Reallocation of compensation releases to restore river flows and improve instream habitat availability in the Upper Derwent catchment, Derbyshire, UK

The Upper Derwent catchment is situated in the Peak District National Park in North Derbysshire, England and includes the Derwent Valley Reservoir Sys tem. The natural inflows to the reservoir system are boosted by flow divers ion schemes from the River Ashop and River Noe, leaving almost dry stretche s in these rivers for long periods of time. Compensation releases are made into Jaggers Clough and the River Derwent. This study examined the possibil ity of altering the operation of the diversion scheme and compensation flow releases, both temporally and spatially to restore flows within these dry reaches. The overall intention was to minimize the ecological impacts of re gulation in the four rivers whilst protecting the yield of this critical pu blic water supply. The study utilized the Physical Habitat Simulation System (PHABSIM) to iden tify and compare feasible operational changes. This technique enables quant itative comparisons of the suitable habitat available under different flow regime scenarios. Brown Trout is the most abundant fish species in the Upper Derwent streams, with Grayling, Brook Lamprey and Bullhead also present. The invertebrate f auna is typical of upland streams with neutral to acid waters. The ecologic al data were assessed to identify suitable target species/life stages for u se with PHABSIM. Brown Trout, Grayling and four invertebrate families (Rhya cophilidae, Leuctridae, Chloroperlidae and Heptageniidae) were selected. Habitat mapping along four stretches of river totalling 10 km was carried o ut in the summer of 1998, followed by PHABSIM fieldwork on 24 transects in the autumn. This information was utilized to examine the tradeoffs in habit at availability between reinstating flows in the dry stretches of river, an d reducing compensation flows elsewhere to minimize the supply impact. Vari ous operating scenarios were examined and two sets of compensation control rules proposed for normal and drought years. Each set included seasonal var iability in the rules. The PHABSIM work described here is the first stage in the process of develo ping a more ecologically acceptable flow regime in the Upper Derwent catchm ent. The decision on the final implementation will be subject to further re source modelling and negotiation between the Environment Agency, the water company and local interested stakeholders. Copyright (C) 2001 John Wiley & Sons, Ltd.