At the northern end of the Sierra del Tigre, the Rio San Bernardino joins the Bavispe River, then heads south to Mexico.

At the very south of the valley, north of Douglas, Arizona–Agua Prieta, Mexico lies the San Bernardino National Wildlife Refuge.

The refuge, grasslands, and wetlands are part of the south and southwest-flowing Yaqui River of Mexico that reaches the northeast region of the Gulf of California.

The San Bernardino Valley floor is covered by basaltic lava dating from 0.27 to 1.0 Ma[3] (million years ago).

The field covers some 850 km2 and consists of olivine basalt flows, tuff rings and maars with 130 identified volcanic vent areas and cinder cones.