Oosterschelde

The Oosterschelde is the largest national park in the Netherlands and almost none of it is land: thirty-seven thousand hectares of tidal channel, sandflat, mussel bed and salt marsh — the last arm of the Zeeland delta where the sea still breathes twice a day. It exists in this form because of the most consequential change of mind in Dutch engineering. The Delta Plan drawn up after the 1953 flood ordered the estuary dammed like its neighbours; by the early seventies the oyster and mussel fishermen of Yerseke and a young environmental movement had forced the question, and in 1976 parliament chose the expensive alternative — a storm-surge barrier that stands open. The Oosterscheldekering, finished in 1986 with its sixty-two gates, closes only when the forecast demands, and everything alive behind it is the dividend: an inland sea with a three-metre tide, saved on purpose.

The life is Atlantic in miniature. Harbour porpoises hunt the deep channels year-round — the most reliably seen cetacean in the country, best scanned for off Zierikzee and the barrier — while harbour and grey seals haul out on the Roggenplaat's banks. On and under the water the park is famous continent-wide among divers: slack-tide descents at Zeelandbrug pilings and the dike toes reveal anemone walls, lobsters, and every May and June the sepia run, when cuttlefish come in from the North Sea to spawn in the shallows and hang in the water like patterned lanterns. Above the tide line, the schorren at Rattekaai and Viane carry sea lavender and roosting waders in the tens of thousands, and the whole basin is a service station on the East Atlantic Flyway — oystercatchers, knot and curlew stacked on the flats as the water makes.

The park's central worry is a debt the barrier itself runs up. By slowing the currents, the Kering starved the flats of sand — zandhonger, sand hunger — and the roosts and feeding plains erode year by year; the answer so far is rainbowed sand, whole plateaus like the Roggenplaat re-nourished by dredger to buy the birds decades. It is a very Oosterschelde condition: a wilderness with a maintenance schedule, and the more moving for it.

What's worth knowing before you go

The park has a rim, not a centre: Zierikzee (harbour-town base, porpoise-watching and seal trips), the barrier itself at Neeltje Jans (the Delta works exhibition, and the plainest look at the gates that define the place), and Yerseke on the south shore, where the mussel and oyster culture that helped save the estuary is lunch. Goes and Middelburg are the rail approaches, with buses over the dikes. Time everything by the tide table: low water spreads the flats and the waders with them, high water pushes seals and porpoises up the channels. Divers need slack-water planning and local briefing — the currents are the point and the danger. The connoisseur's date is a June evening dive (or snorkel from the stairs at Wemeldinge) among spawning cuttlefish; the lay equivalent is October dusk on the Plompe Toren dike, waders coming in over a drowned village's last tower.