Kate Rowntree

Vegetation controls on channel stability in the Bell River, Eastern Cape, South Africa

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. Copyright

AN ASSESSMENT OF THE POTENTIAL IMPACT OF ALIEN INVASIVE VEGETATION ON THE GEOMORPHOLOGY OF RIVER CHANNELS IN SOUTH AFRICA

Summary Invasion of the riparian zone by alien vegetation is recognised as a serious problem in many areas of South Africa. Vegetation is a dynamic component of river channels. It is an important control variable affecting channel form whereas the flow and sediment regime influences vegetation growth. Wherever alien vegetation invades the riparian zone it can be expected that there will be some impact on the physical structure of the riparian habitat. This paper reviews the effect of riparian vegetation on channel processes and channel form and discusses the implications of the invasion of riparian zones by alien vegetation. Woody species in particular are seen as having a significant potential for inducing channel modification, whilst their removal could lead to significant channel instability and mobilisation of sediment. The need for further research into the impact of alien vegetation on the geomorphology of South African river channels is stressed.

Empowerment for development: taking participatory appraisal further in rural South Africa

This paper examines the potential for Participatory Rural Appraisal techniques to contribute to community development and empowerment in a deprived rural community in the Eastern Cape Province in South Africa. A series of participatory workshops was undertaken in which various new techniques were used to identify people-environment relationships and, in particular, the community perception of the value and problems relating to the river and riparian zone. The workshops led to the community taking positive action to address the problems identified. The study indicates the value and role of participatory research among disempowered communities in rural Africa. This article is hosted by our co-publisher Taylor & Francis.

The development of a geomorphological classification system for the longitudinal zonation of South African rivers

ABSTRACT The recognition of the Reserve by the new South African Water Law poses new challenges for river scientists. The ecological water requirement or environmental flow is recognised by that part of the Reserve known as the ecological Reserve. If the ecological Reserve is to be implemented, it must first be defined and quantified for all river systems subject to water related developments. Standard procedures are being developed through the Resource Directed Measures (RDM) of the Department of Water Affairs and Forestry while monitoring the health of South Africas rivers is taking place through the National River Health Programme (NRHP). Cost effective methods of developing spatial frameworks for both the RDM and NRHP are required. Concepts of longitudinal river zonation were developed by river ecologists in the 1960s and 1970s to assist in the classification of ecologically uniform stretches of rivers. This paper reviews the different zonation concepts and presents a geomorphological classification of South African river zones that is being applied to the RDM and NRHP. The classification is tested on three rivers, the Sabie, Buffalo and Olifants.

Hydroclimatic trends, sediment sources and geomorphic response in the Bell River catchment, eastern Cape Drakensberg, South Africa

ABSTRACT The Bell River in the Eastern Cape Drakensberg of South Africa has shown recent signs of channel instability as evidenced by avulsion, sinuosity reductions, gradient steepening and general channel instability. Analysis of rainfall data did not provide evidence for progressive long term change in rainfall pattern for the catchment. However, annual and seasonal rainfall cycles with variance peaks every 16–19 years were found, from which it can be inferred that flood events following years of below average rainfall may cross the threshold limit for channel stability. Channel instability is often followed by a major flood event after sustained low rainfall periods, a sequence referred to as the Flood and Drought Dominated Regimes by Warner (1987). Evidence presented also indicates an increase in catchment erosion in the past century with attendant sediment production increases to the channel. This may be due to catchment land use and management practices (following settlement by commercial farmers) s...

A reconstruction of historical changes in sediment sources, sediment transfer and sediment yield in a small, semi-arid Karoo catchment, semi-arid South Africa

Land degradation is widespread in South Africa but few long-term data sets are available to help identify the timing of this degradation and its impact on catchment sediment yields. We report a case study reconstructing the erosion history of a small upland catchment in the Sneeuberg mountains of South Africa based on the dating of reservoir sediments using 210Pb and 137Cs. Sediment sources have been identified using mineral magnetic measurements. Additional information has been derived from the analysis of aerial photographs and satellite images and the measurement of badland erosion using erosion pins. Results suggest that sediment yields increased dramatically from < 30 t km-2 yr-1 in the early part of the 20th Century to ∼1600 t km-2 yr-1 in the 1970s, subsequently falling to ∼850 t km-2 yr-1 in the last decade. Contemporary erosion pin measurements suggest that badlands are still extremely active and help maintain current high yields. Analysis of aerial photographs and satellite images indicate that badlands and extensive gullies existed in the catchment before 1945. An analysis of mineral magnetic signatures suggested that badlands made some contribution to the reservoir sediment in the 1930s, but did not significantly increase sediment yields. Photographic evidence suggests that a major badland became connected to the main channel network (and to the reservoir) from the early 1960s onwards. This interpretation was confirmed by significant and sustained changes in the mineral magnetic signatures of the reservoir sediments. The results indicated that a delayed increase in sediment yield, despite extensive badland development much earlier, appears to have been caused by changes in catchment connectivity rather than being a direct response to contemporary changes in land degradation within the catchment. These results add additional complications to our understanding of the relationship between the response to, and recovery from, periods of overgrazing in the 1930s that probably initiated the badland development.

Linking the Doing to the Thinking: using criterion-based assessment in role-playing simulations

Abstract This paper uses a case study to show how we developed a criterionbased assessment instrument in the context of a postgraduate level module in a South African University. The module’s pedagogy is based on three inter-linked active learning strategies. The ‘African Catchment Game’ is a role-playing simulation game and is the focus of this article. The other activities were a portfolio and a computer simulation. The assessment was designed to promote conceptual understanding after each activity and so can be positioned within Kolb’s experiential learning cycle. Evaluations from the learners in 2003 showed that the module had been successful in developing their ability to relate experiences from simulations to the theoretical literature.

CONTROLS ON CHANNEL FORM AND CHANNEL CHANGE IN THE BELL RIVER, EASTERN CAPE, SOUTH AFRICA

ABSTRACT Channel instability has occurred in the Bell River in the form of meander cutoffs, incipient meander cutoffs, and general channel instability. This paper examines the incidence of channel instability in relation to the spatial variability of factors controlling channel form. The locations of channel change were mapped from aerial photographs for a 40 year period. Channel form characteristics were measured over a 17 km stretch of channel. Data collected from this stretch included pattern, width, depth, form ratio, channel gradient, particle-size distribution of bed and bank materials and riparian vegetation. Results indicate that the two primary spatial controls on channel form and pattern are riparian vegetation and bed-material size. Evidence indicates that narrow stable stretches are associated with finer bed-material and relatively high levels of riparian vegetation. Riparian vegetation increases bank stability and reduces channel cross-section, thereby inducing stability at flows less than ba...

Water resources in Botswana with particular reference to the savanna regions

ABSTRACT Production and development in the savanna regions of southern Africa are primarily determined by interactions between the limitations imposed by ecological determinants (such as rainfall and soil quality) and the management strategies of the specific region. Good planning, focussing on both the short and long-term effects of water use, is needed in water management strategies. Botswana is already experiencing so-called ‘water stress’ which is related to a number of factors such as rapidly increasing population leading to a sharp increase in water demand, low and variable rainfall, high rates of evaporation, and the high cost of exploiting existing water resources. At the current rates of abstraction, the lifetime of surface and groundwater resources is limited to decades. Botswana shares four river basins with its neighbouring countries. This results in a situation where 94% of the fresh water resources which Botswana can theoretically access originates outside its borders, making water resource management highly complex. Transnational sharing and management of water resources, therefore, plays a major role in securing sustainability of this precious resource.

Rainfall characteristics, rainfall reliability and the definition of drought : Baringo district, Kenya

ABSTRACT Monthly rainfall data for Kabarnet District Office, together with comments taken from the Baringo District Annual Report are used here to examine the relationship between rainfall characteristics and agricultural drought experienced over the period 1915 to 1943 in order to establish the meteorological criteria for a definition of drought applicable to the former Kerio Native Land Unit, Baringo District, Kenya. The concept of reliable rainfall as a drought threshold is also examined. The analysis shows that the occurrence of drought was related to both the annual rainfall total and to the seasonal distribution. The meteorological criteria for drought and for reliable rainfall were found to differ between the upland cultivators and the lowland pastoralists, the latter experiencing a higher frequency of drought.