Here's why and some potential antonyms:
* "Arroyo" as a dry streambed:
* River: A flowing body of water.
* Stream: A smaller flowing body of water.
* Lake: A body of standing water.
* Ocean: A vast body of saltwater.
* "Arroyo" as a gully or ravine:
* Plateau: A flat, elevated area.
* Hill: A rounded, elevated area.
* Plain: A large, flat area of land.
* "Arroyo" as a narrow, winding path:
* Highway: A wide, straight road.
* Boulevard: A wide, landscaped road.
* Trail: A wider, more defined path.
Ultimately, the best antonym for "arroyo" depends on the specific context in which you're using it.