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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 and rock structures.
They would be installed across the stream, bank to bank.
Structures would not be designed to flood water outside the stream channel.

B. Installation, cont.

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.
A structure that arches upstream also could focus pourover, as shown below in exhibit 3.
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.

3. Arched structure focusing scouring

From Low-Tech Process-Based Restoration of Riverscapes, Pocket Field Guide, Utah State University, www.lowtechpbr.restoration.usu.edu.

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.