The Jefferson Watershed (HUC 10020005) spans over 1,700 square miles in western Montana and Idaho. The watershed is part of the Columbia River Basin and provides important habitat for a variety of fish and wildlife species. The hydrology of the watershed is heavily influenced by snowmelt from the surrounding mountains, with snowpack levels fluctuating from year to year. Surface water in the watershed includes the Jefferson River, as well as several smaller tributaries. The reservoirs in the watershed store water for irrigation and hydropower, and can also impact water temperature and nutrient levels. Interesting climatic facts include the watershed's location in a semi-arid region with significant temperature variations between winter and summer. In recent years, the watershed has experienced increased drought conditions and earlier snowmelt, which could have long-term impacts on water availability and ecosystem health.