The study presents the characteristics of the Mściwojów Reservoir equipped with a unique pre-reservoir structure that supports the process of self-purification of waters. The authors present more than ten years of studies, focusing mainly on the issues of water quality and the concentration of phosphorus, which is considered as the main factor influencing water eutrophication process. Quite a high concentration of phosphates was noted in the outflow from the main reservoir, in spite of a lower concentration of these compounds in the water leaving the pre-reservoir. Basing on the conducted analyses, the catchment of the reservoir was qualified as group 4, being very prone to the movement and supply of material to the reservoir. The negative value of the retention coefficient of phosphorus obtained for the main part of the reservoir points to the existence of an internal source of phosphorus supply to the reservoir. During over 10 years of studies, new directions of the development of the rural areas were determined. Future works should be extended so as to cover all elements of the ecosystem of the reservoir. In a longer term it seems natural to extend the research works to cover the whole catchment of Wierzbiak River.