Modifying dam operations to restore rivers: Ecological responses to Tennessee river dam mitigation

The Tennessee Valley Authority (TVA) initiated a Reservoir Releases Improvement Program in 1991 to increase minimum flows and improve water quality by modifying its dam operations. We compiled a comprehensive data set from ecological monitoring below nine dams to evaluate the effects of these modifications on physicochemical conditions and benthic macroinvertebrate assemblages. Abiotic and biotic data were collected in tailwaters by the TVA for three dam operation "treatments" (i.e., before any modifications, following flow modifications, and following both flow and dissolved oxygen [DO] modifications) at three different stations (Upper, Middle, and Lower) located at increasing longitudinal distances below each dam. Analysis of variance was used to test for differences in ecological conditions among treatments and stations. Dam modifications had significant effects on both abiotic and biotic variables, and macroinvertebrate assemblages exhibited significant longitudinal differences. Yearly mean DO and mean minimum velocity increased following dam modifications. Across all sampling stations, macroinvertebrate family richness increased and the percentage of pollution-tolerant macro invertebrates (% Tolerant) decreased after dam modifications. Family richness also increased, and % Tolerant decreased, with increasing distance below the dams. Total abundance of macro invertebrates increased after flow modifications and then decreased following changes in DO. The percentage of individuals belonging to the orders Ephemeroptera, Plecoptera, and Trichoptera (% EPT) increased following flow and DO modifications, but only at the Upper station. EPT family richness was unaffected by increased flow alone but increased following increases in both flow and DO. The design of the reoperation "experiment" made it difficult to ascertain the relative contributions of flow and DO changes to the observed biotic responses, but flow alone appeared to have a smaller beneficial effect than the combined effects of flow and DO.