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Comprehensive Plan Conservation Element proximate to U.S. Highway #1 in Indian River County. In the early -1920's, however, three F. S. 298 Special Water Control Districts operating in Indian River County began constructing a network of drainage canals that artificially expanded the IRL watershed. Now, four major canals in Indian River County outfall into the IRL. These canals are: the North Relief Canal (located just north of the Gifford area); the Main Relief Canal (located just north of the Merrill -Barber Bridge); the South Relief Canal (located just south of 4th Street), and the C-54 Canal (located proximate to the Indian River -Brevard County line). Combined, these canals discharge approximately 188 million gallons per day (MGD) of freshwater to the IRL. • Intracoastal Waterway The deepest part of the IRL is a manmade channel, the Atlantic Intracoastal Waterway (ICW). The ICW was dredged to provide safe passage for watercraft along the east coast. In 1881, the original construction was started by the Florida East Coast Canal Company. Later, the project was adopted by the U.S. Army Corps of Engineers (ALOE). In the mid-1950s, the ICW in Indian River County was dredged to a depth of 14 feet, but has not been dredged since that time. Due to sedimentation, the current depth of the ICW is approximately nine feet. When the ICW channel was dredged, 49 spoil islands were created within the Indian River County portion of the Indian River Lagoon. These artificially created islands are discussed under the heading "Spoil Islands". • St. Sebastian River Located in the northern portion of the county, the St. Sebastian River is a major source of freshwater inflow into the IRL. The South Prong of the St. Sebastian River extends from its headwaters near C.R. 510 (85th Street) to the Indian River -Brevard County Line, where it discharges into the Indian River Lagoon near the Sebastian Inlet. The deepwater portion of the South Prong covers approximately 500 acres. According to U.S. Fish and Wildlife Service National Wetlands Inventory Maps, the St. Sebastian River is classified as a freshwater riverine system, which becomes an estuarine subtidal system as it approaches its confluence with the IRL. Similar to the IRL, the natural watershed boundaries of the St. Sebastian River have been extended by connecting drainage canals to the river. The major canal influence is the east -west C-54 canal, extending from the Upper St. Johns River Basin to the St. Sebastian River, just north of the Indian River County line. Among its various functions, the St. Sebastian River plays host to transit fish normally associated with estuarine systems. While the upper part of the South Prong is freshwater, marine fish such as mullet, spot, lady fish, tarpon, and snook nurse as far up the river as the C.R. 512 over -pass. Four Community Development Department Indian River County 16