a stream or brook
"After the heavy rain, we watched as the dry creek bed quickly filled up to become an arroyo rushing down the mountain."
A dry creek or streambed, a gulch which temporarily or seasonally fills and flows (after sufficient rain).
"We hiked along the arroyo where the last storm had left deep ruts in the dried-up bed."
A surname, from Spanish of Spanish origin.
"The new neighbor told us his last name is Arroyo during the block party introduction."