Dillenbaugh Creek Restoration Proposal

Dillenbaugh Creek and its tributary Berwick Creek is one of the more productive stream systems within the city limits of Chehalis and within the area affected by the proposed Twin Cities Levee Project. However, the lower portions of the Creek, especially after it passes under I-5, have been extensively modified over the years, and the majority of the stream in this area currently runs through ditched wetlands infested with reed canary grass. Without a riparian canopy the waters in this slow-moving section likely reach temperatures in the summer time that are fatal for salmon, and there is insufficient gradient in this mud-bottom section of the stream for any salmon spawning, so this area is likely mostly used as a seasonal migration corridor for juvenile and adult salmon. This function is in danger from the proposed levee system, as it is likely that a flood gate will need to be installed in the lower portion of the creek at or downstream of I-5 to prevent backwater flooding around the levee. This will significantly compromise migration when fish need it most, for adult fish to reach the headwaters for spawning during flood events, and for juvenile fish trying to seek refuge from high velocity floodwaters in the mainstem. Some kind of mitigation is going to be required for these situations, and this proposal is an idea for improvements to the stream that will coincidentally reduce salmon migration impacts from the flood gate while simultaneously providing both new high quality habitat and easier access to habitat outside of the city that is in a much more productive condition.

While not much information has been officially recorded about the various relocations and adjustments to the route that Dillenbaugh Creek used to take to the mainstem Chehalis, it is known that its principle and most productive tributary, Berwick Creek, used to flow into the Newaukum River downstream of I-5 (Bob Amrine, Lewis County Conservation District, personal communication). Dillenbaugh Creek downstream from its present confluence with Berwick comes tantalizingly close to the Newaukum, which at the point that Dillenbaugh emerges from the Rice Road water crossing structure is 1700 feet from a nearly pristine oxbow next to the Newaukum that is all within ownership of WSDOT and under lease to the City of Chehalis as a park. There is a practical route through this park, which is all within the floodplain and floodway of the Chehalis and Newaukum, is used seasonally for ball fields and as a summertime RV park, and both owner and lessee are willing to entertain the idea of a stream restoration project in the park. See the attached photo for a depiction of the project route.

The diversion of the creek at Rice Road and into the Newaukum River would reduce the length of lower Dillenbaugh Creek by 1.93 miles (measured on Salmonscape). As mentioned previously, this entire stretch has no riparian vegetation, little or no in-water structure or woody debris, and is entirely vegetated in canary grass. It is also crossed 6 times in this stretch, by two railroads, I-5 twice, including by on and off ramps, by SR 6, and by a county road, and receives the majority of the stormwater generated by the developed portions of the city of Chehalis. In other words, this is degraded habitat. The diversion would instead add, in addition to 1700 feet of engineered channel within the Chehalis park area that would be equipped with habitat features including riparian plantings and large woody debris as per the WDFW Stream Habitat Restoration Guidelines at http://wdfw.wa.gov/hab/ahg/shrg/index.htm (see an example in Figure 1, an attached graphic from SHRG), permanent access to a nearly pristine 1290 foot heavily vegetated side channel tha