LANDSCAPE STATE CHANGE IN THE SEMI-ARID SABIE RIVER, KRUGER NATIONAL PARK, IN RESPONSE TO FLOOD AND DROUGHT

• Published in: South African geographical journal Vol. 82; no. 3; pp. 173 - 181
• Main Authors: ROUNTREE, M. W., ROGERS, K. H., HERITAGE, G. L.
• Format: Journal Article
• Language: English
• Published: Abingdon Taylor & Francis Group 01.09.2000; Routledge
• Subjects: Africa; Bgi / Prodig; Drought; Ecosystems; Floods; Geology; Landscape; National parks; Physical geography; Rivers; Soil erosion; South Africa; Southern Africa; South Africa; Impact; National park; Climatic variability; Precipitation; Flood; Water regime; Seasonal variation; Riparian vegetation; Arid area; Semi-arid area; Stream; Drought
• ISSN: 0373-6245; 2151-2418
• DOI: 10.1080/03736245.2000.9713711

Semi arid rivers are subject to highly variable flow regimes as a result of strongly seasonal and unpredictable rainfall. Riparian landscape state changes resulting from a prolonged low flow period followed by a high magnitude flood were examined in the semi arid, mixed bedrock/alluvial Sabie River in the Kruger National Park, South Africa. Aerial photographs from 1986 and 1996 were analysed to document the changes in seven landscape states (sand, rock, water, reeds and riverine shrubs, trees, herbaceous vegetation and terrestrial shrubs) at three scales of a geomorphological heirarchy. Three elevation (the active, seasonal and ephemeral) zones were shown to correspond with high, intermediate and low inundation frequencies respectively. Sediment was scoured from the low elevation active zone and a large increase in the surface area of water was observed. Much of the seasonal zone was eroded down to the height of the active zone, and similarly sediment from features formerly classified in the ephemeral zone was eroded, and the new height of the eroded features classified in the seasonal zone. Thus the area of the active zone increased at the expense of the seasonal zone, and the seasonal zone at the expense of the ephemeral zone. In some cases sediment had been deposited onto the reeds and riverine shrubs of the active and seasonal zones, resulting in an increase in height of the associated geomorphic units. The elevation of the geomorphic units are probably increasing during low flow periods, as sediment is deposited in the absence of floods of sufficient magnitude required to erode the features. Then, during infrequent high magnitude floods, the elevation of the geomorphic units is decreased due to scour. At the channel type scale, flood-induced vegetation change corresponded with the energy slopes of the channel types, since more trees were removed from the steeply sloping, high energy bedrock anastomosing channel type. The observed levels of change in the lower gradient pool/rapid and braided channel types were comparable, as are their energy slopes. Many semi arid rivers have been shown to behave in an episodic disequilibrium fashion, whereby the dominant, prolonged accumulation phase (characterised by sediment accumulation) is ended or 'reset' by infrequent, high magnitude floods which strip the accumulated sediment from the channel. Long term vegetation changes suggest that the rivers in the Kruger National Park behave in this fashion. However flow reductions and increased sediment supply in the catchment may have resulted in the development of increasingly stabilised riparian landscapes, with the result that increasingly large floods will be required to scour the accumulated sediment in order to reset the system.