Water that supports or restores riparian and aquatic ecosystems in the 2,250 km Colorado River in the United States is mostly incidental to its conveyance to farms and cities or results from inefficiencies in its use. In the river’s delta, in Mexico, environmental water comes from increased agricultural efficiency or the purchase of water rights.
In 1922, water rights to 18,500 mcm/yr were divided among the seven U.S. basin states. In 1944, Mexico secured a right to an additional 1,850 mcm/yr. Indigenous groups receive their allocations from within the individual states’ allocations. Nature has no water allocation. Long-term flows average 18,100 mcm/yr. The river is in deficit, rarely reaches the sea now, and will carry even less water as climate continues to warm. Natural systems persist near the river’s remote headwaters, while downstream, semi-natural aquatic and riparian ecosystems are supported by a largely uncoordinated patchwork of laws and policies (Endangered Species Act, Grand Canyon Protection Act, Grand Canyon National Park, Lower Colorado River Multi-Species Conservation Plan), and the need to convey water to diversion points near or at the Mexican border. Basin states and the federal government try to balance power production, water diversions and conservation of native species and their habitats.
New treaty agreements with Mexico resulted in an engineered spring flood in 2014 that recharged groundwater and generated goodwill. Subsequent environmental flows are delivered to restoration sites managed by NGOs. One-third of the water (10 mcm/yr) is purchased or leased with philanthropic support. NGOs have been successful at restoring 443 hectares of riparian habitat; the goal is 1,700 ha by 2026. The estuary is a challenge.
Restoration requires frequent intervention and the efficient use of water. Most restored ecosystems are not self-sustaining. ”Farming nature” is the way forward in the Colorado River Basin.