Actions > Refuge

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.

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.

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.

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.