Benefits of Two Engineered Water Quality Improvement Options in a Hydropower Tailwater: An Evaluation of Oxygen Enhancements in a Coldwater Fishery

Water quality in impounded waters degrades over time due to thermal stratification, changes in sediment regimes, and accumulation of pollutants; consequently, reservoir releases affect water quality in tailwaters and other connected water bodies. The objective of this study was to evaluate dissolved-oxygen conditions downstream of a hydropower dam after installation of a Supersaturated Dissolved Oxygen System. Low dissolved oxygen concentrations in the Illinois River downstream of Tenkiller Ferry Reservoir are common during summer due to thermal stratification, limited discharge releases, and a dam design that keeps dissolved oxygen low even when water is released.  Rainbow trout was introduced to the Illinois River as the primary mitigation when the reservoir was established.  However, no water storage was allocated for this purpose leading to violation of water-quality standards and fish kills. Management agencies worked together to install a bypass piping system and a Supersaturated Dissolved Oxygen System to improve water quality while minimizing water use in the downstream river. We calibrated a WASP model for 10 km downstream of the dam to evaluate any water-quality improvements resulting from the engineered structures.  Water-quality modeling will be used to further evaluate discharge options to increase instream water quality and support the cold-water fishery.