Spring Creek Culvert Replacement
The Spring Creek Shasta crayfish population, estimated at 5,000 to 6,000 individuals, constitutes about 50 percent of the total species population. The Shasta crayfish population in Spring Creek is located upstream of the four culverts at the Spring Creek Road crossing. For more than ten years signal crayfish were only found in lower Spring Creek (i.e., downstream of the culverts), while an allopatric population of Shasta crayfish was found in upper Spring Creek (i.e., upstream of the culverts).
Although not designed for this purpose, the culverts at the Spring Creek Road crossing originally constituted an effective barrier due to high water velocity, relatively smooth culvert surfaces, and overhanging culvert ends. Over the years, however, corrosion and other factors resulted in partial filling of the culverts with embedded gravel and cobble, filling-in of culvert overhangs, and partial collapse of culvert walls. The material deposited in the culverts increased the roughness at the bottom of the culvert and created a boundary layer that allowed signal crayfish to migrate through the culverts into upper Spring Creek. In July of 1997, a few signal crayfish were found upstream of the culverts with only 800 meters of open stream separating them from the downstream-most Shasta crayfish population.
The two parts of this project were the replacement of the culverts and the eradication of signal crayfish upstream of the culverts. The initial construction plan for culvert replacement involved isolation and dewatering of the excavation area with gravity block cofferdams and a one-week road closure. Because of the porosity of the roadbed and the condition of the culverts, this plan proved impossible. The construction had to be done "wet," which greatly increased the duration and complexity of the project and required extensive use of scuba gear to work in the turbid, cold water. The road crossing now has four new culverts that are longer with culvert openings that extend well above the streambed at a distance from the road. Overhanging culvert ends are necessary to ensure that the culverts remain effective barriers even under the no-flow conditions that can occur during flooding of the Fall River.
In addition to re-creating an effective barrier to signal crayfish, all signal crayfish upstream of the Spring Creek Road crossing had to be eradicated to achieve the overall project goal, which is to maintain an allopatric population of Shasta crayfish in upper Spring Creek. Successful eradication of signal crayfish requires extensive hand removal with either mask and snorkel or scuba. Trapping can be a useful part of a crayfish eradication program, but hand removal is the only way to target all age classes. Eradication is greatly facilitated if the eradication area is confined. Immediately upstream of the Spring Creek Road crossing there is a relatively small area of good crayfish habitat comprised of lava cobbles and boulders that is separated from the rest of upper Spring Creek by an extensive bed of aquatic vegetation with no available lava substrate.
Prior to construction we had successfully removed all signal crayfish from upstream of the culverts. Unfortunately, it was impossible to keep signal crayfish from entering upper Spring Creek during the removal of the old culverts. Most signal crayfish that escaped upstream of the road crossing during construction were removed when the cofferdams were removed. Scuba surveys to remove signal crayfish were continued until no crayfish were found for several surveys over a several month period. An annual survey in the vicinity of the Spring Creek culverts will be conducted to ensure that the culvert ends have adequate overhang above the stream channel and that no signal crayfish are found upstream of the culverts.