Why does the pavement on Redwood Highway right at the Greenbrae Boardwalk go under water on a clear, dry Tuesday in December, weeks after the last rain? Anyone who walks, bikes, or drives that stretch regularly has seen it: sunshine overhead, a puddle across the road that has nothing to do with weather at all.
Marin County's own Department of Public Works has an answer, and it names this exact spot by name. In its seasonal high tide advisory, the county lists a short roster of Marin addresses most prone to tidal flooding: North San Pedro Road through China Camp, Greenwood Cove Drive in Tiburon, and two Greenbrae locations specifically, Lucky Drive and Redwood Highway at the Greenbrae Boardwalk. That list has repeated, address for address, across back-to-back years of county advisories. This isn't a one-time drainage complaint. It's a known pattern tied to the lunar calendar, not the forecast.
The moon, not the sky
The California Coastal Commission's King Tides Project explains the mechanism in plain terms: king tides happen when the Earth, moon, and sun align in a way that pulls ocean water a foot or two higher than a typical high tide. They're not caused by storms and they're not caused by sea level rise, though the Commission notes they offer a preview of what higher seas will look like on a daily basis within a few decades. Marin County's public works advisory flags the season for this every year, warning that tide levels high enough to cause roadway flooding in coastal and bayside areas typically arrive across a window spanning November through January, with the risk compounding sharply if a storm happens to land on top of an already-high tide.
For a road, that's a nuisance to plan around. For the ground underneath the Greenbrae Boardwalk, it's the entire reason the neighborhood looks the way it does.
Ground that started as boards across pickleweed
The Boardwalk holds forty-nine houses on pilings above a salt marsh, and it's reachable only by foot or bike over a wooden walkway, according to research published by UC Berkeley's Department of Architecture. That's not a modern design choice. It's a direct continuation of what was there before any house existed.
In the early 1900s, small floating houses called arks were common off the shores of Larkspur. The Boardwalk itself started as a series of narrow boards, twelve by two, laid across pickleweed marsh, used for catching bass and duck hunting long before it carried foot traffic to anyone's front door. By the 1910s, three houses had gone up along that same walkway, among the first homes built in what would become Greenbrae. Basic infrastructure arrived slowly and in a specific order: a waterline in 1921, electricity in 1946, postal delivery in 1948, and sewage service by 1961. Each of those dates marks a point where a community built on marsh caught up, piece by piece, to what land elsewhere in Marin already had.
That history matters for anyone walking the same route today. The reason water shows up on Redwood Highway isn't a broken storm drain. It's tidal ground remembering what it is.
The marsh on the other side of the fence
Walk or bike the Boardwalk far enough and you're skirting the edge of the Corte Madera Marsh Ecological Reserve, a stretch of tidal marsh managed by the California Department of Fish and Wildlife, with public access from the Boardwalk itself as well as Industrial Way, Redwood Highway, and Channel Drive, according to Marin Audubon Society's own birding location guide. Two named sections make up most of it, and they got there in very different ways.
Heerdt Marsh is the older of the two, a 95-acre stretch of ancient tidal marsh that's never been diked, meaning it has never been walled off from the tides the way most of the Bay's original marshland was over the past century and a half. Muzzi Marsh sits just south of it, also roughly 200 acres, and it tells the opposite story: pastureland for decades before being deliberately restored to tidal marsh in the mid-1970s. One patch of ground was left alone long enough to stay wild. The other was farmland that engineers later reopened to the tide on purpose.
The flood nobody complains about
That distinction, between marsh that was never touched and marsh that was intentionally reopened, points to something worth knowing about a separate project just up the shoreline. In a restoration completed around January 2021, engineers at WRA excavated several acres of previously filled land next to the Ecological Reserve, reconnected it to San Francisco Bay through a newly built tidal channel, and let the site flood on every high tide by design. A new public trail loops around the restored marsh plain and ties into the existing bayfront path, with benches and interpretive signage along the way. The project's stated goal wasn't just habitat. It was designed specifically to preserve low-lying land that will convert to tidal marsh as sea levels rise, keeping the habitat intact rather than losing it to erosion.
Put those two facts side by side and the contrast is hard to miss. A few hundred feet from where Redwood Highway floods and county crews treat it as a problem to advise around, a nearly identical flood happens on purpose, engineered into the landscape as long-term insurance against the same rising water. The tide doesn't distinguish between the two. The people managing the land increasingly do.
Who actually keeps track of the creek
None of this upkeep happens by accident, and it isn't run by a government agency alone. Friends of Corte Madera Creek Watershed is an all-volunteer nonprofit founded in 1995 specifically to protect and restore the creek and its wetlands. The group organizes creek clean-up days, hosts bioblitzes to catalog the plants and animals living in the watershed, and currently has active projects underway well beyond Greenbrae itself, including restoration work at College of Marin and efforts to improve fish passage in Kentfield and San Anselmo. For a watershed that stretches roughly 28 square miles from redwood forest down to tidal wetland, keeping any of it functional takes exactly this kind of unglamorous, recurring volunteer labor, not a single fix.
Reading the corridor like it actually works
None of this changes what the water does. It only changes what you check before you plan around it. A weather app will tell you whether it's going to rain. It won't tell you whether the marsh is going to rise over Redwood Highway at 4pm on a Thursday in December, because that has nothing to do with rain at all. NOAA's tide predictions, which the county's own advisory draws from, will tell you that instead, and the county revisits this same warning every autumn as the November-through-January window approaches.
For anyone who walks the Corte Madera Creek Path regularly, that same tidal rhythm shapes what you'll actually see out there. The path itself runs about 3.5 miles from Remillard Park in Larkspur to Lagunitas Road and Sylvan Lane in Ross, passing through Kentfield along the way, with a stretch that briefly carries walkers out over the creek on a boardwalk section near the Highway 101 interchange. Marin Audubon lists the adjoining marsh as a genuine birding destination, and a marsh that's built to flood at high tide is, by definition, a marsh that exposes mudflats at low tide. That's the same water working both sides of the same daily clock. Whether you catch it as an obstacle on your commute or as the reason a heron is standing exactly where it is, it's the same tide either way.
If you're curious enough to want to see it for yourself with your own timestamp, the California Coastal Commission's King Tides Project invites residents to photograph these highest tides each winter and add them to a public map tracking shoreline change over time, which is a genuinely useful way to watch this stretch of Greenbrae shift year over year rather than just hearing about it secondhand.
If you're weighing a move to a stretch of Marin where the shoreline, the tide chart, and daily life are this closely linked, Rick van der Wal can walk you through what that actually means for a specific address, not just the neighborhood in general.
