Underground river

From Dwarf Fortress Wiki

Underground rivers occur in almost every region with a mountain biome, similar to the appearance patterns of adamantine.

In maps without any underground soil or aboveground herbs to harvest (so you can therefore get their seeds and begin farming aboveground) these can be a blessing, but on others it can be a hindrance.

The underground river is commonly filled with aggressive creatures such as snakemen, olmmen, giant olms and cave crocodiles. These can interrupt the work of any dwarf within line of sight, and can thus be a real nuisance.

Cave spiders are a type of vermin that occurs near underground rivers. They leave small webs strewn around, which can be collected for silk weaving.

The water sources for underground rivers are waterfall tiles.

Underground rivers are 2 to 5 tiles wide and while they often flow in a straight line, they can twist and turn across the entire region tile they are in.