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  • Natural processes leave refuge for trout.

  • That would be locations of deeper water while flow elsewhere is temporarily too shallow or lost.

  • It results from the tendency of water levels to rise directly upstream of across-channel structures.

  • And, more consequentially in volume, for plunge pools to develop directly downstream due to streambed scouring.

  • The scouring is most intense during seasonal runoff from snowmelt.

  A. Examples​

  • The plunge pool in exhibit 1 below the fallen tree was approximately 2 ft deep, 5 ft wide, and 5 ft long, or 50 cu ft.

  • In exhibit 2, the plunge pool was 2 ft deep, 6 ft wide, and 8 ft long, roughly 100 cu ft.

  • For comparison, the pool in exhibit 2 was equivalent in volume to 50 ft of stream length.

1. Fallen tree creating trout refuge


2. Refuge from two fallen trees


  B. Installation​

  • Additional refuge could be created from an organized placement of wood structures.

  • More details are at "Installation" in Augmentation.

  • Structures would be situated across the stream, bank to bank.

  • They would be designed not to flood water outside the stream channel.

  • They would be high enough to make some refuge upstream and to increase pourover momentum.

  • Shaping or a cutout would direct the pourover for maximum scouring.

  • Most streams in the study area have abundant wood and rock along their channels.

  • Structures would not be tall and would not seal, accommodating the passage of water and trout.

  • The tools necessary for installation are simple and portable.

  C. Preferences​

  • A preferred site results in a directly downstream body of water that is deeper and has an upstream bed slope that is steeper than at other sites.

  • That is, deeper in order to be the least likely place to go dry during dewatering conditions.

  • And a steeper upstream bed slope to bring more water momentum for scouring the streambed directly downstream of the structure.

  D. Scenario​

  • Will creation of stream refuge as pools, or pockets, significantly reduce the amount of flow entering the main stem from the stream?

  • The question is answered by posing and solving a worst-case, hypothetical scenario.

  • In this scenario, none of the water pooled as refuge enters the main stem.

  • That is, hypothetically, all the water is lost to evapotranspiration (or, much less likely, to infiltration that does not re-enter as baseflow).

  • The specific question is how many created pools, or pockets, would it take to reduce flow at the mouth of stream by 0.1 percent (0.001)?

  • Assume the pools are 4' wide by 4' long by 2' deep.

  • Evapotranspiration is greatest in August due to highest air temperatures, so August flow is assumed, too, which is the lowest of the 3 summer months.

  • Assumed, also, is a small stream size, 3 cfs, which is consistent with developing a worst-case scenario.

  • From scenario calculation, it was determined to require an amount of water loss equivalent to 251 pools to reduce a 3-cfs August flow by 0.1 percent.

  • That is, the volume of 251 pools, 4' x 4' x 2' in size, would be 0.1 percent of the total amount of water discharged over 31 days at 3 cfs.

  E. Interpretation​​

  • What is an example upper Dolores basin stream setting for interpreting these results?

  • Wildcat is a small stream with 3 cfs mean August flow (as in the scenario), based on application of the U.S. Geological Survey program StreamStats.

  • A reasonable objective could be installation of 30 structures in the lower 1.75 miles of the stream, which would create 60 new refuge pools.

  • Applying the worst-case scenario assumptions, total water loss at those 60 pools hypothetically would reduce August flow by 0.02 percent.

  • If drought conditions diminished mean flow to 1 cfs, or 1/3 of the StreamStats value, the August stream flow would be reduced by 0.07 percent.

  • That assumes the same amount of water loss as in non-drought conditions, which is additionally conservative since pools will have smaller hypothetical volumes to lose due to drought.

  • Neither of those hypothetical flow decreases, 0.02 or 0.07 percent, are problem reductions.

  • As well, no new water loss is expected since only depth is increased from pools, not surface area, so no opportunity for evapotranspiration to increase.

  • Installation of 30 structures is an estimated one week of work for a crew of 4 personnel.

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