MORPHOLOGICAL AND SEDIMENTOLOGICAL CHANGES IN A GRAVEL-BED RIVER FOLLOWING 12 YEARS OF FLOW REGULATION FOR HYDROPOWER

The flow regime of the gravel-bedded river North Tyne has been regulated by the Kielder reservoir for the past 12 years; for the past nine years, regulation has been dominated by hydropower generation. Diurnal stage fluctuations of up to 0.6 m are experienced during periods of peak hydropower flows. The main morphological and sedimentological impacts of this regulation are identified and physical explanations provided for the observed adjustments. The main morphological adjustments are identified as the degradation of riffle spawning grounds, the development of fine sediment berms along channel margins, the aggradation of pools, vegetation of former gravel shoals and the growth of tributary confluence bars. Sedimentological adjustments are subtle and are characterized by higher percentages of fines within spawning gravels, coarsening of surface gravels and the development of a stable, strong bed fabric. The physical explanations for these adjustments relate to changes in the sediment transport regime controlled by the hydraulics associated with the pool-riffle sequence during hydropower generation.