The Collins Watershed covers an area of 207 square miles in southwestern Montana. The hydrology of the watershed is heavily influenced by snowpack, which typically accumulates from November through April and provides the majority of the water supply for the area. Surface water in the watershed includes streams and rivers, with the Big Hole River being the largest and most important. Reservoirs in the watershed are primarily used for irrigation and flood control purposes. Interesting climatic facts include a high frequency of thunderstorms during the summer months and occasional winter inversions that can trap pollution in the valley. In recent years, the watershed has experienced increases in average temperatures and decreased snowpack, leading to concerns about future water availability for both human and wildlife populations.