Baseline Conditions: Aquatic

Baseline Conditions: Aquatic

Both sides of the river were, until recently, occupied by residential structures which were protected by levees and revetments. The Orting Hill Levee presently runs along the RB from the new 154^th Street Bridge downstream to the abutment of the old Elliott Bridge. This levee/revetment restricts channel migration and, to a limited degree, prevents flooding of the properties landward. However, these properties still experience relatively frequent flooding and so were purchased by King County as part of its Flood-Prone Property Buyout program over the last several years. All of the residential structures in this area have since been removed.

Another levee, known as the Elliott Bridge Levee, begins on the LB about 1,000 feet downstream of the 154^th Place SE Bridge and forces the river to turn slightly to the north. Formerly, this levee constrained the river as it approached the old and now-removed Elliott Bridge and it ends after about 500 feet at the remaining concrete abutment and fill prism that was the southern approach to the old bridge. A corresponding fill prism exists across the river on the RB. The LB is unprotected and relatively natural downstream of these old bridge approaches and abutments.

The Punnett Briggs Revetment armors the RB beginning about 400 feet downstream of the old bridge approaches and protects several residential structures landward. Between the old bridge approaches and this revetment are two remaining residential structures with relatively low, unprotected banks.

The historical presence of bridges over the river in this location (including a predecessor of the Elliott Bridge that was located near the upstream end of the Punnett Briggs Revetment) and the use of levees and revetments to restrict channel migration in their vicinity have caused the river channel to become narrower and simpler through this reach. Historical sources going back to the original Government Land Office maps of the late 1800’s show that the channel used to migrate frequently across its floodplain, especially to the south of its present location. The narrower and deeper channel that has resulted also has steeper, often rock-protected banks, all of which have decreased the frequency with which trees mobilized by the river stick and persist in this reach of the river. During the planning phases of this project, virtually no large wood was present in the channel in this reach of the river.

Fish Use

The Cedar River is used by coho, sockeye and ESA-listed chinook salmon, coastal cutthroat, rainbow and ESA-listed steelhead trout, and other species. The greater Elliott Bridge Reach (extending upstream and downstream from the project area) contains heavily-used salmon spawning grounds (just downstream of the 154^th Place SE bridge), previous side/backwater channel restoration projects (downstream of the project site), and log jams resulting from landslides (downstream of the project site).

In addition to the levees and revetments described above, there are numerous residential structures located immediately on the river channel banks upstream of the 154^th Place SE Bridge and more levees and revetments both upstream and downstream of the project site. The project site is just outside the city limits of Renton, Washington and the entire lower and middle Cedar River have been subject to residential, commercial and infrastructure development for well over 100 years. The resulting river is constrained and constricted for most of its length and lacking in off-channel habitats that are critical for rearing of several species of juvenile salmonids.

Development of the Cedar River valley and floodplain has also affected riparian vegetation and function. Few stands of mature conifer trees remain on the river banks and consequently comparatively few such trees are recruited and mobilized by the river during floods and storms. Degradation of riparian vegetation communities, along with residential, commercial and infrastructure development and restriction of channel migration have resulted in a mainstem channel that is simplified and lacks the hydraulic diversity that would be present in a channel allowed to migrate naturally.

Historically the mainstem channel migrated in this reach as shown in Figure 8.

6. Mitigation Work Plan

1. Right Bank Wetland, Buffer, and River Margin

Mitigation actions on the right bank include: establishment of 2.16 acres of wetland and 0.28 acres of river margin, enhancement of 2.65 acres of riparian upland enhancement (of which 1.15 acres are non-credit generating), removal of 300 feet of existing rock revetment including a portion of the existing Orting Hill Levee, and placement of habitat elements. Habitat elements include the placement of snags, debris piles and large woody debris. An in-stream scour structure that will be installed in the RB reach is discussed separately below.

The design of the RB wetland and river margin habitat is based on interaction with the Cedar River and associated groundwater to support a wetland mosaic of forest, scrub-shrub, and emergent vegetation communities and to reconnect the river to its floodplain within this reach. The right bank wetland will be created by excavating approximately 18,000 cubic yards of existing material. The wetland will be mostly flat, with gradient that slopes gently downstream and towards the Cedar River. In order to establish wetland hydrology, the elevation of the RB wetland is such that flood flows on the Cedar River will occasionally inundate the entire wetland.

The RB wetland will be planted with a mosaic of wetland buffer, forested wetland, scrub-shrub, and emergent vegetation communities as detailed in the EBR Permit Plan Set (Attachment 1). Planting will be accomplished in stages with the initial planting focusing on robust, fast growing species that will establish quickly and provide shade and cover over the site. Later stage planting will focus on slower growing trees, shrubs and emergent species that are not as suited to post construction conditions.

The removal of existing revetment rock and excavation of the RB wetland will increase the connectivity between the river and its floodplain. This interaction will be most persistent along the RB river margin. The river margin will be an area of low-angle banks that transition from the river to the wetland. The area will be vegetated with fast growing woody-stemmed species such as willow that will provide cover and shade. Woody debris consisting of conifer logs with intact branches will be placed along the margin to provide cover, habitat complexity and allochthonous input. These habitat logs will be anchored to the bank using large rock (below grade) attached with chains, and oriented with their tips facing toward the river (see Attachment 1, sheet 9). This edge habitat along the river margin is important to juvenile chinook salmon and other fish known to spawn within this reach of the Cedar River.

Habitat elements including the placement of snags, debris piles, and LWD will be installed in the RB wetland and buffer to enhance the ecological benefits of the site. Snags, consisting of conifer logs buried in growth position will be placed in both the wetland and the buffer. Debris piles consisting of woody debris and brush will be interspersed throughout the wetland to provide terrestrial wildlife habitat and topographical diversity. Logs with root wads will be placed along the river margin and also embedded into the existing berm that was the road prism of the former Elliott Bridge at the downstream end of RB wetland where it joins the Cedar River. These logs will protect that topographic feature, which provides some flood protection to downstream properties, from erosion during high flows.

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