Quality of water in Reservoir II in Siemiatycze and its tributaries have been monitored before and after bathing season 2011. State of water quality appeared to be poor and insufficient to fit bathing and irrigation requirements. Two inflows: Kamianka River and Mahomet River are the important source of reservoir pollution (e.g. organics, total nitrogen, nitrates, total phosphorus). Kamianka River receives wastewater from municipal treatment plant and Mahomet River collects runoff from cultivated land. Biogens incoming with the inflows result in annually cyjanophyta bloom in the Reservoir II and depreciation its utilitarian value. At the actual state of wastewater management and agricultural practices in the catchment area, Reservoir II can be use only for retention or fire protection purpose.