Project Objectives: Replace a culvert that is currently blocking 3.5 miles of anadromous habitat with a structure that allows passage for all aquatic species.

Current limiting factors within the subbasin include erosion/sedimentation (Wallowa County- Nez Perce Tribe Salmon Habitat Recovery Plan, revised 1999, p 25), decreased flow, fish passage restriction and loss of habitat complexity (Imnaha Subbasin Assessment, 2002, p 147 & 162).  On pages 35-37 of the Imnaha Subbasin Plan (2004), under Aquatic Objective 10A, connectivity is specifically discussed.  It further states that connectivity is essential to salmonids and represent "one of the most important and readily addressable factors currently limiting aquatic focal species in the subbasin" (Imnaha Subbasin Plan, 2004, p36).  The draft Snake River Recovery Plans (In Press) for spring/summer Chinook and steelhead also speak to addressing fish passage issues within the Imnaha Subbasin.

The culvert, located on Mahogany Creek is a total migration barrier to adult and juvenile steelhead.  This structure blocks at least 3.5 miles of steelhead spawning and rearing habitat and up to 3.5 miles of Chinook and Bull Trout rearing habitat.  The replacement of this structure will allow proper road drainage, and hence, decrease sedimentation. Without replacement, fill from the roadway will continue to wash into the stream creating sedimentation and water quality issues.  Furthermore, this specific site was identified as a physical barrier that should be addressed in the Imnaha Biological Assessment, 2003 (p 316) and by the Nez Perce Tribe (Wallowa County Culvert Inventory, 2004 Draft).  The Imnaha Biological Assessment also rated this stream as "functional at risk" due to this barrier structure (in Imnaha Subbasin Plan, 2004, p36).

The main objectives of this project are to:
• Allow passage to all aquatic species;
• Increase habitat complexity in this area by re-opening more complex habitat areas;
• Increase access to refugia areas during high and low flow events;
• Decrease erosion from the road; and,
• Provide educational opportunities to local high school students.

Project Description:

Introduction  
For several years, numerous agencies, other than the Nez Perce Tribe, have had the desire to replace this known fish passage barrier (Ken Bronec, Forest Service, Pers. Comm., 2004; Brad Smith, ODFW, Pers. Comm., 2005; Gretchen Sausen, USFWS, Pers. Comm. 2005).  Furthermore, it has been stated that this culvert is the highest priority culvert replacement in Wallowa county (Ken Bronec, Forest Service, Pers. Comm., 2005).  The intent of this project is to replace one culvert currently acting as a fish migration barrier with a structure that allows passage to all aquatic species.  The culvert will be replaced with an Open-bottom arch culvert, installed at stream grade to simulate natural channel conditions.  

Existing condition  
As previously mentioned, the Mahogany Creek culvert blocks 3.5 miles of spawning and rearing habitat.  Bankfull width upstream of the culvert is 10.5 feet on average.  Gradient is approximately 3% with the majority of substrate being boulders or cobble with some gravel.  The current outlet drop of this structure is approximately 2 feet.  The USDA- Forest Service Hydrologist has completed both a Wolman Pebble Count and flow calculations near this structure.  The current culvert width is only 6 feet.   This is approximately half of the actual bankfull stream width. Due to inadequate installation of the existing culvert, fill is now eroding over the culvert causing sediment to wash into the stream.  The inadequate width and relatively steep culvert gradient was exacerbated during the extreme flood event that occurred in January, 1997.  During that high flow event, down cutting at the outlet of the structure resulted in the current situation.

One of the major issues that has been repeatedly discussed is the "flashy" nature of this particular stream.  The one hundred year flow calculations that the Wallowa Mountains Zone Hydrologist (Dana Orrick, Forest Service, Unpublished Data, November 17, 2004) calculated for this stream was 174 cfs.  With a 20% allowance of error, the 100 flow would be 209 cfs.  This would indicate that a culvert width of 12 feet, with a rise of 5 feet, would be adequate for this site (Brett Moore, Anderson, Perry and Assoc, Pers. Comm., 2005).   However, this width would not fully allow for natural channel simulation.   This fact coupled with the nature of the system led to the decision to pursue replacing the existing structure with a 15 feet wide culvert.

Benefits:

Replacing this structure will create proper drainage during a one hundred year flood event and allow access to 3.5 miles of habitat.  The benefits of this project are re-opening of critical habitat in the Imnaha sub-basin as well as road stabilization.  The removal of this barrier will result in increased access to more complex habitat types within the subbasin, decreased sedimentation and increased refugia areas during seasonal periods of unfavorable flow conditions (extreme high flows or warm mainstem Imnaha River temperatures).