The Arroyo Colorado flows through Hidalgo, Cameron and Willacy Counties in the Lower Rio Grande Valley of Texas into the Laguna Madre and is the major source of fresh water to the lower Laguna Madre. The Arroyo Colorado is an economically and ecologically important resource to the region, having water exchange with the Gulf of Mexico. One third of the stream is also used for shipping from the Gulf Intracoastal Waterway to the Port of Harlingen. Most of the flow water in the Arroyo Colorado is also sustained by wastewater discharges, agricultural irrigation return flows, urban runoff, and base flows from shallow groundwater (Webster et al. 2000; Filteau 1995; Charbonnet et al. 2006; Rosenthal and Garza 2006). The Arroyo Colorado watershed has been on the state's list of impaired water bodies for low dissolved oxygen since the state began assessing water bodies in 1974. Moreover, the Laguna Atascosa National Wildlife Refuge and several county and city parks are located within the Arroyo watershed; its mild climate, semi-