Reserva de la Biosfera Alto Golfo de California y Delta del Río Colorado

This reserve covers the northern tip of the Gulf of California, where the Colorado River reaches the sea between Sonora and Baja California. It was decreed on June 10, 1993, joined UNESCO's Man and the Biosphere programme in 1995, and in 2005 became part of the Islands and Protected Areas of the Gulf of California World Heritage Site, now on the List of World Heritage in Danger. It spans 934,756 hectares, nearly all of it sea. The 164,779-hectare core zone is the delta itself, a roadless plain of mud that floods at high tide. Its land falls in the municipalities of Mexicali, San Luis Río Colorado and Puerto Peñasco.

This is the only home of the vaquita, a small, endangered porpoise found nowhere else. Vaquitas drown in gillnets set illegally for totoaba, a fish whose swim bladder fetches high prices in Asia. The 2025 survey, using acoustic detectors from May to September and ships in September, put the population at 7 to 10 animals and spotted at least one calf. The same waters hold about 18 kinds of marine mammals, blue whales among them, 149 fish species, five sea turtles and some 315 bird species.

On land, the Ciénega de Santa Clara is the delta's largest surviving wetland, about 18,000 hectares fed by salty farm runoff carried from Arizona through a canal built in 1972. It holds the largest population of the Yuma Ridgway's rail and draws hundreds of thousands of migrating birds, and the Luis Encinas Johnson ejido runs it for ecotourism. The way in is El Golfo de Santa Clara, a fishing settlement founded in 1966 with 4,618 people in 2020, known for seafood and sand dunes such as El Machorro, about 50 m high.