Avulsion mechanisms on the Okavango fan, Botswana: the control of a fluvial system by vegetation

Abstract

A study of the avulsion of a major distributory channel on the alluvial fan (22 000 km2 in area) of the Okavango River in northern Botswana has revealed that channels serve as arterial systems distributing water which sustains large areas of permanent swamp. The channels are vegetatively confined. A primary channel, defined here as a channel which receives water and sediment directly from the fan apex, aggrades vertically as a result of bedload deposition. The rate of aggradation increases downchannel and may exceed 5 cm yr−1 in the distal reaches. Rapid aggradation is associated with a decline in flow velocity. This initiates a series of feedback mechanisms involving invasion of the channel by aquatic plants which trap floating plant debris, further reducing flow rate and causing the channel water surface to become elevated, thereby increasing rate of water loss from the channel, accelerating blockage and aggradation. The channel ultimately fails. Enhanced water loss from the channel promotes the growth of flanking swamp vegetation, which confines the failing channel. Increased flow through the swamp erodes pre‐existing hippopotamus trails, producing a secondary channel system which overlaps but does not connect directly to the failing reach of the primary channel. The region of failure of the primary channel migrates upstream, accompanied by headward propagation of the secondary channel system. The swamp distal to the failed primary channel dessicates and is destroyed by peat fires. Secondary channels are stable and not prone to blockage. Comparison with avulsions described in other river systems indicates that the influence of plants in the Okavango River system is exceptionally strong.

Model-Based Analysis of Macrophytes Role in the Flow Distribution in the Anastomosing River System

“…Marcinkowski et al [63] recognized that anabranches loss in the anastomosing section of the River Narew is periodically triggered during the summer low flow conditions, when water discharge in the anabranches promotes sediment vertical accretion. Other studies on anastomosing rivers functioning indicate that localized flow blockages caused by vegetation are one of the key factors controlling their formation [20,22]. Therefore, it can be assumed that the most important factors controlling anabranches maintenance and persistence are related to water and sediment supply.…”

“…The impact of vegetation on anastomosing system flow characteristics is to date recognized mainly in theory [20,22,[26][27][28]. According to theoretical understanding, vegetation encroachment causes flow velocity decrease and sediment aggradation, which initiates feedback mechanisms involving further invasion of aquatic plants in the channel and water loss [26][27][28].…”

“…Numerous hypotheses explain avulsion triggering, which is indispensable for the creation and maintenance of anabranches. Among them the most widely applicable are a highly variable flood-prone flow regime [19,23] and localized flow vegetation obstacles [20,22]. The role of vegetation in influencing anastomosing river hydrology and geomorphology is complex, and-as stated by Tooth and Nanson [25]-needs to be assessed more fully, based on the data from a variety of rivers, before any generalizations can be made.…”

“…The role of vegetation in influencing anastomosing river hydrology and geomorphology is complex, and-as stated by Tooth and Nanson [25]-needs to be assessed more fully, based on the data from a variety of rivers, before any generalizations can be made. The most comprehensive studies describing the impact of vegetation on anastomosing system formation concerned the River Okavango (Botswana) [20,22,[26][27][28]. The authors, in their studies, created a conceptual model of the formation of the anastomosing system, which was assumed to be mainly controlled by the growth of bank and in-channel vegetation.…”

The micro‐topography of the wetlands of the Okavango Delta, Botswana

“…Channels leak because they are vegetatively confined (McCarthy et al, 1992), resulting in wide distribution of water in the areas around distributary channels. Raised tracts such as inverted channels or anthills form islands in these swamps, and these accumulate precipitates from the water due to transpiration by trees on the islands (McCarthy et al, 1993a).…”