Thermal habitat assessment of alternative flow scenarios in a tailwater fishery

Abstract

Tailwaters below hydropower dams can create desirable coldwater trout fisheries; however, a flow regime ideal for hydropower often presents challenges for management of the fishery. The Smith River tailwater (Henry County, VA) offers a self-sustaining brown trout fishery managed for trophy trout (! 406 mm), yet trophy-sized fish are rare. Slow growth and small size are likely caused by any one or a combination of thermal habitat, limited food resources, and/or physical habitat. To evaluate the potential for thermal habitat improvement, temperature changes resulting from alternative flows were assessed with a one-dimensional hydrodynamic model coupled with a water temperature model. Simulated temperatures from each flow scenario were assessed every 2 river kilometres over a 24 kilometre river section below the dam for occurrence of optimal growth temperatures, as well as compliance with Virginia Department of Environmental Quality hourly temperature change and daily maximum temperature standards. The occurrence of optimal growth temperatures increased up to 11.8% over existing conditions by releasing water in the morning, decreasing the duration of release, and not increasing baseflow. Incidences of hourly temperature changes greater than 2 C were reduced from 4% to 0-1.2% by non-peaking releases, increasing baseflow, morning releases, and decreasing the duration of release. Maximum temperature occurrence (>21 C) decreased from 1.3% to 0-0.1% by releasing flows daily to prevent elevated temperatures on non-generation days, increasing baseflow, increasing duration of release, and releasing in the morning rather than evening. Despite conflicting adjustments to best improve all thermal criteria concurrently, a 7-day/week, morning, one hour release regime was determined to improve all criteria throughout the tailwater compared to existing conditions.

Modelling Tools to Analyze and Assess the Ecological Impact of Hydropower Dams

“…As damming affects the morphological and hydrological conditions of the river, changes in sediment transport [74], water depth, water temperature [50] and stream velocity [32,[75][76][77] are observed. Moreover, in some cases, huge amounts of sediment that are highly contaminated with heavy metals, organic matter and pesticides can be deposited in the reservoir [78,79].…”

“…In some cases, outputs from water quality or hydrodynamic models have been used as the input variables to predict the impact of dams on habitat suitability. For example, Krause et al [77] used models to assess the impact of flow manipulation by hydropower on water temperature. Subsequently, they used the simulated water temperature as an input variable to assess the habitat suitability of fish.…”

“…Classic model validation included a model comparison between predictions and observations from an independent dataset [62]. Alternatively, models were validated by comparing results with field observations from a similar ecosystem [37,77].…”

“…Understanding which factors are important, as well as being able to describe the interaction between factors will help managers to determine the potential management strategies [77]. For instance, flow alteration causes changes in water temperature [76,77], and this change in temperature also affects other variables such as pH and dissolved oxygen.…”

“…For instance, flow alteration causes changes in water temperature [76,77], and this change in temperature also affects other variables such as pH and dissolved oxygen. These relational shifts can influence the survival, growth and reproduction of aquatic species [26].…”