The refuge was a response to decades of bay fill that had reduced San Francisco Bay's surface area by a third. Salt production, landfill, and development had consumed thousands of acres of tidal marsh. Don Edwards championed the idea that what remained could be protected -- not in spite of being surrounded by cities but because of it. The refuge now manages lands in Fremont, Newark, Milpitas, San Jose, Sunnyvale, and Alviso, making it the largest contiguous protected area in the South Bay.

The refuge's tidal marshes and salt ponds lie on the Pacific Flyway, one of the four major bird migration routes in North America. Hundreds of thousands of shorebirds, waterfowl, and wading birds pass through or winter in the refuge each year. Species include the endangered California clapper rail, the salt marsh harvest mouse, and vast flocks of western sandpipers that can turn the mudflats into a shimmering carpet of movement during peak migration.

From altitude, the refuge is unmistakable: a patchwork of geometric salt pond basins in shades of red, green, and brown, bordered by the dark green of tidal marsh and the gray of surrounding development. The contrast between the protected wetlands and the urbanized shoreline is one of the most striking aerial features of the Bay Area.