Lake Shannon is a long, narrow reservoir on the Baker River in Skagit County in the U.S. state of Washington. Formed in the 1920s by the construction of an arch dam just above the river's mouth, the lake is approximately 7.5 miles (12 km) long and averages 0.6 miles (1 km) wide at full extent. Lake Shannon is part of a cascade of two reservoirs, the Baker River Hydroelectric Project, built on the lower half of the Baker River by Puget Sound Energy for generating hydroelectricity.