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 site-specific selenium guidelines for the protection of aquatic life. A conceptual model framework was published in 2017 (Jenni, Naftz, and Presser, 2017) to serve as a coherent and consistent structure for organizing relevant scientific information, to provide an appropriate context for interpreting new information as datasets and site parameters are developed, and to identify data and science gaps that limit understanding of the implications of alternative Se criteria. This Data Release makes available measurements taken in May, July, and October, 2015; April, May, July, and September, 2016; and monthly from April through October, 2017. 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). In 2017, samples were additionally collected during June, July, and September at a site south of the Elk River inflow by the British Columbia Ministry of Environment & Climate Change Strategy. 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. Also included is an example sampling matrix that illustrates the types of data needed within Lake Koocanusa to support robust ecosystem-scale selenium modeling. For this 2017 update of the Data Release, the same methodologies were followed as in 2016, except sampling occurred over seven consecutive months. This allowed documentation of a more detailed view of the dissolve and particulate selenium dynamics of Lake Koocanusa to help identify trends and connect to other ecological and management variables. This Data Release will be updated and modified if additional sampling and analysis takes place.