Temporal variability in catchment sediment yield determined from repeated bathymetric surveys: Abbeystead Reservoir, U.K.

Situated an upland area of Northwest England, Abbeystead Reservoir has experienced severe sedimentation problems. Presently, only 6 % of the original capacity and 30 % of the original surface area remains and the deposition process is considered to have reached a quasi-equilibrium condition. Temporal variations in storage capacity losses were determined from morphometric analysis of 7 bathymetric surveys (1851-1991) and indirectly using a time-dependent relationship between storage capacity and water surface area. The sediment yield between successive surveys was obtained by gravimetric conversion of storage capacity losses, corrected for untrapped sediment lost directly over the spillway. The analysis highlights uncertainties in the use of empirical trap efficiency relationships, particularly in very dynamic systems. The long-term (140 years) sediment yield of the Upper Wyre catchment was estimated as 192 t km^-2 yr^-1, containing a mean organic matter content of 18.5 %. Decadal-scale variations in sediment yield ranged from 78 to 390 t km^-2 yr^-1. Two periods with elevated yields were distinguished and linked to phases of construction and land drainage within the catchment.