Jackson Lake, the largest of the park’s many lakes formed by glaciers, was expanded when the Bureau of Reclamation built a dam in 1907, creating a reservoir on top of a natural lake. Water behind the dam provides irrigation water to communities in Idaho through the Minidoka Project, and Jackson Lake Dam allows the Bureau of Reclamation to manage the lake’s water level. At times the level is kept low to capture meltwater, while at other times it is low because of lack of precipitation. The water level varies from year to year due to seasonal precipitation, winter snowpack, and irrigation demands; in some years the reservoir begins full of meltwater from deep winter snow and ends the summer nearly empty after being drained to meet downstream water needs. The dam increased the lake’s capacity by 30 percent and raised the water level nearly 40 feet, though while reservoir levels vary, the natural lake is always full. Research also indicates that the park’s glaciers have receded significantly in the last 100 years, reflecting the effects of climate change.