The effect of regulation by two impoundments on an acid, blackwater, Cape mountain stream

Doctoral Thesis


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University of Cape Town

The world-wide proliferation of dams in the last few decades has necessitated a fuller understanding of the effects of stream regulation but has also provided a setting for testing and developing basic theories of stream ecology. Two important concepts in stream ecology, the River Continuum Concept (RCC) and the Serial Discontinuity Concept (SOC), required investigation as to their applicability to entire river systems in the Southern Hemisphere. The RCC hypothesises that biological communities are structured in a predictable fashion from the source to the mouth of a river in relation to physical environmental gradients along the stream continuum. The SOC proposes that a major impoundment at any position on a river system will cause a longitudinal shift in a given parameter (physical or biological) which will cause a discontinuity in the stream continuum. A certain distance is then required for the stream to return to its normal position in the continuum, and this is termed a 'discontinuity distance'. The effect of regulation by multiple impoundment on the 'discontinuity' (recovery) 'distances' for various physical, chemical and biological attributes of the Palmiet River (a short, steep gradient, cool, acid, low nutrient system) was investigated in a two-year study. Two zones of the river were identified for study: one below an upper-reach impoundment, Nuweberg, and one below a middle-reach impoundment, Arieskraal, as they provided sufficiently long stretches of unperturbed river (7 km and 37 km) over which recovery distances could be investigated. Sampling was undertaken monthly for 27 months from February 1986 to April 1988. Eleven sites were chosen along the length of the river: one in the pristine upper reaches, one site above and 3 sites below Nuweberg, a site on the river as it passes through the village of Grabouw, one site above and three sites below Arieskraal and a site on a tributary, the Klein Palmiet, from which water is abstracted into Arieskraal. The values of the variables downstream of the impoundment are said to increase or decrease with respect to the "above-impoundment" value. Median temperatures, pH and alkalinity increased below Nuweberg, but decreased below Arieskraal, whereas the converse was true for soluble reactive phosphate. Seasonal flow regime reversal occurred below Nuweberg and seasonal flow constancy below Arieskraal; there were seasonal shifts in temperature and a dampening of the annual temperature range below both impoundments; and seasonal shifts in alkalinity below Arieskraal. At least partial recovery was noted in all cases, but in some instances, recovery was incomplete to the estuary.