Aspects of the physico-chemical environment of the Rwenzori rivers, Uganda

A number of permanent rivers flow from the well-watered Rwenzori Mountains, which form part of the wider catchment of the River Nile. A study was carried out on the physical and chemical aspects of three rivers on the Ugandan side and in particular the Mubuku-Bujuku system where access up to 4200 m a.s.l. is possible, to assess changes with altitude. At high altitudes above the foothills gradients were steep with frequent waterfalls and cascades over bedrock. boulders and unstable cobbles. The water was cold, between 4 and 14 degreesC. well oxygenated and dilute with conductivities generally less than 1.00uS/cm and pH values of 6.3-6.5. Below 1750 m a.s.l. streams Eire characterized by lessening gradients, wider dimensions, continuing unstable boulder and stony beds but also sand banks, and on the flats some muddy substrates. There is continued predominance of turbulent flow with rapids and riffles, but with an increasing occurrence of smooth glides and occasional backwaters and quieter margins. The water temperature is warm between 12 and 24 degreesC with generally raised ionic concentrations of about 200uS/cm, but still of a low order of magnitude. It rains with variable intensity in the mountains through the year with some noticeable rains during the dry season. The study suggests that altitude related factors such as the increasing physical harshness including decreasing temperatures from downstream to upstream would be responsible for governing the distribution of biota including fish.