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Natural processes leave refuge for trout.
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That would be locations of deeper water while flow elsewhere is temporarily too shallow or lost.
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It results from the tendency of water levels to rise directly upstream of across-channel structures.
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And, more consequentially in volume, for plunge pools to develop directly downstream due to streambed scouring.
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The scouring is most intense during seasonal runoff from snowmelt.
A. Examples
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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.
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In exhibit 2, the plunge pool was 2 ft deep, 6 ft wide, and 8 ft long, roughly 100 cu ft.
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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
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Additional refuge could be created from an organized placement of wood and rock structures.
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They would be installed across the stream, bank to bank.
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Structures would not be designed to flood water outside the stream channel.
B. Installation, cont.
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They would be high enough to make some refuge upstream and to increase pourover momentum.
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Shaping or a cutout would direct the pourover for maximum scouring.
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A structure that arches upstream also could focus pourover, as shown below in exhibit 3.
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Most streams in the study area have abundant wood and rock along their channels.
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Structures would not be tall and would not seal, accommodating the passage of water and trout.
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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.
C. Preferences
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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.
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That is, deeper in order to be the least likely place to go dry during dewatering conditions.
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And a steeper upstream bed slope to bring more water momentum for scouring the streambed directly downstream of the structure.
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