Refuge-reserve

A. Depiction

As described, tributaries with outfalls above 7500 ft were cold-enough, based on July-August 2018, -19, and -20 data.
Depicted below, the main stem also was cold enough in 2018 above 7500 ft on July 15. (See Plot 1, 7500 ft.)
At the peak of 2018 warm-water conditions on July 26, the main stem was cold-enough only above 8550 ft.
Tributaries, on the other hand, stayed cold enough, despite the 2018 drought conditions.

B. Interpretation

The combined flows of the cold-enough tributaries and upper main stem...
Are greater than the flow at the upper extent of the too-warm conditions in the main stem.
That is, the total flow for the cold-enough tributaries from 7500 to 8550 ft, expressed as the sum of X cfs...
Plus the flow in the upper main stem are greater than the main-stem flow at 8550 ft, which is Y cfs.
So, more cold-water flow always is available to trout in tributaries with outfalls above 7500 ft...
And the upper main stem, in total, than at the upper elevation of too-warm temperatures in the main-stem.
This applies throughout flow conditions, including drought, so long as water remains in those channels.
It is because study data show that tributaries remain colder than the CO chronic criterion and the main stem.

C. Function

Maybe main-stem trout use the tributaries as off-ramps for access to cold-water thermal relief.
Or perhaps functioning as reserve is the key role of tributaries and the upper main stem.
That is, hosting populations that assist in rebuilding after thermal stress in the lower main stem.
Such stress is reasonably expected to become more frequent and severe due to climate change.
Either way, actions to protect and preserve habitat in tributaries and upper main stem appear warranted.
And they may be important, perhaps key, in maintaining trout populations in the main stem.