A sediment record of floods ca. 1820–1950 for the River Isla, eastern Scotland

Abstract

A 2.4 m long highly stratified sediment stratigraphy was recovered from an abandoned meander in the lowland reach of the River Isla in Perthshire, in eastern Scotland, 200 m from the river, closely connected to it through having no flood embankment. The meander was isolated from the active channel around 1820. Sediments were sampled by Eijelkamp peat gouge and Wink vibro‐corer, described from photographs in great detail in the laboratory, then subsampled at contiguous 1.0 cm intervals for particle size distributions and dated by ²¹⁰ Pb and ¹³⁷ Cs. Flood units were defined visually as mineral sediment bands ≥3 mm thick with sharp boundaries usually capped post‐flood by fine‐grained and often organic‐rich laminae and bands. Changes in flood regimes were defined from particle size distributions and occurred in the late 1820s, the mid‐1830s, the late 1830s, the mid‐1850s, the late 1860s, the late 1880s and the mid‐1900s. Large magnitude floods occurred in the late 1840s and early 1850s and from the 1880s to ca. 1910. Stratification was lost in sediments formed after 1950. The chronology of floods ca. 1820–1950 corresponds in part to that from a new documentary record of flood frequency derived from local and regional newspapers 1820–1960. There are more sediment flood bands ( n  = 146) than reported floods ( n  = 99); newspapers probably underrepresent the numbers of floods. Flood frequency is best calculated from precisely dated documentary data, but the sediment record provides more and better estimates of flood magnitude than newspaper accounts.