The all-purpose reply–
In really low flows Sand Wash to Mexican should be okay, in a small raft without 10,000 beers. But it might end up as a long day or even an overnight run. On the full-length run, the last day above Clay Hills can be a skank at low flows because of the silting and braiding: if you pick the wrong channel, you can have a strenuous drag.
But, it varies tremendously. In early September (flashflood season) it can jump from 600 cfs to 10,000 overnight. Check the hydrographs for September 2002, and for September 2005.
The gage website is <http:nwis.waterdata.usgs.gov/ut/nwis/discharge/? site_no=09379500>. If that doesn't work, go to usgs.gov and select Utah and click on the map dot.
Like I said, it varies.