Blue Cypress Lake, originally called Lake Wilmington, is a lake in Indian River County of the Treasure Coast in Florida. It is the largest lake in the Treasure Coast and Indian River County. It is home to the origin of the St. Johns River. It is believed that the source is on the northeastern portion of the lake. The lake is over 6,500 acres (26 km²) in size, 21 mi (34 km) in circumference, and has an average depth of 8 feet (2 m). The lake is 2,100 acres (8.7 km²) larger than Lake Washington, 27 mi (43 km) north of this lake. The lake's name comes from the blue appearance of the cypress trees as the morning sun's rays reflect off the water.