Field and model assessment of the ecological impacts of redesigning compensation flow releases

Abstract

Rivers around the world have been regulated for a variety of reasons. Historically the flows released from these reservoirs have been set with minimal attention to ecology. The aim of this study was to use both field and modelling techniques to assess the ecological implications for altering compensation flow regimes in upland millstone grit catchments. A simple paired-catchment approach was chosen. The study catchments used were the regulated Rivelin and Loxley (tributaries of the River Don), and the unregulated Hipper. Before the 1st of April 2004， the steady state compensation releases from the Rivelin and Loxley were set at 2,6 Thousand Cubic Metres per Day (TCMD) and 28 TCMD respectively. After the Ր՛ of April 2004 the compensation flows were altered on the Rivelin and Loxley to 8.6 TCMD and 22 TCMD. This study used three broad scales of enquiry: macroinvertebrates; fisheries and modelling. Detailed macroinvertebrate surveys were conducted over a four year period (2002-2005), with samples taken in the spring, summer and autumn of each of those years. Surber samples were taken, along with associated environmental variables. Data analysis was conducted using both univariate and multivariate techniques. The invertebrate results showed that the role of flow variability is key within both of the regulated systems. The influence of flow variability is different on each of the study rivers, and the influence changes due to the alteration in compensation flows. Fisheries surveys were also conducted over this four year study period (2002-2005). Nine sites were surveyed yearly on the Rivelin and Loxley; and three on the Hipper. The fish populations in each of the rivers are dominated by brown trout (Salmo trutta). The importance of the interaction of reach scale morphology and discharge was evident in the fisheries populations with the Rivelin having similar observed densities of brown trout as the Loxley. Altering the compensation flow regimes appears to lead to an increase in the growth rate of older trout within the Rivelin.A two-dimensional hydraulic model was used to simulate the distributions of depths and velocities on four study reaches (two on the Rivelin; two on the Loxley). The model was calibrated to three separate calibration data sets. Subsequently, steady state simulations were conduced for the pre and post change compensation flows in each of the reaches. This output was subsequently linked to a fuzzy-logic based habitat model in order to generate predictions of available habitat. Predictions of available habitat were generated for four life stages of brown trout and for functional feeding groups of macroinvertebrates.