No abstract

No abstract

This work has drawn upon previous attempts to define geomorphic provinces, but also on more recent work on the geological and geomorphological evolution of southern African fluvial systems. It has also used Digital Terrain Model (DTM)-derived data and statistical techniques to determine 34 geomorphic provinces and 12 sub-provinces within South Africa, Lesotho and Swaziland. Ninety-nine main stem river longitudinal profiles and valley cross-sectional profiles were generated from the DTM-derived data, and a statistical technique, the Worsley likelihood ratio test (WLRT), was applied to define statistically significant changes in slope and valley cross-sectional width along the river continuum. This isolated 471 macro-reaches for the 99 main stem rivers. Each macro-reach was then analysed using a variety of descriptors including shape, best fit curve, slope, sediment storage potential and valley width. Principal component analysis was applied to the data set to determine whether significant groupings existed, indicating significant similarities in the data by way of area, and conversely, whether distinct differences between groups of data were evident. The scores for the whole data set showed a large grouping around the origin with some scatter along the PC1 axis. Distinct groups were, however, evident for macro-reaches within each province. These reflect the extent of uniformity in the slopes, valley widths, altitudes and shape descriptors of each province. A description of each of the 34 provinces and 12 sub-provinces is presented.

Channel instability has occurred in the Bell River in the form of meander cutoffs, a number of which have occurred since 1952. Increased sediment loading from widespread gully erosion in the catchment has been proposed as the trigger for this instability. Willow species of the Salix family, in particular S. caprea, have been planted along the banks in an effort to prevent further channel shifting. This study reports the results of an investigation into the effect of vegetation on channel form and stability over a 17 km stretch of channel. Results indicate that riparian vegetation has significant effects on channel form which have implications for channel stability. Riparian vegetation increases bank stability and reduces channel cross-sectional area, thereby inducing stability at flows less than bankfull. Evidence indicates that narrow stable stretches are associated with relatively high levels of riparian vegetation. Wider, unstable channels are associated with relatively less riparian vegetation. The effectiveness of riparian vegetation relative to bank sediments was investigated. A dense growth of willows was found to have an equivalent effect to banks with a silt±clay ratio of about 70 per cent. The channel narrowing induced by vegetation may contribute to channel shifting at high flows. The reduced channel capacity is thought to result in more frequent overbank flooding which may ultimately lead to channel avulsion. Thus where increased sediment loading is pushing the channel towards instability, vegetation may be effective in imparting local stability, but it is unable to prevent long-term channel shifts, and may rather help to push the system towards more frequent avulsions.

Channel instability has occurred in the Bell River in the form of meander cutoffs, a number of which have occurred since 1952. Increased sediment loading from widespread gully erosion in the catchment has been proposed as the trigger for this instability. Willow species of the Salix family, in particular S. caprea, have been planted along the banks in an effort to prevent further channel shifting. This study reports the results of an investigation into the effect of vegetation on channel form and stability over a 17 km stretch of channel. Results indicate that riparian vegetation has significant effects on channel form which have implications for channel stability. Riparian vegetation increases bank stability and reduces channel cross-sectional area, thereby inducing stability at flows less than bankfull. Evidence indicates that narrow stable stretches are associated with relatively high levels of riparian vegetation. Wider, unstable channels are associated with relatively less riparian vegetation. The effectiveness of riparian vegetation relative to bank sediments was investigated. A dense growth of willows was found to have an equivalent effect to banks with a silt±clay ratio of about 70 per cent. The channel narrowing induced by vegetation may contribute to channel shifting at high flows. The reduced channel capacity is thought to result in more frequent overbank flooding which may ultimately lead to channel avulsion. Thus where increased sediment loading is pushing the channel towards instability, vegetation may be effective in imparting local stability, but it is unable to prevent long-term channel shifts, and may rather help to push the system towards more frequent avulsions.