Coastline

The strip between the high tide mark and the low one, and the brackish country behind it: beach, rock platform, salt marsh, mangrove and river mouth. It is rich in forage, but a route across it is open for only part of the day, so a party has to time the tide. Nearly every settlement on a water world is on it.

Regional environment Terrestrial (land)

Tropical Dry Temperate Continental Polar

Lowland

Crossing it

Navigate: DC 10
Forage: DC 8
Notice: DC 12

A route across is open six hours in twelve, and the tide table decides when to cross.

In play

Tide

A route across flats or a marsh is passable for about six hours in twelve. A party caught mid-crossing is swimming, in water that is rising 3 feet an hour.
Footing

Wet rock, mud and root are difficult terrain, and a Dash across them calls for a DC 12 Dexterity save against falling Prone.
Brackish

Water here is undrinkable, and boiling does not fix it. Fresh water must come from inland or be brought in.

Variations

Tidal flat

Sand or mud, and miles of it at low water. Open, level and entirely exposed.
Salt marsh

Temperate and cold. Channels that cut the ground into islands at every high tide.
Mangrove

Tropical. Root above the waterline in every direction, and sightlines of 30 feet.
River mouth

Fresh water meeting salt. Traffic, silt and the deepest channel in the region.

Worlds it occurs on

Terran (Arctic) A rocky world covered in ice for most of the year but habitable, with open water at the margin in summer.
Terran (Oceanic) A world of ocean from pole to pole, with land only where a volcano has kept up.
Terran (Verdant) A rocky world with liquid water, breathable air and a magnetic field to protect them, rare and the cause of most wars.

Built for this