The U.S. federal government has built a number of large dams in the San Juan River system to control floods and to provide irrigation and domestic water supply. In addition, the lower part of the river is inundated by Lake Powell, one of the largest reservoirs in the United States. Efficient management is crucial to ensuring enough water supply for not just farms and urban areas but also for recreational boating, fisheries, and environmental restoration. However, heavy water use has significantly reduced the flow of the San Juan River by as much as 25 percent since pre-development conditions. In addition, warming temperatures in the Rocky Mountains are projected to have a further negative effect on snowpack and thus stream flow.