A long-term estimate of bedload and suspended sediment yield derived from reservoir deposits

The draw-down of Pinmacher Reservoir (Ayrshire, west Scotland) in 1988 exposed sedimentary sequences entrapped since impoundment in 1903. The sole surface influent, an ungauged second-order stream, drains a mainly undisturbed, upland catchment. Most of the floor of the small basin is coated with mud. However, below the inflow a normally subaqueous delta of gravelly sand has accumulated. Examination of sections has enabled the construction of an isopachyte map from which the volume of deposits has been evaluated. Taking account of trap efficiency, sediment water content and bulk densities, the total sediment yield from the Pinmacher catchment is 51.1 t km-2 year-1. On the assumption that the deltaic and lacustrine sequences, respectively, represent the bed and suspended fractions of the influent stream load, long-term (85 years) average yields for the two components of 23.3 t km-2 year-1 and 27.8 t km-2 year-1 may be discriminated. Thus, bedload accounts for ca. 46% of the total load. Whilst the quantification of long-term bedload transport remains elusive, this study provides an unusually long record. Moreover, analysis of the delta stratigraphy demonstrates two major and 21 additional flood events during the history of impoundment.