An Important Water Catchment Area
Baviaanskloof, Kouga and Kromme Rivers
These rivers all flow roughly west to east along parallel valleys created by uplift and faulting of the Table Mountain Group rock followed by erosion and floodplain formation. The catchment areas feeding these rivers are 1,234 km2, 2,820 km2, and 1,560 km2 respectively. All three rivers have perennial surface flows in their lower reaches. However the Baviaanskloof River has substantial river reaches along the floodplain that are seasonally dry. The Baviaanskloof River intersects the Kouga River near the head of the Kouga Dam while the Kromme River feeds the Churchill and Impofu Dams. These dams provide over 70% of the water supply to the Nelson Mandela Metropolitan Area in addition to supplying irrigation water to major agricultural areas. It is estimated that the Kouga provides 125-135 million m3/year of water, the Baviaanskloof 35-45 million m3/year (Jansen 2008), and the Kromme 90 million m3/year (WR2005), however the interannual variability in this supply is very high. Due to the steep mountainous nature of much of the region, the infrequent, but periodic intense rain events result in high velocity runoff and stormflow in the rivers. Occasional floods are natural phenomena for these catchments, but often result in farm losses, infrastructural damage, and safety risks.
The rivers of the area support valley bottom, floodplain, and groundwater seepage wetlands which have been subject to varying levels of human impact. The Kromme catchment receives the most rain and supports the most extensive system of wetlands, including multiple peat forming, Palmiet (Prionium serratum) dominated wetlands along the main stem of the river. The drier Baviaanskloof supports more seasonally inundated riparian and floodplain wetlands (Haigh et al 2004). In the Kouga catchment, many of the wetlands in the wetter, western areas have been drained for commercial fruit farming. Wetland areas in all the catchments have been purposefully drained and/or impacted by channel incision. The Kromme and Kouga catchments also have significant encroachment of invasive woody species on what had been herbaceous wetlands. As a result of excessive water usage, these tree species contribute to the drying of former wetlands. Wetlands can aid in maintaining water quality, regulate streamflow, store carbon to mitigate climate change, and provide habitat to many wetland specific plants and animals.
Alluvial fans are geomorphological features present in all the catchments. Alluvial fans are sediment deposits that form on relatively level areas at the feet of steeper slopes where tributary valleys intersect the flatter main valley floor. These fans can promote groundwater recharge and act as a hydrological buffer between tributaries and the main river because flows coming from the tributary valley spread out and quickly infiltrate into the fan and may not reach the main river as surface flow.
There are two main groundwater systems in the region: the Table Mountain Group (TMG) fractured bedrock aquifer and the conglomerate alluvial aquifer (Jansen 2008). The fractured bedrock aquifer exists as a network of fractures and joints that going hundreds of meters deep into the TMG quartzitic sandstones that dominate the catchment geology. These are filled with water during rainfall on fractured rock faces and infiltration through the shallow soils of the Baviaanskloof and Kouga Mountains. Because the network of water filled cracks is irregular and not continuously connected, not all locations are favourable for extraction from this aquifer, however it does feed surface water springs located both in tributary kloofs that intersect the mountain water table and hillslope locations where shale layers block the downward flow of water in the fractured sandstone (e.g. at Sewefontein).
An alluvial aquifer exists in the deep sediment deposits (sand, gravels, cobbles) of the Baviaanskloof River floodplain, located between the Baviaanskloof and Kouga Mountains. The groundwater in the alluvial aquifer is recharged by infiltrating surface runoff from the mountain slopes, occasional rainfall in the valley, adjacent springs, and potentially subsurface seepage from the TMG aquifer.
Author: Julia Glenday (2012)