expression, godan verb (-ru)
go down a river; descend a river
Used for traveling downstream by boat or following the river's course toward its mouth.
彼らはカヌーで川を下った。
They went down the river by canoe.
この川を下ると大きな湖に出る。
If you go down this river, you'll reach a large lake.
川を上る means to go upstream, while 川を下る means to go downstream.
Compound of 川 (river) + を (object marker) + 下る (go down). The expression is transparent from its components.