Reconnaissance sediment budgets for Lynn Valley, British Columbia: Holocene and contemporary time scales

Abstract

Volumes of erosional and depositional landforms were estimated by reconnaissance methods in a steep, forested drainage basin in the Coast Mountains, British Columbia, to examine what insight might be gained into the sediment budget in the absence of direct measurements of sediment transfer processes. Sediment transfers in postglacial and contemporary time were inferred on hillslopes, from hillslopes to stream channels, and within stream channels. Data were collected by aerial photographic analysis and field observation. Postglacial sediment sources are major gullies created by debris slides and flows, failures in glaciolacustrine terraces, and stream channel degradation. Depositional landforms consist of talus slopes and colluvial and alluvial fans. Contemporary sediment sources include debris slides and flows in established gullies and minor processes on hillslopes. Debris slide and debris flow volumes were calculated, and other processes were estimated from regional values. Erosion rate averaged over postglacial time is 276 t·km–2·a–1 (0.15 mm·a–1 surface lowering), with gullies and stream channel degradation contributing 170 and 82 t·km–2·a–1, respectively. A terminal alluvial fan provides an independent check of the results. In contemporary time, erosion rates are calculated to be 350 t·km–2·a–1, with debris flows and slides contributing nearly all of this sediment. The contemporary rate is probably perturbed by recent land use history. Mass-movement processes appear to be the dominant mechanism of sediment transfer and, contributions from Pleistocene valley deposits have declined significantly during Holocene time.