The U.S. Geological Survey is studying Lake Koocanusa, a border reservoir between British Columbia in Canada and Montana, in collaboration with the Montana Department of Environmental Quality and the U.S. Environmental Protection Agency (Region 8). This work is part of a multi-agency project to apply an ecosystem-scale selenium modeling methodology (Presser and Luoma, 2010) to the lake to support development of a site-specific selenium guideline for the protection of aquatic life. This Data Release makes available measurements taken during the periods of May, 2015 to October, 2015 and April, 2016 to September, 2016. Spatially and temporally matched sets of dissolved and suspended particulate material selenium concentrations were collected mainly at two locations (forebay and international boundary) and at two depths (epilimnion, 3 meters below lake surface; hypolimnion, 3 meters above lake bottom). These data allow calculation of an environmental partitioning factor (i.e., Kd) between dissolved and suspended particulate material selenium concentrations as an initial modeling step. This Data Release contains 1) a location map; 2) a modeling schematic; 3) collection methods, analytical methods, and quality control information; 4) spreadsheets and graphical displays of selenium data; and 5) hydrological parameters related to sampling conditions at the lake. This Data Release will be updated and modified as additional sampling and analysis takes place.