood waves are created when ice-jams fail or the stream is blocked by avalanches resulting in temporary loss of ice cover. The frequency of these extreme events increased in areas that had recently been impacted by wildfire. This previously undetected phenomena in the region demonstrated that the stream ecology must be resilient to these extreme fluctuations and constant winter baseflow may not represent pre-dam conditions. In regulated reaches, the reservoir release is typically warmer than the pre-dam condition and has the potential to open leads in the ice cover. The removal of the insulating layer of snow and ice cover leaves the stream vulnerable to the formation of frazil ice under the conditions of extreme diel cycles in ambient temperature. The loss of ice cover and risk of frazil or anchor ice are evaluated under a range of managed winter discharges
Year: 2017