The impact of heavy grazing on an ephemeral river system in the succulent karoo, South Africa

The impact of long-term high grazing pressure on geomorphological features, plant community composition and cover and soil characteristics of an ephemeral river system in Namaqualand, South Africa, was compared to that of lighter grazing pressure across fencelines which crossed the river system. Under heavy grazing the river system was less braided and a higher proportion of the river width was unvegetated runoff channels. The river system was generally dominated by very sandy soil. However, more silt and organic matter, and less sand were found in soil under plants growing in the river bed. Since vegetation cover was much higher under light grazing, river soil was more fertile when grazing was light. A plant species compositional shift when grazing pressure was high resulted in riparian vegetation which more closely resembled the surrounding rangelands. Graminoid growth forms were encountered more frequently in the lightly grazed river. A rest from grazing of 33 months resulted in increased plant cover in another section of the river system. Heavy grazing alters the physical and soil features of this river system by reducing plant cover. Riparian vegetation, by slowing flow rates and catching light soil particles increases landscape heterogeneity and creates productive sites in the landscape.