SITE CHARACTERISTICS: Slope/Valley gradient: 1% Valley Type: B2 Elevations: 2640
Proximity to water: Instream Channel type: F3 (pre-treatment); C3 (post-treatment)
Other: Project treats 2,280 linear feet of stream channel and associated floodplain
PROBLEM DESCRIPTION: A splash dam and flume were constructed on this site between the years 1915 and 1920 to convey logs through the Benewah valley downstream to Benewah Lake and the St. Joe River. Following the dismantling of the splash dam, sometime in the 1930s, the creek was straightened and the natural floodplains cleared and drained to develop cropland and pastures adjacent to the creek. Straightening increased the channel gradient, which in turn, increased the channels ability to convey bed material and subsequently caused the channel to degrade. This deeper, incised channel was vertically separated from its floodplain and unable to sub-irrigate the riparian vegetation it once depended upon for stability. Recent grazing pressure intensified the problem by reducing plant density and diversity. Streambanks were extremely unstable and instream habitats have little value as summer rearing for cutthroat trout.
Results of the pretreatment channel survey help illustrate these problems: sinuosity=1.06; flood prone width at twice maximum bankfull depth (dmaxbf )=80.2ft.; Entrenchment ratio=1.92; mean bankfull depth (dbf)=1.8ft.; widthbf/depthbf ratio=23.2.
DESCRIPTION OF TREATMENT: Previous work on this site involved implementation of a stream channel design which converted the existing degraded channel from an F4 to C4 stream type by increasing the meander width ratio and sinuosity, lowering the bankfull width/depth ratio, and reducing the channel entrenchment ratio. A new meandering channel, which added nearly 500 ft. of channel length, was constructed and much of the existing unstable channel and floodplain was filled and regraded. The new channel was built just large enough to convey the bankfull discharge within its banks. The controlling riffle elevations were set at a consistent gradient and the bank heights at all the riffles and bends were built so that the banks would overtop simultaneously during flood events. During construction, ten riffles, four j-hook structures, and more than 40 pieces of large wood were placed to enhance streambank stability and instream habitat diversity. Additional implementation work conducted from 2000-2001 included riparian planting and fence construction.
During the first two years following construction at the project site moderate bank erosion was observed at two meanders and material was scoured from several of the constructed riffles, which lowered the controlling elevations for the channel at several key points. Several causes for the failures were identified, including 1) assuming during the planning and design phase that sod would be available on site to serve as a short-term bank protection measure when competent sod never existed, and 2) underestimating the need for large wood throughout the site for increased hydraulic friction, bed load sorting and habitat enhancement.
Maintenance work was initiated in 2002 to address the sources of these problems. Work consisted of stabilizing the two meander bends in question and reestablishing grade control at key riffle locations. A total of 160 yds3 of rock and 6Mbf of large wood was hauled to the site and used to construct four J-hook type flow deflectors and 4 cross-vane grade control structures after Rosgen (2000). Remaining rock was used to lengthen and increase the coefficient of friction at four of the constructed riffles, and large wood was partially buried in stream banks and placed adjacent to the bankfull channel to provide habitat enhancement and increase the protection of floodprone areas.
PROJECT TIMELINE: Project implementation required a site inspection by a certified archaeologist and subsequent clearance by the Tribal Cultural Officer and the SHPO (6/00), as well as a wetland delineation (8/00) and USACOE 404 permit (9/00). Phase I channel reconstruction was completed 10/00 and the remaining Phase II channel work was finished by 7/01. Plantings were completed in fall 2000 and also in 2001. Riparian fencing was completed 9/01. The maintenance work conducted in 2002 was completed under USACOE permit NWW No. 001201070 prior to its expiration.
PROJECT GOALS & OBJECTIVES: Restore the channel and floodplain to a naturally appearing and functioning geometry using native materials. Create a stable creek and riparian environment that will naturally develop into optimal fish habitat. Restore a proper bedload balance within the project reach and minimize the flood potential for adjacent cropland.
Project effectiveness should be evaluated by follow-up measurements of channel bed-form, substrate and fisheries response consistent with recommendations in the RM&E Plan.
RELATIONSHIP TO SCOPE OF WORK: The operations and maintenance work completed at this site fulfills the Program commitments for Objective 4, task 4c under the Phase I FY 2002 Scope of Work and Budget Request (Inter-Governmental Contract #10885).
Pre- and Post-Construction Images
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