M.T. Greenwood

Channel changes and invertebrate faunas below Nant-Y-Moch dam, River Rheidol, Wales, UK

River impoundment is known to affect the physical, chemical, and biological characteristics of the downstream river, but little is known of the way in which changes of these characteristics interact or the time-scale required to attain a new quasi-equilibrium state. The data presented herein show that channel sedimentation below a tributary confluence, resulting from reduced flow competence and capacity within the regulated mainstream, has produced morphological changes associated with substrate particle-size alteration. Four physical compartments are identified, each representing a different stage of readjustment to the hydrological conditions imposed by the dam. Each stage is shown to have a characteristic fauna. Moreover, the species diversity, composition, and biomass were found to be most different from the natural data during the early stages of readjustment and to approach the natural data more closely as the channel attained a quasi-equilibrium form.

Channel changes and invertebrate faunas below Nant‐Y‐Môch Dam, River Rheidol, Wales, UK: 35 Years On

River regulation initiates a succession of changes within lotic ecosystems driven by the adjustment of channel form to the imposed flow and sediment transport regimes. One typical sequence of channel changes involves channel adjustment to reduced flows. This temporal succession of channel changes is also found as a spatial sequence of channel forms, in different stages of adjustment during the relaxation period. This paper describes channel changes within a short reach of the River Rheidol, Wales, which was regulated in 1961. The ecological implications of channel change are assessed by examination of the benthic macroinvertebrate communities. Survey data from 1996 are compared with data from the early 1980s. A model of ecological change is presented comprising two main stages: (i) an accommodation phase, during which flows are accommodated within the pre-existing channel form and (ii) an adjusting phase, the period of channel change. Thirty-five years after dam closure, most of the River Rheidol is experiencing an accommodation response and changes of the macroinvertebrate community are related to stable flows and local siltation in the pre-regulation channel. Channel change is limited to a 200 m reach below a tributary confluence, i.e. below a sediment source. In terms of the number of taxa, more were found in the regulated river than in unregulated sites and the greatest number occurred in the short reach that was in a late stage of channel change.