Just got back from a two week trip uplake traveling uplake as far as the Escalante. I found the coolest water temperatures in the early morning at 68-69 F. Normal daily temperature was 71-73 F. Temperatures were fairly consistent each day. The average temperature at the beginning of our trip was 73 F A week later the average temperature was 71 F.
I normally take the daily water temperature from the ramp at Wahweap. As I traveled uplake I found the daily temperature was not much different lakewide.