For most of the year the Rillito is a pale road of sand. You can walk the bed, if you want to, and hear only the mesquite and the occasional bicycle bell on the path above. Then the monsoons come, and for a few weeks the wash remembers that it is a river.

I walked the path from Campbell to First Avenue six mornings in a row after the last heavy rain. On Monday the water was still a sheet, wide…