Effects of damming on the ecological condition of urban wastewater polluted rivers

Increasing damming of rivers is a global challenge and many studies have highlighted the ecohydrological effects particularly on migratory fauna. However, there are fewer studies that have explored the effects of the reservoirs on the ecological condition of urban wastewater polluted rivers. Therefore, damming of polluted rivers that drain urban wastewater from the City of Bulawayo, Zimbabwe represents an opportunity to understand the effects of interactions between damming and ecological condition of the affected rivers. Data on environmental variables and macroinvertebrates were collected from twelve sites selected from three rivers: Umzingwane - a river that does not receive wastewater from the city; and Umguza and Khami - both rivers draining urban wastewater from the city. One site each on Khami and Umguza rivers were located before the city whereas the rest of the sites were either located immediately after the city but upstream or downstream of the dams. Sampled sites were assessed for similarity based on macroinvertebrate data and three clusters comprising of (i) less impacted sites, (ii) sites immediately upstream and (iii) those immediately downstream of dams were produced. The assessed environmental variables (ANOVA, p < 0.05) and macroinvertebrate community structure (ANOSIM, p < 0.05) indicated significantly deteriorated conditions at sites located immediately upstream of the dams and these variables significantly improved soon after the dams. Sites located immediately upstream of the dams were typified by poor water clarity, high nutrients, high sedimentation, dissolved oxygen deficiency, and high levels of COD and BOD plus significantly degraded macroinvertebrate communities. Except for salinity that increased downstream of dams, the present results indicate that dams on urban wastewater polluted rivers improves the ecological condition of the affected rivers. In general, this study suggest that pollutants may be retained within the dams and therefore the utilisation of water from these reservoirs for human consumption, crop and livestock production may be risky as pollutants are likely to be retained within these dams.