Interpretation: Water temperature

A. Patterns

Main-stem warm-weather water temperatures decreased with increasing elevation. See WAT plots.
Tributary water temperatures were colder than at equivalent elevations in the main stem.
They stayed colder up to approximately 10,000 ft, where main-stem and tributary temperatures converged.
With prior, extended exposure to direct solar radiation, temperatures at higher elevations were higher and variable.
Without such exposure, water temperatures and temperature variation were moderate, by comparison.
Tributaries with smaller flows had higher temperature variation.
Logically, their lesser water mass left them more susceptible to diurnal fluctuations in ambient air temperatures.
The smallest tributary, an exception, was protected from high temperature variation by deep vegetation cover.
The tributary with the largest flow had the lowest and most stable water temperatures of the tributaries.
This was along with a small tributary where temperature apparently was dominated subsurface water input.
Water diversion left downstream water temperatures susceptible to warm ambient air temperatures.
Drought conditions also resulted in elevated water temperatures.

Water temperatures in a significant portion of the main stem were too warm during warm weather.
That is, they were too warm in the lower two-thirds in 2018, during drought, and in the lower one-third in 2019.
In contrast, tributaries large and small were cold enough throughout their lengths during warm weather.
This included during drought conditions.
The exception was Stoner, which had warm-weather water diversion for agriculture.
Stoner had outfall temperatures that reached the CO chronic criterion and were above the CO acute criterion.
They were higher temperatures than observed at other tributaries.
By contrast, water temperatures were stable and stayed well below the CO chronic and acute criteria at Bear.
Bear has the largest July-August and annual mean flows of the study area tributaries.
Where streams had prior, extended exposure to direct solar radiation, high-elevation temperatures were variable.
They were at the CO acute criterion or just below it, but were well below the CO chronic criterion.
Absent prior, extended exposure, high-elevation temperatures were lower than the CO chronic and acute criteria.

Thermal relief for trout in nearby deeper, faster, or shaded flow conditions does not appear to exist.
This is based on limited investigation using installed sensors at an upstream elevation in the main stem.
It also includes application of a hand-held sensor at downstream sections of three tributaries.
Study data indicate, however, that cold-water refuge is available in tributaries and upstream in the main stem.
This includes throughout drought conditions.
Researchers at other study areas have documented that trout will move a considerable distance seeking refuge.
The presence of colder water in large tributaries in this study area was signaled by cold-water plumes.
The largest plume extended 20 ft downstream in the main stem below the tributary discharge.
Cold-enough water in tributaries and the main stem for thermal relief, refuge, and reserve is in this configuration.
Notably, more cold-enough water always is available to trout in total in tributaries and the upper main stem...
Than at the upper elevation of too-warm temperatures in the main stem.
This apples throughout flow conditions, including drought, so long as water remains in those channels.

Tributaries with larger flows have habitat that is more resilient to dewatering than smaller ones.
This is because they have more water to lose before habitat is lost.
Larger-flow tributaries also have larger drainage areas and higher maximum elevations.
These likely are important characteristics in their resilience to dewatering.
High maximum elevations generally result in contributions from snowmelt to stream flow, including baseflow.
As noted above, it also can mean potential exposure of water at the higher elevations to direct solar radiation.
Larger-flow tributaries tend to have longer stream lengths.
This can mean greater capacities to moderate warm temperatures after upstream exposure to direct solar radiation.
Larger-flow tributaries have more volume than those smaller for trout seeking thermal relief from the main stem.
They also can continue accommodating populations already resident there.
Larger-flow tributaries may be favorable streams for applying resources to protect and preserve trout habitat.
Other considerations figure, too, however, such as current land use and protection needs for native trout species.

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