Dam function rules based on brown trout flow requirements: design of environmental flow regimes in regulated streams

Abstract

The operation of small hydroelectric dams built on mountain streams induce changes in stream flow regimes that are manifested not only in the intensity of flow events, but also in the variability and frequency of high- and low-flow episodes. Former studies have shown the influence of flow variability upon the dynamics of a resident brown trout population, especially that related to the stream flow regime during spawning, incubation and emerging periods. As these life-stages are known to determine the population dynamics in further ages, stream flow variability appears to be a major influence on the regulation of a wild brown trout population. Thus, mean flow discharge should not be the only parameter taken into account when establishing ecological flow regimes to support rehabilitation of degraded trout populations in mountain streams. Ecological stream flow regime characteristics are proposed as a basis for the design of environmental flow regimes in mountain reaches downstream of hydroelectric or water supply dams. Case studies were conducted in a high mountain basin in Central Spain (River Tormes) for a period of 5 years showing that relationship between duration and frequency of high and low flow episodes during egg incubation could be linked to young-of-the-year recruitment and quantified in terms of flow management units. Duration and frequency of flow discharges could be manipulated so as to create favourable hydrological conditions for restoring sustainable populations of brown trout in rivers affected by flow regulation