In this study, a riverine constructed wetland system (RCWS) was monitored with the aim to investigate its functionality and cost-effectiveness in treating domestic wastewater. Turkey’s second biggest freshwater lake, Eğirdir lake, is becoming polluted directly and indirectly with both domestic and diffuse pollution sources. In this paper we: (1) describe a water quality monitoring campaign on an existing long drainage channel where the majority of pollution originates from untreated urban sewage (2) discuss the design parameters and some considerations for a new RCWS to diminish and eliminate domestic and diffuse pollution loads originating from Gelendost and Yaka. In our design considerations, (1) we projected population increase over a 30-year period, which resulted in total 12,000 population equivalent, including the drainage network run-off originating from agriculture; (2) we aim to achieve 90% reduction in chemical oxygen demand, 70% of nitrogen (N) and 40% of total phosphorus. To achieve design objectives, we propose that the new RCWS consists of six horizontal flow CW and seven FWS systems, laid out within the drainage channel bed itself. This project would represent one of the few systems specifically designed to provide diffuse pollution treatment, and would be the first system of this kind in Turkey.