The hydrological characteristics of the Berg River

Abstract

This study aimed to provide a comprehensive description of the natural and present-day hydrology of the Berg River, focusing on the hydrological importance of the upper river, i.e., the subcatchment to be dammed by the Berg River Dam, to its lower reaches. Considerable between-year variability was demonstrated in all aspects of flow, including baseflows and the range of different magnitudes of floods. Flood frequencies are high (average of 7 to 8 floods above a disturbance threshold per annum; range 3–15). Flooding is strongly seasonal with very low levels of flooding from November to April, alongside a period characterised by high flood frequencies, from May to September, with one or more floods occurring on average per month. This pattern of seasonal disturbance has a fairly high predictability (52%), based on Colwell's measures. However, within the flood season, flood frequency per month is unpredictable (17%). Colwell's Contingency, or the measure of how repetitive the pattern is, is particularly low. Floods constituting disturbance events in the Berg River show intra-annual variability in their timing and inter-annual variability in the frequency (number per year) and periodicity (number per month). The upper Berg River plays a critical role in contributing to both base flows and small floods in the lower river, contributing on average 72% of the flood peak in the lower reaches, for floods <20 m3/s and contributing disproportionally to all intra-annual flood events in the lower reaches. Base flow level has a major influence on the extent of floodplain inundation achieved by floods in the estuary. The disproportionate contribution of this subcatchment is a reflection of both its natural characteristics (high rainfall, water-producing area within the catchment) but also, and importantly, a consequence of the extent of development elsewhere in the catchment that has removed orreduced smaller flood flows. This contribution of the upper river to floods in the lower reaches appears to be substantially greater during drought years. The Berg River Dam is likely to diminish this contribution of the upper catchment to base flows and smaller floods along the full length of the river and will quite probably remove this ‘flood buffer’ provided by the upper river during dry or drought years.