These were mostly perching on rocks (apple-to-melon sized), logs, or sticks on the ground around the dry wash, within the buffer zone ~10 m from the edge. About 30% were on bare sand, but most were on perches that gave them 1-4 cm of elevation. Those are individual grass blades in the photo! I also found them in similar perches upstream (by Pauma Valley city) where there was actually water flowing.