The Colorado freshwater ecoregion, FEOW ecoregion 130, is a xeric freshwater ecoregion of western North America that includes the Colorado River basin and, in Mexico, the lower Colorado River, the Colorado River Delta, and associated wetlands of Baja California and Sonora. The Colorado River historically flowed to the Gulf of California through a vast delta of marshes, backwaters, riparian forests, tidal channels, brackish wetlands, and floodplain habitats. During the last century, dams, irrigation projects, and water extraction greatly reduced freshwater flow to the delta.

The broader ecoregion extends from the Rocky Mountains and Colorado Plateau southward to the northeastern tip of Baja California. The Colorado River is about 2,282 km long and has major upper-basin tributaries such as the Green, Gunnison, Dolores, and San Juan rivers, and lower-basin tributaries such as the Little Colorado, Bill Williams, and Gila rivers. FEOW treats the Gila and Virgin systems as separate neighboring ecoregions because their fish faunas are distinctive. In Mexico, the most important parts of the ecoregion are the lower Colorado River, delta wetlands, irrigation drains, marshes, shallow backwaters, and coastal-brackish transition zones.

Historically, the Colorado River supported warm, silty, highly variable large-river habitats, together with floodplain wetlands, backwaters, oxbows, riparian forests, springs, and delta marshes. Seasonal floods, sediment transport, warm water, and floodplain connectivity were central to the system's original ecology. Today, much of the lower river and delta has been transformed by dams, reservoirs, canals, irrigation return flows, salinity, drought, and reduced freshwater discharge. The Ciénega de Santa Clara and related lower-delta wetlands are especially important modern refuges in the Mexican portion of the basin.

The native fish fauna of the broader Colorado River basin is famous for specialized large-river fishes. Important species include Gila cypha, the humpback chub; Gila elegans, the bonytail; Ptychocheilus lucius, the Colorado pikeminnow; and Xyrauchen texanus, the razorback sucker. These fishes evolved in warm, turbid, seasonally variable rivers with strong floods, shifting sandbars, backwaters, and floodplain habitats. Many have declined sharply due to dams, altered flows, cold reservoir releases, habitat loss, and the introduction of fishes.

In the Mexican lower Colorado region, one of the most important native fishes is Cyprinodon macularius, the desert pupfish. It occurs in shallow marginal habitats, marshes, drains, and wetland refuges in Baja California and Sonora, including the Ciénega de Santa Clara and nearby lower-basin habitats. Other native or historically important fishes of the broader ecoregion include suckers, cyprinids, spring fishes, and headwater species, but in Mexico, the strongest conservation emphasis is on the lower river, delta wetlands, desert pupfish habitats, salinity, and the loss or transformation of the former delta ecosystem.

Cichlids are not native to the Colorado ecoregion. Any cichlid records from the lower Colorado region should be treated as introduced. Tilapiines such as Oreochromis niloticus, Oreochromis aureus, Oreochromis mossambicus, or Coptodon zillii / Tilapia zillii-type fishes may occur in modified warm waters, reservoirs, canals, drains, aquaculture systems, or irrigation habitats, but they are not part of the native fish fauna. For FFM, the Colorado should not be presented as a native cichlid ecoregion.

Introduced fishes are a major conservation issue. Non-native fishes may compete with, prey on, or displace native fishes, especially in small wetlands, drains, marshes, and refuge habitats. In the Mexican Lower Colorado Basin, introduced fishes have been recorded alongside Cyprinodon macularius, and habitat alteration and exotic species are major factors affecting desert pupfish distribution and abundance. Across the broader Colorado River system, dams and non-native fishes are among the principal causes of native fish decline.

Conservation priorities include protecting and restoring delta wetlands, maintaining environmental flows, improving floodplain and marsh connectivity, managing salinity, conserving desert pupfish refuges, reducing the impacts of invasive fish, and improving water quality. Major threats include dams, flow regulation, water extraction, irrigation withdrawals, groundwater use, salinization, channelization, loss of floodplain wetlands, invasive fishes, agricultural drainage, pollution, climate-driven drought, and the disconnection of the river from the Gulf of California. For FFM, the Colorado ecoregion should be treated as a highly altered but biologically important desert-river and delta-wetland system of northwestern Mexico.

This account is based in part on information from Freshwater Ecoregions of the World (FEOW), © WWF/TNC, used as a reference for educational and scientific purposes. See: http://www.feow.org.