The substantially natural hydrography of the upper Gila River supports one of the highest levels of aquatic and riparian biodiversity in the region, including the largest complement of native fishes and some of the best remaining riparian habitat in the lower Colorado River Basin. Native vegetation dominates the broad and structurally diverse floodplain, creating habitat for hundreds of birds and other wildlife. Two of the Gila’s fish species, spikedace and loach minnow, and a neotropical migratory bird, the southwestern willow flycatcher, are federally listed as endangered. The yellow-billed cuckoo, a candidate species for listing, nests in the Cliff-Gila Valley. Changes to the river’s hydrology, including peak flows, base flows and groundwater levels, may significantly degrade the aquatic and riparian ecosystem. The Arizona Water Settlements Act (AWSA) authorizes the expenditure of up to $128 million in federal funds to build a New Mexico Unit that could divert up to 14,000 acre-feet annually.