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Environmental Flow Research in the Tennessee River Basin

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More about … Streamflow influences fish health and abundance in Tennessee

Graph showing fish abundanceInsectivorous fishes, such as Percinidae darters, Cyprinidae minnows, and Noturus madtoms, are among the most jeopardized fish in the Tennessee River Valley. Insectivorous fish represent a middle ground in the trophic structure of a stream, feeding on invertebrates while being prey for predator species. For their eggs to hatch insectivorous fish must lay them in gravel beds that are relatively clear of sediment. Insectivorous fish are sight-feeding and need clear water for feeding.

Constancy is a measure of flow stability—the consistency of streamflow from one day to the next. Constancy reflects average conditions and is most closely associated with base flows. Persistence of streamflow at base-flow levels determines the available wetted perimeter of the channel. Stability of wetted perimeter corresponds to stable and available habitat for invertebrate colonization and subsequent uptake by insectivorous fish. Equally important, stability of base-flow is critical to maintaining water-quality conditions such as dissolved oxygen, temperature, and basic water chemistry. Insectivorous fish scores increased with increasing constancy.

Frequency of moderate flooding is defined as the average number of occurrences per year of floods with magnitudes that are at least three times the median annual flow. It is speculated that the velocity and stream power associated with floods of this magnitude are sufficient to remove silt from the substrate and moderately disturb the bed material. Decreased siltation and increased water clarity from decreased silt have proven to be beneficial to sight-feeding fishes. Average stream velocities for the streamflow three times the median annual are 3.9, 1.6, and 0.65 feet per second for the Oconoluftee River, North Fork Holston River, and Wartrace Creek. Insectivorous fish scores increased with the decreasing frequency of moderate flooding. The negative correlation seen in our study indicates that insectivorous fish respond positively to decreased disturbance. Habitat becomes increasingly unstable with increasing frequency of moderate floods, resulting in the decline of insectivorous fishes and invertebrates. 

Rate of streamflow recession is a measure of how fast or slow streamflow recedes to baseflow following a flood peak. The rate of streamflow recession from runoff events can provide habitat-limiting factors for different segments of the fish community. High recession rates elicit several consequences in the stream, including stranding fish in isolated pools made available during high flows and limiting the amount of time for passage of fish between potential spawning and feeding areas. High streamflow recession rates have been associated with saturated streambank failure, potentially increasing embeddedness through higher sediment loadings and decreasing the clarity of the water. Sediment deposition from bank failure also disrupts and diminishes available spawning and feeding habitat. Diminished habitat results in crowded conditions that result in hybridization between fish species, which is an undesirable occurrence. In this study, insectivorous fish scores increased as recession rates decreased. Additional details regarding the correlation between fish community structure and streamflow characteristics can be found in the journal article titled “Relating streamflow characteristics to specialized insectivores in the Tennessee River Valley: a regional approach,” published in Ecohydrology.

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