Why does the marsh look like swiss cheese at low tide?

Up close they’re shallow pools, some holding just an inch or two of salty water. From above they form a pattern that’s both beautiful and functional, showing the balance between salt, soil, and tide.

Those “holes” are called salt pannes and pools. They form where the ground sits slightly lower and water lingers after each tide. As that water evaporates, salt builds up in the soil and prevents most plants from growing. What’s left behind is a bare patch rimmed by marsh grass, a small feature that plays a big role in the ecosystem.

If you look closely, some of the pannes in this image have slightly raised edges, a ridge that forms as plants grow around the rim over time. It’s one of the subtle signs of a healthy, stable marsh.

These areas aren’t empty. Crabs, snails, and small fish move through them with the tides, and shorebirds feed along the edges. Algae and salt-tolerant plants colonize the margins, creating color and texture when seen from above.

Over time, some pannes fill back in while others deepen into permanent pools. Together they help the marsh balance salt, water, and nutrients, a quiet rhythm that keeps the ecosystem in motion.

Next time you’re walking near the marsh or looking at it from above, see if you can spot the raised rims or the bright halos of vegetation around these pools.

Rumney Marsh - October 2025