Abstract. The Incomati is a semi-arid trans-boundary river basin in southern Africa, with a high variability of streamflow and competing water demands from irrigated agriculture, energy, forestry and industries. These sectors compete with environmental flows and basic human water needs, resulting in a "stressed" water resources system. The impacts of these demands, relative to the natural flow regime, appear significant. However, despite being a relatively well-gauged basin in South Africa, the natural flow regime and its spatial and temporal variability are poorly understood and remain poorly described, resulting in a limited knowledge base for water resources planning and management decisions. Thus, there is an opportunity to improve water management, if it can be underpinned by a better scientific understanding of the drivers of streamflow availability and variability in the catchment. In this study, long-term rainfall and streamflow records were analysed. Statistical analysis, using annual anomalies, was conducted on 20 rainfall stations, for the period of 1950 to 2011. The Spearman Test was used to identify any trends in the records at annual and monthly time scales. The variability of rainfall across the basin was confirmed to be high, both intra- and inter-annually. The statistical analysis of rainfall data revealed no significant trend of increase or decrease for the studied period. Observed flow data from 33 gauges was screened and analyzed, using the Indicators of Hydrologic Alteration (IHA) approach. Long-term analyses were conducted to identify temporal/spatial variability and trends in streamflow records. Temporal variability was high, with the coefficient of variation of annual flows in the range of 1 to 3.6. Significant declining trends in October flows, and low flows indicators were also identified at most gauging stations of the Komati and Crocodile sub-catchments, however no trends were evident on the other parameters, including high flows. The trends were mapped, using GIS and were compared to historical and current land use. These results suggest that land use and flow regulation are larger drivers of temporal changes in the streamflow than climatic forces. Indeed, over the past 40 years, the areas under commercial forestry and irrigated agriculture have increased over four times.