Pocket refuge

  A. Introduction

  1. Pocket refuge refers to habitat that has more volume per unit length than elsewhere in a stream.

  2. It typically results when flow encounters in-stream structures.

  3. Those structures may be placed logs, fallen trees, beaver dams or leftover debris from them, and rocks.

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Log placement that creates pocket refuge
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  B. Function​

  1. Streambed scouring directly downstream from the structures increases localized water volume.

  2. Access to the higher volumes during low flows may reduce trout population stress.

  3. More critically, during periods of lost stream connectivity, they may figure in survival of population members.

  4. That is, pocket volumes can be consequential, despite being relatively small percentages of total stream volumes.

  5. Pockets are opportunities for retreat, but are not replacement of larger habitat for meeting trout life-cycle needs.

  C. Explanation

  1. The scouring results from constriction and elevation drop.

  2. That is, as seen in the photo above, the cutout causes constricted flow.

  3. This increases the velocity and, thereby, the momentum of the water at the pourover.

  4. The distance the water falls, the difference in elevation head, is the further factor in the depth of the scouring.

  D. Application​​

  1. Creating more refuge can be simple to plan and create, and can be done soon and in numbers of streams in a basin.

  2. Additional pockets would not modify the amounts and timing of downstream flows, avoiding water rights issues.

  3. They can be created anywhere along a stream channel, and are not reduced in volume by upstream pockets.

  4. They are compatible with beaver dam analogs (BDAs) or post-assisted log structures (PALS) in the stream.

  5. The pocket refuge idea gives attention to water directly downstream, distinctly, not upstream like at BDAs and PALS.

  6. Water directly upstream will elevate, but volume can be kept small (or made large) by site choice and structure height.

  7. Key is the pocket that develops directly below a structure due to the natural process of scouring from flow.

  8. Scouring is greatest during the high flows that result from snowmelt during May and June in the study area.

  9. Cutout placement determines where streambed is scoured; cutout size and shape determine scouring intensity.

  10. What does pocket refuge look like, and can development of it be part of forward thinking for trout preservation?​

  E. Examination

  1. The lower, approximately 1-mile section of Ryman was examined on May 16, 2021, for locations of pocket refuge.

  2. Ryman's stream length is 5.1 mi; drainage area, 5.7 sq mi; mean annual flow, 5.4 cfs; and mean slope, 46 per cent.

  3. It is part of the Divide range allotment, described as vacant. Signs of past grazing are evident in the 1-mi section.

  4. For comparison, Wildcat, a candidate for proposal as Outstanding Waters, has very similar features.

  5. Four types of observations were made at Ryman: installed logs, fallen trees, beaver dams, and candidate sites.

  6. They were photographed and pocket characteristics recorded, as described below.

F. Considerations​

  1. The examination of Ryman was done during the spring, high-flow conditions that occur from snow melt.

  2. As a result, the photographs show the turbulence and flow energy that scours, creating and deepening pockets.

  3. Likely there will be little additional scouring of pocket depth until the next spring, high-flow conditions.

G. Proportions

  1. Pocket depths directly below the log placement, fallen tree, and beaver dam structures at Ryman were 1-4 ft.​

  2. Pocket volumes were 50-200 cu ft, with the largest at location "10. Beaver dam," described below.

  3. Long stretches of the stream were roughly 8 in deep and 3 ft wide.

  4. A middle value for a representative pocket volume is 100 cu ft, or roughly 2 ft deep, 6 ft wide, and 8 ft long.

  5. This pocket volume is equivalent to approximately 50 ft of stream length that is 8 in deep and 3 ft wide.

  6. If stream depth decreased by half to 4 in from dewatering, the pocket depth remains considerably greater at 1 ft.

  7. For simplicity, this characterization is based on changes in depth, but not in width and length.

  8. In this sketch of proportions, 8 ft of pocket length is equivalent in volume to 50 ft of stream...

  9. With the advantage of the pocket being that it is deep enough for refuge from stream dewatering.

  H. Installation

  1. Materials for in-stream structures are trees, logs, and rocks available streamside.

  2. Equipment for installation are simple, portable tools like shovels, axes, and saws.

  I. Observations

  1. ​Eleven observations about pocket refuge in the lower 1-mi section are described below, with photographs.

  2. They are shown in the order in which they occur, moving upstream.

Descriptions of existing and candidate pocket refuge at Ryman

 
IMG_4654_edited.jpg
1. Log placement
  • Pocket below log: depth, 2 ft; width, 5 ft; length, 5 ft.

  • Log: length, 22 ft; diameter, 1 ft; cutout width, 1 ft.

  • Note: The cutout on the top of the log concentrates the water's scouring effect on the streambed directly below. For comparison, long stretches of the stream were roughly 8 in deep and 3 ft wide.

IMG_4662_edited.jpg
2. Beaver dam
  • Pocket below debris: depth, 2.5 ft; width, 7 ft; length, 10 ft.

  • Water above debris: depth, 8 in.

  • Note: This pocket was created by residual woody debris that remained after flow washed out much of a beaver dam.

IMG_4668_edited.jpg
3. Log placement
  • Pocket below log: depth, 1 ft; width, 7 ft; length, 8 ft.

  • Water above log: depth, 2 ft.

  • Log: length, 16 ft; diameter, 1 ft; cutout width, 1 ft.

IMG_4672_edited.jpg
4. Fallen tree
  • Pocket below tree: depth, 2 ft; width, 5 ft; length, 5 ft.

  • Water above tree: depth, 1 ft.

  • Note: This is an example of pocket refuge that has developed naturally.

IMG_4676_edited.jpg
5. Fallen tree
  • Pocket below tree: depth, 2 ft; width, 6 ft; length, 8 ft.

  • Water above tree: depth, 1 ft.

  • Note: This is another example of a naturally developed refuge pocket.

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6. Candidate site
  • The log, extending across the stream, could be used to brace additional materials, such as other logs and rocks.

  • This could create pocket refuge just downstream of it.

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7. Fallen tree
  • Pocket below tree: depth, 1.5 ft.

  • Water above tree: depth, 1 ft.

  • Note: This is an example of a naturally developed pocket refuge.

Cand2_edited.jpg
8. Candidate site
  • The log, with a cutout and likely resting on the streambed at one time, could be used to brace materials such as other logs and rocks to create pocket refuge below and above it.

  • The rock overhang just upstream and shading the stream could be part of developing pocket refuge at this site.

Cand3_edited.jpg
9. Candidate site
  • The elevated flow on the right, which clearly is scouring under the high-flow conditions, could be strengthened with wood and rocks to preserve the pocket refuge.

  • Flow on the left could be similarly elevated to create another pocket.

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10. Beaver dam
  • Pocket below dam: depth, 4 ft; width, 5 ft; length, 10 ft.

  • Water above dam: depth, 1 ft.

  • Note: The pocket refuge scoured below this intact beaver dam is the deepest observed, 4 ft, of the 11 log placement, fallen tree, beaver dam, and candidate locations.

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11. Log placement
  • Pocket below log: depth, 2 ft.

  • Water above log: depth, 2 ft.

  • Log: length, 12 ft; diameter, 1 ft; cutout width, 1 ft.

  • Note: The log is anchored on both ends with rocks.

 

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