The Upper Heart Watershed (HUC 10130202) spans an area of approximately 1,324 square miles in Montana and Wyoming. The hydrology of the area is primarily driven by snowmelt, as the watershed receives significant snowfall during the winter months. Snowpack levels in the area vary year to year, but have generally been decreasing over the past few decades as a result of climate change. Surface water in the watershed is largely comprised of streams and rivers that feed into various reservoirs, including the Tongue River Reservoir and the Yellowtail Reservoir. These reservoirs provide important water resources for irrigation, recreation, and municipal use. Interesting climatic facts about the area include its location in a semi-arid region, which is prone to drought and wildfire. Trends in the region include increased temperatures and decreased precipitation, which have implications for water availability and management.