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

Previous 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 2·0), but further downstream in the Fan, the narrower ( 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.