The effects of impoundment on the physicochemistry of two contrasting southern african river systems

The downstream effects of six impoundments on their respective physicochemical conditions over two years are compared for two contrasting for two contrasting rivers. The impoundments studied included two on a cool, clear, clean, acid, and short system with a steep gradient (Palmiet River), and four on a warm, turbid, polluted, alkaline, and relatively long system (Buffalo River). Where possible, results are also compared against predictions of the Serial Discontinuity Concept (SDC) of Ward and Stanford (1983), and the ‘recovery’ (discontinuity) distances are estimated for a number of variables. Major effects centred on changes in median spot temperatures and annual temperature ranges, particularly for dams regulating the upper reaches of both rivers. These upper river impoundments behaved similarly despite different release patterns; and caused only weak chemical effects, most of which recovered rapidly (within 3 km). Middle reach impoundments also behaved similarly despite different release characteristics, but they caused more pronounced changes and recovery distances were longer (up to 30 km). Observed exceptions to the predictions of the SDC included alterations to nutrient regimes of receiving reaches downstream of all dams, annual temperature range was affected downstream of headwater dams on both rivers, and all impoundments on both systems depressed flow fluctuations. Although the specific predictions of the SDC model are not generally borne out, it is regarded as a useful tool for testing the effects of impoundment on rivers, and the concept of ‘recovery distance’ forms a useful practical framework for management of releases.