EFFECTS OF RESERVOIR STORAGE ON STREAMFLOW IN WESTERN CAPE, SOUTH AFRICA

Construction of dams can significantly alter river flow, thus affecting downstream aquatic and riparian environments. As human populations continue to grow, the demand for water also grows. Cape Town in the Western Cape Province, South Africa relies heavily on reservoirs for water supply. Topography, drainage basin size, and seasonal precipitation all affect the relatively small and seasonal discharges in Western Cape rivers. Therefore, reservoirs are essential to store water during rainy seasons to use during dry months. The objective of this study is to determine how regulation and water use impacts streamflow discharge and riparian inundation frequency downstream of reservoirs. Discharge and precipitation data were acquired from South Africa’s Hydrologic Information System (HIS) and Climate Systems Analysis Group (CSAG). I used a paired watershed approach to examine discharge and annual volume of flow in gauged dammed and undammed rivers with similar basin areas for the period of gauged records. Flow duration analysis (probability of daily discharge) was used to compare the range of discharges for dammed and undammed streams. Sequential air photos were used to examine wetted river width in gauged rivers and changes in wetted width with discharge. Width and discharge data were used to define hydraulic geometry relationships (Width = a Q^b). These hydraulic geometry relationships along with the flow duration analysis were used to determine wetted width probability distributions for selected dammed and undammed rivers. Trend analysis of precipitation, unit discharge (m³/s per km²), and Mean Annual Runoff (MAR) were conducted on the time series data. Results indicate no significant trend in annual precipitation over the past 30 years, but significant declines in streamflow in dammed river systems. Dammed rivers had lower annual runoff compared to undammed rivers of similar sizes. River width inundation data indicate a significant reduction in riparian flooding. This would result in significant decreases in riparian zone groundwater recharge. Water availability is becoming increasingly unpredictable with climate change and population growth, thus it is essential to understand how human infrastructure and the consequences of water retention and use affect downstream water availability and river habitats.