Long-term flood controls on semi-arid river form: evidence from the Sabie and Olifants rivers, eastern South Africa

G Heritage,N Entwistle,S Tooth,D Milan

doi:10.5194/piahs-367-141-2015

G Heritage, N Entwistle + Show 2 more

Open Access

https://doi.org/10.5194/piahs-367-141-2015

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Abstract

Abstract. Rivers in the Kruger National Park, eastern South Africa, are characterised by bedrock-influenced "macrochannels" containing variable alluvial thicknesses and riparian vegetation assemblages. Evidence from the Sabie and Olifants rivers suggests that flows up to moderate floods (<3500 m3 s−1) tend to result in net alluviation, with sediments gradually covering the underlying bedrock. More extreme floods strip alluvium and erode bedrock, effectively exerting the primary control over long-term river morphologic development. On the Olifants River, post-flood aerial LIDAR imagery reveals that the 2012 extreme flood (~14000 m3 s−1) resulted in extensive stripping of stored alluvial sediment, exposing and eroding the underlying weathered bedrock. On the Sabie River, preliminary optically stimulated luminescence ages for remnant alluvium are all less than 1000 years, highlighting typical timescales of sediment storage. Together, these results suggest that while periods of general alluviation occur on these systems, long-term river development results from extreme flood-generated bedrock erosion.

Highlights

Drylands cover ~50% of the Earth’s surface and sustain ~20% of the world’s population
Dryland rivers are characterised by long periods of very low or no flow, interspersed with infrequent short-lived, extreme flood events (Tooth, 2000)
These variable flow regimes combine with erratic sediment supply and diverse riparian vegetation assemblages, and produce channel morphologies and dynamics that differ markedly from humid temperate rivers

Summary

Introduction

Drylands (hyperarid, arid, semi-arid and dry-subhumid regions) cover ~50% of the Earth’s surface and sustain ~20% of the world’s population. The diverse underlying geology results in frequent, abrupt changes in macrochannel slope and associated sediment deposition patterns, creating diverse channel types that include alluvial single thread and braided channels, and bedrock-influenced anastomosed and pool-rapid channels (van Niekerk et al, 1995).

Results

Conclusion