Last postglacial sea-level fluctuations control fluvial morphodynamics along the West Coast of Sumatra, Indonesia

Abstract

The west Coast of central Sumatra is situated above the Sumatra subduction zone and is characterized by short (< 100 km) and very steep (> 20°) bed rock channels draining the volcanic arc towards the Indian Ocean. Incision and fluvial adaptation of these catchments are commonly attributed to tectonic uplift along the subduction zone and emplacement of magma bodies within the volcanic arc. Recent recognition of a regional sea-level high stand between 4,500 to 5,000 years ago and the low shelf depth of ~70 m below sea level, may indicate that base-level fluctuations had a stronger control on river network dynamics than previously thought. Here, we explore the impact of these processes on the fluvial morphology and the incision history of 31 catchments draining the volcanic arc of west-central Sumatra. Landscape evolution simulations using PyBadlands, geomorphic metrics of the normalized-steepness index, knickpoint detection and c-analysis derived from 30 m SRTM-satellite data demonstrate that the morphometric response of smaller catchments (drainage area: < 500 km2) is different from that of larger catchments (drainage area: > 500 km2). The reduction in overall drainage area due to the last postglacial sea-level rise forced smaller catchments to oversteepen to adjust to the new conditions. Instead, larger catchments responded by upstream drainage area expansion and capture of previously eastward flowing rivers, thus maintaining an overall lower gradient. Furthermore, the mid-Holocene high-stand and subsequent sea-level drop resulted in a major regional base-level fall and creation of a knickzone along the entire West Coast of Sumatra that is currently migrating up the fluvial network and is located at an elevation of around 200 m. Our results imply that (1) the tempo of fluvial incision between catchments along the West Coast of Sumatra may be out of phase with the uplift of the volcanic arc; (2) the drainage area reduction due to postglacial sea-level rise controlled fluvial catchment evolution; and (3) we observe a catchment size depended threshold at which catchments either oversteepen or incise headward to adjust for drainage area loss. This response should be applicable to all natural occurring fluvial bedrock channels that experience drainage area loss and should be modulated by runoff and erodibility. The process is exceptionally well visible in Sumatra as there exists no orographic rainfall gradient along the mountain front. Keywords: Postglacial sea-level rise, fluvial geomorphology, landscape evolution, erosion, subduction zone, SE Asia