Reading a tide table before a walk on the salt marsh

Start with the right station, not the nearest town

The Wadden Sea does not have one tide. High water arrives at the western basins well before it reaches the eastern ones, and the difference across a single morning's travel can be more than an hour. I learned this by arriving at a marsh edge confidently late, having read the figures for a harbour that happened to be printed at the top of the page. The water was already in the creeks and the walk I had planned had quietly become a walk along the dike.

Now I pick the published station closest to the stretch of shore I actually intend to stand on, and I write its name in the notebook beside the times. If the nearest station is some distance away, the official tables give a correction in minutes for intermediate points; I apply it, then round against myself. Being twenty minutes early on a marsh costs nothing. Being twenty minutes late can cost the whole plan.

What the numbers are telling you

Dutch tables give times for high and low water and a height in centimetres relative to NAP, the national ordnance datum. The height matters more on a salt marsh than almost anywhere else, because the marsh surface itself sits in a narrow band around mean high water. A modest high tide wets only the lowest pioneer zone and the creek mouths. A high tide fifty or sixty centimetres above the usual figure will float across the whole plain, and the difference on the ground is not gradual: it is the difference between damp boots and no route home.

I copy four things into the notebook: the time of high water, its height, the time of the preceding low water, and the spring or neap position in the cycle. Around new and full moon the range widens, so both the reach of the water and the extent of exposed flat increase. Those are the best days for watching the tide work, and the least forgiving days to be careless about where you stand.

• Station name written beside the times, always
• High water time and height in centimetres relative to NAP
• Correction in minutes applied for intermediate points
• Spring or neap noted, because the range changes what is walkable

Wind is the correction the table cannot make

Predicted tides assume an atmosphere that behaves. A sustained westerly or north-westerly pushes water into the Wadden Sea and holds it there, and the observed level can sit well above the printed figure for hours; the ebb comes late and lazily. An easterly does the reverse, draining the basin so that low water exposes flats that are not usually walkable and lulls you into thinking the crossing is easier than it is.

On 7 October 2024 I stood on the dike at Zwarte Haan with a table promising high water at 11.20 and a blunt wind off the sea. By half past ten the creeks on the marsh were already brimming and the outer path was gone. Nothing had gone wrong; the table was simply describing a calmer day than the one I had. I now read the wind forecast immediately after the tide, and if the two disagree I believe the wind.

The margins I actually keep

My rule is to be off the outer marsh and back inside the seawall two hours before predicted high water, and to add an hour to that margin for any wind over about four Beaufort from the western half of the compass. This sounds excessively cautious on a still June evening and entirely reasonable in November. Because the margin is a rule rather than a judgement, I do not have to make the decision while cold, tired and pleased with a bird I have just found.

The second habit is a landmark. Before walking out I choose one fixed thing on the seawall behind me, a bend, a sluice, a numbered post, and I check it each time I look up. It gives me a bearing in flat light and a sense of how far the return actually is. Salt marsh distances lie: an even green plain with no trees reads as smaller than it is, right up until you try to cross it against a rising tide.