Cold-enough water

A. Data

As seen in the WAT plots, there were locations where water temperatures were continuously cold enough.
This would be during summertime warm-weather conditions, that is, July August.
The cold-enough water temperatures were in the tributaries and at higher-elevation main stem sites.
As shown below in figure 1, tributary MWAT values were at or below the CO chronic criterion.
Best-fit regression lines were applied to the 2018 and 2019 main-stem MWAT values.
They enabled interpolation of water temperatures between the elevations at which they were measured.
The TAT plots show the same pattern of colder temperatures in tributaries and the upper main stem.
Almost all the 2018 and 2019 tributary MTAT values were below the CO acute criterion (figure 2 below).
The exceptions were at Taylor, 8487 ft, in 2018 and at Stoner, 7467 ft, in 2019.

Water was continuously cold enough in the main stem in 2018 above 8550 ft; and in 2019, above 7300 ft.
This is based on comparison of MWAT values with the chronic criterion line, shown above in figure 1.
That is, two-thirds, or 40 of the main stem's 60 miles, is below 8550 ft; and one third, below 7300 ft.
In 2018, there was continuously cold-enough water throughout July-August at all the tributaries measured.
They were 3-10 F below the chronic criterion.
Tributary temperatures were colder than the main stem, given elevation, up to 10,000 ft.
In 2019, there was continuously cold-enough water throughout July-August at all the tributaries measured.
They were 6-16 F colder than the chronic criterion.
The exception was Stoner, outfall elevation 7467 ft, with an MWAT value the same as the criterion.
In 2019, there was continuously cold-enough water throughout July-August at all the tributaries measured,
They were 5-11 F colder than the chronic criterion.
Figure 2 above shows interpretation of cold-enough water based on MTAT values.

The regression lines used to interpret elevations of cold-enough water showed excellent fit with the data.
They had r-squared values of 1.0 with the MWAT data; and values of 0.93 and 1.0 with the MWAT data.
That and the judgement of sufficient normality support confidence in application of the regression lines.
Cold-enough water can function as refuge-reserve for trout.