Controls on the transition from meandering to straight channels in the wetlands of the Okavango Delta, Botswana

Abstract

AbstractPrevious studies of alluvial rivers have shown that channel patterns form a continuum controlled by interactions among factors such as gradient, discharge, sediment size, and bank strength. Data from channels in the permanent wetlands of the Okavango Delta add to these findings by focusing on pattern transitions in channels with banks formed by sedges and grasses that are rooted in peat and underlain by unconsolidated sediment. Channels are well defined, and transport fine–medium sand as bedload between the vertical, vegetation‐lined banks. Water depths, velocities, grain sizes, and bankline vegetation do not vary significantly or systematically downstream, but the permeable banks allow water to leak from the channels, contributing to an overall downstream decrease in discharge and width. In addition, as the Okavango River flows from the <12 km wide ‘Panhandle’ and splits into distributaries in the broader ‘Fan’, valley gradient steepens by c. 60 per cent.These downstream changes result in channel pattern adjustments. In the Panhandle, the Okavango River is a relatively wide (c. 30–100 m), actively meandering, sinuous channel (P > 2·0), but further downstream in the Fan, the narrower (<40 m) distributaries follow laterally stable, less sinuous (‘straight’) courses (P < 1·75). Some channel pattern discrimination diagrams based on simple indices of gradient, discharge, sediment size or stream power are inadequate for analysing the meandering–straight transition in the Okavango but Parker's (1976) approach, based on ratios of depth–width and slope–Froude number, accurately characterizes the transition. Our field observations, combined with the results from previous experimental studies, suggest that in relatively wide channels (w/d > 10), thalweg meandering results in scour of the unconsolidated sediment at the bank base, leading to undermining and collapse of the vegetation, and to slow meander migration. However, as channels narrow downstream (w/d < 10) with discharge losses, proportionally increasing sidewall drag exerted by bankline vegetation suppresses thalweg meandering and bank scour, and channels follow stable, less sinuous courses. Copyright © 2004 John Wiley & Sons, Ltd.