The Napa Sonoma Marsh sprawls across 48,000 acres at the northern edge of San Pablo Bay, the shallow upper arm of the San Francisco Bay. Three waterways converge here: Sonoma Creek, draining the Sonoma Valley; Tolay Creek, originating in the Tolay Lake basin; and the Napa River, carrying the runoff of the entire Napa Valley. The result is a vast estuarine landscape that stretches north as far as State Route 12 but remains, as a practical matter, accessible almost exclusively by boat. From the air, the marsh is unmistakable -- a patchwork of open water, mudflats, salt ponds, and dense cordgrass that shifts color with the tides and seasons. Thirteen thousand acres of abandoned salt evaporation ponds checker the landscape, geometric remnants of an industry that once extracted sea salt from the shallows.

Around 1860, the Napa Sonoma Marsh was one of the most productive wetlands on the Pacific Coast, a stopover and breeding ground for millions of birds along the Pacific Flyway. Then came a century of filling, draining, diking, and industrial use that devastated the San Francisco Bay's perimeter wetlands. By the mid-1980s, over 91 percent of the Bay's original wetlands had been destroyed. The Napa Sonoma Marsh survived in diminished form, one of the few expanses large enough to make restoration feasible. The U.S. government recognized this potential by designating 13,000 acres as the San Pablo Bay National Wildlife Refuge. The marsh still shelters endangered species including the California clapper rail, a secretive bird that nests in the cordgrass, and the California freshwater shrimp, found in only a handful of coastal streams.

The marsh's military chapter is one of its strangest. During the Vietnam War, the Navy needed a place to train crews for its new Patrol Craft Fast -- the Swift boats that would become synonymous with riverine warfare in Southeast Asia -- and the Patrol Boat, River craft that operated in the Mekong Delta's narrow channels. The Napa Sonoma Marsh offered the right conditions: shallow, winding waterways hemmed in by dense vegetation, with tidal currents that made navigation unpredictable. Mare Island Naval Shipyard, sitting at the marsh's southern terminus where the Napa River empties into San Pablo Bay, served as the staging point. Crews learned to handle high-speed boats in confined waters before deploying to a war zone that, despite being half a world away, bore an eerie geographical resemblance to the marshland where they had trained.

The Point Reyes Bird Observatory selected the Napa Sonoma Marsh as one of only seven Bay Area marshes for intensive long-term monitoring, out of fifty discrete wetlands they surveyed around the San Francisco Bay. The distinction reflects the marsh's outsized importance as bird habitat despite its degraded state. Restoration efforts focus on breaching the levees of abandoned salt ponds to allow tidal flows to re-enter, gradually converting industrial landscapes back into functioning wetland. The process is slow -- it takes years for sediment to accumulate, for native plants to colonize, for the intricate food web of an estuarine ecosystem to reassemble itself. But the scale of the opportunity is rare. Few places around the Bay still have enough contiguous acreage to support the kind of large-scale restoration that could meaningfully reverse a century of habitat loss.