Most appropriate tag is “waterway=stream”; but, it suggests running water. How do I specify that “nahal” is dry except for few days in the year?
Atually, waterway=stream is the correct tagging.
From its wiki page:
If it’s always running, then use waterway=river
If you want to have nice wide rivers, then draw them as a closed loop tagged waterway=riverbank
(e.g. Meshushim and Jordan where they meet the Sea of Galilee)