I've done the Dirty Devil, and I'm suspicious of the flow numbers in that report. Relatively recent irrigation changes make the historical flows a bit harder to interpret, and perhaps lower than you might think.
The number I heard to be able to get close to paddling the river was 200 CFS.
When I did it, in May, I had between 25 and 50 cfs over two weeks. I walked probably 75% of the river, on Sand, towing a whitewater open canoe. That flow range is available all year, during May temps were over a nice range and splashing through the sand bars was fun, if a bit slow. I was able to carry enough camp gear and food to be very comfortable.
I will do this one again, and it will be more likely that I'll risk the flash floods of late summer than the cooler weather of March - unless my schedule gets so flexible as to match what are very rare trully paddleable dates.
Here are my pics:
Two Weeks in the Dirty Devil Canyon (Motley Blog)