For fly fishing on tail waters (a river coming out of a reservoir such as the
Middle Provo or Lower Provo), the best time to fish is when the insect hatches
take place. Sometimes, this can be during the hottest part of the day - for
example in June and July when the stonefly and green drake hatches take place -
from about 11 am to 3 pm. Or, in late July and into August and September, when
trying to catch fish with a grass hopper imitation, I will fish the hottest,
sunniest part of the day. Otherwise, in these tail waters, fishing
during the cooler parts of the day is preferable.
I'd like to see someone write an informed piece on the best places to fish
where the fish are likelier to be healthy to eat. I've read that almost all
locations in Utah will have fish with high mercury levels.