Findings, summary

I. Briefly

  1. Drought conditions in 2018 resulted in warmer water temperatures in the main stem of the Dolores River than the following year, by 6-7 F. 

  2. In fact, the lower two-thirds of the river in 2018 was too warm, as compared with the applicable Colorado chronic criterion, which was set to protect cutthroat trout, a native species. The lower one-third was too warm in 2019.

  3. Tributary water temperatures in the basin, on the other hand, did not rise above either Colorado's chronic or acute criteria, neither during the drought nor the following year.

  4. It meant that the tributaries, along with the upper one-third of the main stem in 2018 and the upper two-thirds in 2019, effectively were available as thermal refuge for trout.

  5. Researchers have documented that trout will move seeking thermal relief.

  6. This study found cold-water plumes where large tributaries enter the main stem, which may signal refuge for trout.

  7. Tributaries, possibly functioning as refuge, also may serve as reserve, holding resident trout populations that can assist repopulation in the main stem following thermal stress.

  8. As drought becomes more frequent and intense due to climate change, large-flow tributaries appear to become more important in preserving and protecting trout populations--because they have both the most water to lose before trout habitat is lost and the most resilience to dewatering.

  9. The basin's tributaries do not seem to have habitat problems from dissolved metals and non-metals constituent concentrations, based on comparison with Colorado's Outstanding Water standards.

  10. There is some concern, however, that sedimentation from grazing can result in streambed cover that reduces trout food supply and lowers successful reproduction.

II. More detail

A. Water temperature

  1. Warm-weather water temperatures in the main stem were warmer, by 6-7 F, during the 2018 drought, as compared with the following year, 2019. (See Statistics and WAT-TAT summaries in Data. See Acronyms to be familiar with their use.)

  2. In fact, from 7000 to 8550 ft, which extends over a 40-mile length of the 60-mile main stem, temperatures were higher than the Colorado (CO) chronic criterion. (See Criteria and "Water temperature" in Regulatory environment.)

  3. That is, only above 8550 ft was the river continuously cold enough to provide temperature-safe trout habitat during the 2018 summer, based on comparison of water temperatures with that chronic criterion. (See Cold-enough water in Interpretations.)

  4. Colorado's chronic criterion value was set to protect, in particular, its native trout species, cutthroat.

  5. It likely was brook and rainbow trout, however, and, possibly, brown trout that were dealing with the elevated temperatures in the main stem. Cutthroat trout probably were absent, except at higher elevations, because they do not have a history of successfully competing with non-native species for habitat.

  6. For comparison of 2018 main-stem temperatures with chronic criteria for brook, rainbow, and brown trout, see WAT, image 1, in Plots.

  7. Tributary temperatures rose during the 2018 drought, but never were higher than the CO chronic criterion. (See values in the Maximum WAT column of the WAT-TAT summary in Data.)

B. Refuge-reserve​

  1. Is there thermal refuge available for trout when main-stem temperatures are too high?

  2. Refuge does not appear to exist locally, that is, not nearby in deeper, faster, or shaded flow. (See Nearby relief in Interpretations.)

  3. In results from this study, water in the main stem and tributaries appears too well mixed for localized colder water temperatures to develop.

  4. Researchers at other study areas have demonstrated that trout seeking thermal relief will move considerable distances to locations of colder water. (See Movement to refuge in Interpretations.)

  5. Limited investigation in this study indicates the presence of cold-water plumes in the main stem where colder-water tributaries enter it, which effectively may signal to trout where thermal relief is available. (See Cold-water signal in Interpretations.)

  6. Based on results for this basin, it appears that the upper portion of the main stem and tributaries are functionally thermal refuge, even during drought. (See Schematic in Interpretations.)

  7. From the schematic, it is evident that the sum of cold-water flow, or refuge, available for trout in the upper main stem and in tributaries exceeds the flow at the highest elevation of the main-stem temperature stress.

  8. It always will be true for whatever main-stem elevation of stress develops so long as water remains in those channels.

  9. This is because refuge includes the continuously cold-enough tributaries entering the main stem below its highest elevation of temperature stress.

  10. Maybe trout use tributaries as off-ramps for temporary access to cold-enough water when temperatures are too warm in the main stem.

  11. Or perhaps functioning as reserve is the key role of tributaries, hosting populations that can assist in rebuilding those diminished in the main stem by thermal stress.

  12. Either way, attention to protecting and preserving habitat in tributaries appears warranted and may be important, perhaps key, in maintaining trout populations in the main stem as climate changes and the basin becomes stressed, hydrologically and, as a result, thermally. (See Refuge-reserve in Interpretations.)

C. Large-flow tributaries

  1. Can streams resilient to drought be identified? In the study area, some have continued with more flow than others during the 2020 drought. This is evident from comparison of images 1 and 2 with 12 and 13 at Drought flows, October 2020 in Gallery.

  2. As apparent from the text with the images, and not surprisingly, streams with the largest estimated mean annual flows, determined from application of the USGS program StreamStats, tended to have the largest drought-condition flows. An exception is in image 10, showing a dry streambed.

  3. Streams with the largest flows are valuable because they have the most water to lose before trout habitat is lost.

  4. Also, not surprisingly, and as noted in this study, larger-flow tributaries correlate with larger drainage areas and higher maximum elevations. It means likely more snowmelt and precipitation are collected, which recharge subsurface water and maintain baseflow, which should provide greater resilience against dewatering.

  5. In addition, those larger-flow streams are associated with longer stream lengths, much of it shaded by vegetation in this study area, which can moderate temperatures in water warmed by direct solar radiation, for example, arriving from above the tree line.

  6. Since they have more volume than smaller ones, streams with larger flows can better accommodate both the trout temporarily seeking thermal relief from main-stem temperatures and the populations already resident in the tributary.

  7. Larger-flow tributaries appear to be attractive candidates for consideration in the planning of resources to be directed at trout habitat protection and preservation. (See Favorable streams in Interpretations.)

  8. Five large-flow tributaries have 40 percent of the flow at the 42 perennial, trout-bearing streams in the Dolores River basin. Ten tributaries account for 60 percent.

D. Water quality​

  1. Tributaries in the study area do not appear to have water quality problems, aside from high coliform bacteria counts. (See Non-metals results and Metals results in Data.)

  2. That is, pH and dissolved oxygen are suitable for trout habitat. And there is no indication of dissolved heavy metals in the stream water, as might result from mining, for example, the concern being potential toxicity to aquatic life.

  3. The high coliform bacteria counts in 9 of the 12 tributaries sampled in July 2020 are due to pasturing of farm animals.

E. Streambed conditions​

  1. Nutrients and waste solids conveyed in runoff from pastures can result in streambed coverings of overgrown algae and fecal matter. (See 5, 15, and 16 in Streambed conditions in Gallery.)

  2. Reduction in streamside vegetation from grazing and trampling means erosion can carry soil material into streams, which can settle into streambeds. (See 7 and 9-11 in Streambed conditions.)

  3. Deposition of sediments can smother macroinvertebrates, which can diminish their diversity and density and decrease the food supply for trout.

  4. Sedimentation also can clog interstitial spaces in substrate harboring incubating eggs, at redds, potentially suffocating embryos or blocking emersion of alevins, reducing reproduction.

F. Other factors​

  1. Other factors besides large flows are necessary to consider in identifying tributaries for actions and resources intended to protect and preserve  trout populations.

  2. For example, the tributaries nominally identified using StreamStats as having the third and fourth largest mean annual flows in the basin have ongoing water diversions for agricultural use that lower their actual warm-weather flows. This reduces volume for trout habitat and results, as well, in elevated water temperatures.

  3. The second and sixth largest-flow tributaries are at high elevations at the head of the basin and are both closer than most of the other basin tributaries to Telluride-based outfitters and to a campground. It results in the trout populations in those streams being diminished by overfishing.

  4. Trout populations in the ninth and tenth largest-flow tributaries are similarly reduced by the same factors.

  5. The fifth largest-flow tributary is on Colorado's "monitoring and evaluation list" for possible arsenic and copper contamination. Analysis results from sampling in July 2020 as part of this study showed no detection of those metals. Undoubtedly, however, additional testing would be needed to resolve the concern for stream contamination, likely including sampling at other flow conditions and times of the year. As well, the success of trout populations in that stream would need to be confirmed.

  6. Most of the flow in the streams in this study area is across federal land managed by the U.S. Forest Service. There is a long history of the permitted grazing of farm animals on Forest Service land. Any thought of modifying this practice in the Dolores River basin, for instance, to improve the aesthetics at watersheds, including in stream channels, is almost overwhelming to contemplate. It surely would be met with great resistance. Is there evidence, however, that streambed sedimentation that results from grazing is reducing food supply and reproduction at trout habitat?

  7. Colorado has programs in place intended to assist in protecting streams, for example, its Instream Flow Program and its system of designating Outstanding Water. Are they sufficient for providing adequate preservation of trout habitat?

  8. Finally, what are the actions that could be taken to effectively and efficiently improve the "protection and preservation" of trout habitat? Are they mostly administrative, such as limits to fishing or to further diversions from stream flow? Are there also documented, science-based measures to consider?