Connecting the disconnected: longitudinal correlation of river terrace remnants

Terrace sequences provide insights into flood plain development. Many studies have examined the cross-sectional morphology and correlation of terraces, but this is only part of the story. Longitudinal correlation can provide a far greater insight into flood plain development processes and the spatial significance of these processes. Here we examine flood plain development from the longitudinal correlation of river terrace sequences in a sand-dominated flood plain. The terrace remnants are discontinuous, having been separated longitudinally by the partial erosion of the flood plain. Terraces along the study reach ranged from recently abandoned (490 ± 60 yBP),poorly developed, vertically accreted flood plains to weathered, early Holocene deposits (10 050 ± 260 yBP). They occurred as inset, fill features which indicated successive phases of alluvial erosion and deposition as well as burial of previous terrace surfaces. Terrace morphology alone did not provide sufficient evidence of longitudinal correlation. Sedimentology and chronology were also vital in correlating these remnants. Four terraces (the Baramul Sequence) were identified in the reach showing - progressively younger characteristics and ages. The longitudinal correlation of each discontinuous terrace remnant shows considerable variation over time in response to gradient changes, discharge, sediment size and stream sinuosity. Results show that climate and the local exceedence of geomorphic thresholds have influenced river terrace formation and highlight the significance of chronology in establishing longitudinal correlation of terrace remnants withvarying morphology. The Baramul Sequence shows a progressive relative fall in bed-level and reduction in slope over the Holocene. This is likely to have been accompanied by progressive contraction of the channel and indicates afeduction in mean discharge over the last 6-7 ka