The Arroyo Colorado (AC) is located in the Lower Rio Grande Valley of South Texas and flows through the middle of Hidalgo and Cameron counties. The lower 16 miles of the AC form the boundary between Cameron and Willacy counties. The AC drainage area is a sub-watershed of the Nueces-Rio Grande Coastal Basin, also known as the Lower Laguna Madre Watershed. The streams of the Nueces-Rio Grande Coastal Basin, including the AC, drain to the Laguna Madre, which is considered to be one of the most productive hyper-saline lagoon systems in the world. The Rio Grande is the largest fluvial system of the lower coast of Texas and forms the border between the United States and Mexico. The Lower Rio Grande Valley comprises the northern part of the Rio Grande Delta, a broad fluvio-deltaic plain laid down over tens of thousands of years by the ancestral Rio Grande. The AC extends approximately 90 miles from its headwaters southwest of the city of Mission, to its confluence with the Lower Laguna Madre in the northeast portion of Cameron County. For much of its course, the AC is a floodway and a conduit used for wastewater conveyance. The lower third of the stream serves as an inland waterway for commercial barge traffic and as a recreational area for boating and fishing. Near the coast, the AC also serves as an important nursery and foraging area for numerous species of marine fish, shrimp and crab.