CHAPTER 2 the Sabie River Catchment and Study Area
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Birds as bio-indicators of the ecological integrity of the Sabie River, Mpumalanga CHAPTER 2 The Sabie River catchment and study area 2.1 GENERAL DESCRIPTION OF THE SABIE RIVER CATCHMENT 2.1.1 TOPOGRAPHY A catchment can be defined as the land area from which a river or reservoir is fed (Davies & Day, 1998). The Sabie River catchment falls within the Incomati River basin, which is an international drainage basin occupied by South Africa, Swaziland and Mozambique (Figure 2.1). The area of the Sabie River catchment is 7096 km2, of which 6347 km2 falls within South Africa. The Sabie River is the main stream of the catchment, with the Sand and Marite Rivers acting as major tributaries, and the Mac Mac River being a tertiary drainage. The Sabie River rises in the west of the catchment in the Drakensberg mountains north of the Long Tom pass at an altitude of about 2200 m. The Sand River rises about 50 km further to the northeast in the Blyderivierspoort Nature Reserve, at an altitude of 1500 m a.m.s.l., and has a total length of about 125 km to its confluence with the Sabie River, whilst the Marite River has a total length of about 58 km to its confluence with the Sabie River. The Sabie River has a total length of about 230 km to its confluence with the Incomati River in Mozambique, at an altitude of about 40m a.m.s.l. (Deacon, 1996). The topographical features of the Sabie River catchment have been used to divide it into two distinct topographical regions, namely, the Middleveld and Lowveld regions. The Middleveld region, which occupies the western portion of the catchment, is generally characterized by an undulating topography that is more mountainous in the west. The Lowveld region is characterized by a flat to Chapter 2: Sabie River catchment and study area 1 Birds as bio-indicators of the ecological integrity of the Sabie River, Mpumalanga Figure 2.1: Map of Sabie River Catchment and Tributaries Chapter 2: Sabie River catchment and study area 2 Birds as bio-indicators of the ecological integrity of the Sabie River, Mpumalanga gently undulating topography, except in the east in the vicinity of the Lebombo mountains. There are no large flood plains, wetlands or swamps in the Sabie River catchment (Deacon, 1996). 2.1.2 GEOLOGY AND GEOMORPHOLOGY Increased demands on limited water resources and changes in land usage lead to modifications in the flow regime and consequently, the morphology of river systems. This leads to changes in available habitat for different faunal and floral species dependent on the river. The organization, structure and development of stream communities are largely determined by the organization, structure and dynamics of the physical stream habitat, together with the pool of species available for colonization. It therefore becomes necessary to understand these physical patterns across space and time and relate them to biotic patterns in order to establish the ecological effects of changes to the physical system (van Niekerk & Heritage, 1993). Using classification systems based on differences measured from aerial photographs, topographical maps, geological maps, satellite imagery and extensive validation in the field, geomorphic patterns may be causally linked to physical factors regulating instream characteristics, and therefore the biota. Recently developed stream classification systems have had a hierarchical perspective which links large regional scales (catchments) with small microhabitat scales. This approach provides a useful framework for studying components of a river system over the wide range of scales which are important to ecologists (van Niekerk & Heritage, 1993). A suitable stream classification system was successfully developed by Frissel et al. in 1986, using a spatially nested hierarchy in which a system at one level for the environment of its subsystems at lower levels (van Niekerk & Heritage, 1993). The basic building blocks are the morphological units, each of which has Chapter 2: Sabie River catchment and study area 3 Birds as bio-indicators of the ecological integrity of the Sabie River, Mpumalanga distinctive substrate characteristics or microhabitats. The morphological units develop in different combinations within a variety of channel types. Channel types that have a functional relationship to each other are grouped into reaches. A segment (or macro-reach) may comprise one or more reaches and have distinctive geological, hydrological, sedimentological, morphological and vegetational characteristics. The segments are associated with differing patterns of sedimentation. A zone has boundaries defined by major breaks in slope, which are usually associated with major changes in geology. The river incorporates the riparian margin from source to mouth, incorporating all of the zones. The catchment, consisting of all tributaries and sub-catchments is the collection of areas having distinct patterns of climate, geology, soils, vegetation and land-use which contribute unique runoff and/or sediment discharges into the tributaries and main river (van Niekerk & Heritage, 1993). The Sabie River is underlain by a wide variety of bedrock lithologies, comprising sedimentary, intrusive and extrusive igneous and metamorphic rocks. Lithological differences in the geology control the longitudinal slope profile of the Sabie River and consequently also the geomorphological form. The Mpumalanga region has been subject to uplift in the recent geological past (10Ka to 100Ka), resulting in the incision of the Sabie River into bedrock. This has created a channel that has a 'floodplain' restricted by the width of incision into bedrock. This incised feature has been termed the macro-channel, as opposed to the smaller, active, perennially flowing channels and seasonally flooded features within its confines (van Niekerk & Heritage, 1993). The Sabie is a physically diverse river system that displays marked changes in channel type as the distribution of sediment over bedrock varies (Heritage et al., 2001). Sediment deposition upstream of bedrock outcrops has led to some extensive alluvial sections developing in the river, whilst elsewhere the river channel is formed predominantly in bedrock. Sediment movement into the river system is episodic, associated with summer rainfall events, and typically Chapter 2: Sabie River catchment and study area 4 Birds as bio-indicators of the ecological integrity of the Sabie River, Mpumalanga accumulates in low gradient sections. The sediment in the macro channel is supplied mainly from sand bed tributaries and is reworked within the channel during medium and high flow periods. Under favourable (low to medium flow) conditions, sediment deposits become colonized by vegetation (e.g. the reed Phragmites mauritianus) and become stable, relatively permanent features (Rountree, 1997). The mineral and hence mining potential of the Sabie River is of a limited nature, occurring mainly in the east and in the west of the catchment. Present mining and industrial activity is limited in extent and size (Chunnet et al., 1990). 2.1.3 RIPARIAN VEGETATION A riparian zone encompasses the stream channel and that portion of the terrestrial vegetation where vegetation may be influenced by elevated water tables during floods, and occupies one of the most dynamic areas of the landscape. As such, they are diverse and complex biophysical habitats, subject to frequent, high magnitude disturbances. However, riparian systems have been noted for their resilience, i.e. their ability to recover quickly from disturbance. Rapid growth rates, high fecundity, and capacity for asexual reproduction are among the factors that allow for rapid recovery of riparian plants after disturbance (Rountree, 1997). The riparian zone plays many essential roles in the functioning of the river ecosystem, such as flow regulation, water quality regulation, habitat provision for many plants and animals, and lastly, serving as a corridor between different habitat types (WRC, 2001). Riparian zones and their associated biota are influenced and altered by two main factors, namely floods and the introduction of alien vegetation. Floods are the primary source of disturbance and are considered to be the driving force responsible for the existence, persistence, productivity and interactions of the Chapter 2: Sabie River catchment and study area 5 Birds as bio-indicators of the ecological integrity of the Sabie River, Mpumalanga major biota in rivers. Associated with these high levels of disturbance are very high levels of plant biodiversity. When these disturbances are suppressed or removed, substantial vegetational changes result. However, frequent flooding discourages the establishment of terrestrial vegetation, both by surface erosion and physiological effects of inundation (Rountree, 1997). Riparian zones are particularly vulnerable to invasion by alien vegetation (because they are good dispersal routes for seeds) and this is becoming a huge ecological problem in South Africa. Alien vegetation is a problem because it takes up more water than indigenous vegetation, it also takes up habitat for indigenous vegetation. Alien vegetation changes the aesthetic characteristics of the riparian zone and damages buffering capabilities (WRC, 2001). A national monitoring programme, the River Health Programme (RHP) has been designed for South Africa, and focuses on measuring and assessing the ecological state of riverine ecosystems. This programme included the need to develop and apply a rapid index to assess the condition of riparian vegetation.