Five islands make up the park: San Lorenzo, Las Animas, Salsipuedes, Rasa, and Partida. The Salsipuedes Channel separates them from the Baja California mainland to the west. Deep, cold water surrounds these rugged outcrops, fed by winter winds that drive Pacific nutrients into the Gulf. The islands themselves are stark, their irregular coastlines eroded into sea cliffs, their elevations rising from sea level to 485 meters on San Lorenzo's southern heights. Most of the park is maritime area; the small terrestrial portions consist of harsh desert terrain where only scattered shrubs and cacti survive.

Rasa Island alone hosts an estimated 260,000 Heermann's gulls and 200,000 elegant terns during breeding season. American oystercatchers, Craveri's murrelets, and brown pelicans add to the cacophony. On San Lorenzo, Salsipuedes, and Las Animas, between 6,000 and 18,000 nesting pairs of brown pelicans crowd the rocks. Cooper's hawks, golden eagles, peregrine falcons, magnificent frigatebirds, and Nazca boobies patrol these waters. The archipelago serves as both permanent home and migratory waystation for species drawn to the Gulf's extraordinary productivity.

Beneath the surface, the Gulf's bounty supports creatures far larger than terns and pelicans. Blue whales, humpback whales, killer whales, and sperm whales pass through these waters. Green turtles, hawksbill turtles, and olive ridley turtles swim among more than 800 fish species and 2,000 invertebrates. The totoaba, a critically endangered fish endemic to the Gulf of California, finds refuge here. Swordfish, hake, and Risso's dolphins inhabit the deeper channels. This biodiversity made federal protection imperative.

The climate is desert harsh: annual rainfall averages just 2.54 inches, temperatures during July and August reach 95 to 104 degrees Fahrenheit, and high solar radiation combines with constant winds to create intense evaporation. Winter brings northwest winds that push cold, nutrient-rich Pacific water into the Gulf, while summer's southerly winds reverse the currents. These seasonal shifts drive the biological cycles that make the archipelago so productive. The marine algae anchoring this food web provides habitat for the small species that larger predators depend upon.

Mexico established the Parque Nacional Marino Archipielago de San Lorenzo by national decree on April 25, 2005. The park falls under protection of Norma Oficial Mexicana NOM-059-SEMARNAT-2001, the federal standard for endangered species habitat. Though administratively part of Mexicali Municipality, the islands remain largely uninhabited, their value measured in ecological rather than economic terms. The park protects not just the birds and whales that draw international attention, but an entire marine ecosystem that remains one of the most productive in the eastern Pacific.