I ran Coldwater Rumble 100 in Goodyear Arizona, near Phoenix. This is one of the many events put on by Aravaipa Running, the same group that puts on Across the Years, Javalina Jundred (100), and others. They always do a great job on their races.
Coldwater Rumble provides races of various lengths: 4 miles, 20K, 20 miles, 52K, 52 miles, and 100 miles. They all start on the same day with staggered start times. About 400 runners would be on the course that is laid out on various desert trails in Estrella Mountain Regional Park. You could clearly tell which race a runner was in by the color and number on their race bib.