Influence of the manner of water discharge from dam reservoirs on downstream water quality

The study was carried out in the area of three dam reservoirs: Blizne and Maziarnia (Voivodeship of Podkarpackie) and Nielisz (Voivodeship of Lublin). The main parameter differentiating the reservoirs was the water retention time and the manner of water discharge from the reservoirs. Three test sites were designated in the area of each reservoir: in the river zone of the reservoir, in the central part of the reservoir, and near the reservoir dam. At these sites, the concentrations of suspended sediment in the water and the content of organic matter in it, the concentrations of total phosphorus and total nitrogen, as well as chlorophyll a were monitored. In addition, two control sites were established: on the river upstream of the reservoir and on the river downstream of the dam, respectively. At these points, the concentrations of suspended sediments in the water and their organic matter content were recorded. The obtained results of the study and multivariate analysis of the data showed that morphometric parameters (including water retention time) of reservoirs and the method of water discharge influence water quality in downstream rivers. It was found that by using lower discharge and ensuring a sufficiently long retention time of water in the reservoir, it is possible to effectively limit the negative aspects of hydrotechnical structures’ impact on the natural environment. In practice, the observed relationships may constitute an important and missing link in the aspect of minimising undesirable side effects of this type of hydrotechnical objects.