The U.S. Department of the Interior has investigated 25 areas in the western USA to determine whether irrigation drainage has caused harmful effects on wildlife or has reduced subsequent beneficial uses of the water. A database of chemical analyses of water, sediment, and biota from the 25 areas was created and supplemented with geologic, climatologic, and hydrologic data. The data were evaluated to identify common features among study areas and principal factors that result in Se contamination of water in lakes, ponds, and streams downgradient of irrigated areas. From the analysis of data, a decision tree that uses readily available geologic, climatologic, and hydrologic data was derived for use by resource managers as a screening tool to predict the likelihood that irrigation drainage will result in Se contamination in areas of the western USA. Irrigation in areas that are not associated with marine sedimentary rocks of late Cretaceous age is unlikely to cause Se contamination. Irrigation in very arid areas that are associated with these Cretaceous sediments is almost certain to cause Se contamination if the irrigation water drains to terminal lakes and ponds. The likelihood that an area will be contaminated with Se because of irrigation drainage can change, particularly with changes in precipitation. During normal or wet periods, Se contamination may not occur in an area, even though it has seleniferous soils, but reduced water deliveries during a drought in such an area may result in Se contamination.