Rapids
From UnReal World Wiki
Rapids can often be found in rivers. You can see them on the wilderness map as water tiles with some white dots in it. They do not freeze in Winter and therefore can be fished in the whole year.
In former versions of URW, more fish was to catch in calmer waters than in Rapids. If anyone has knowledge if that is still true, please share here.
Water tile with white and black dots in it are a ford/passage. You can cross the river here without watercraft or swimming.