Transported material in a small river with multiple impoundments

SUMMARY.

• 1 The transport of particulate material in a multiply-impounded southern African river was studied from source to mouth over a period of 2 years during base flow conditions.
• 2 More than 95% of the dissolved and particulate material in transport was ultra fine (<80 μm). We conclude that too much emphasis has been placed on coarse material (>1 mm) in theories of river ecosystem functioning.
• 3 The ratio of CPOM:FPOM (1000–4000:250–1000 μm) in transport decreased with increasing stream order as a result of inputs of FPOM from pollution sources and from plankton blooms discharged from reservoirs.
• 4 The downstream effects of impoundment depended largely on the type of release and the quality of the inflowing water: surface-releasing reservoirs in the clean, upper reaches of the river had the least effect on transported material. A polluted surface-release impoundment in the mid-reaches of the river converted the particle size spectrum of the river from small (<80 μm) to large (250–1000 μm). Bottom-released water carried high concentrations (36–190 g m³) of small (<5 μm) and largely inorganic (86–93%) material.
• 5 The distance required for zooplankton densities to reduce by 95% was between 4 and 8 km downstream of an impoundment, except below a polluted impoundment where zooplankton densities took 32–35 km to reduce by 95%.
• 6 Most of the downstream changes in transported organic matter in the Buffalo River are due to inflows of agricultural and urban effluent. These disturbances to the river cause greater perturbations than do the impoundments.