Conlon Creek Restoration Project

Conlon Creek Restoration Project

The Project will restore long-term channel stability, ecological functions, and aquatic habitat along Conlon Creek. The Project is situated within Muir Woods National Monument and located on Conlon Creek (Creek), a tributary to Redwood Creek in Marin County (37.890932; -122.564626). The Creek supports breeding and rearing habitat for native fish and amphibian species, such as California giant salamanders (Dicamptodon ensatus) and juvenile steelhead (Oncorhynchus mykiss). In 2015, a damaged culvert failed and caused severe erosion to an unnamed tributary resulting in temporary, emergency stabilization repairs to occur in 2016 (RGP 5 SPN No. 2016-00033). The proposed Project will provide a long-term solution to the culvert failure by restabilizing about 250 feet of Creek and 75 feet of an eroding gully that is actively contributing fine sediments downstream to Redwood Creek. Project Restoration activities will include the following: 1) The existing and damaged, 80-foot-long corrugated metal pipe (CMP) culvert will be permanently removed and disposed of at an appropriate offsite facility. All concrete headwall materials will be hauled away and will not be repurposed onsite. An earthen channel will be constructed in the former culvert alignment (between river stations 170 and 250) with a similar channel geometry to the existing grade. 2) The 75-foot-long actively eroding gully will be stabilized and backfilled with onsite material to reduce fine sediment delivery. 3) The footprint of the culvert failure and headcut will be stabilized by installing a combined engineered, roughened rock ramp and 105-foot-long rock cascade pool structure in the over-steepened channel reach, located upstream of the culvert removal and at the existing road crossing. The rock cascade-pool structure will consist of a series of rock cross-vanes and three, intervening 20-foot-long rock-lined pools created between the six rock weirs with a one-foot drop as described in the KHE Design Memo (Kamman Hydrology and Engineering, Inc.; May 7, 2020) and 90% design plans dated April 15, 2024. About 412 cubic yards of gravel/cobbles and about 415 cubic yards of Class III and V rock riprap will be placed in the Creek to create the rock ramp, rock pools, and weirs. The rock pool design will provide improved fish passage and aquatic habitat conditions for special status species. 4) Large rock boulders will be integrated into the rock ramp and keyed in at the confluence of the gully and Creek to minimize erosion downstream (i.e., downcutting) and to enhance fish passage. The rocks will provide grade control and roughness elements that will generate mixed areas of high velocity and sheltered low velocity resting sites for fish habitat. 5) A minimum of two bay trees will be repurposed and used as large woody enhancement along the Creek channel. The tree trunk will be positioned toward the Creek bank/terrace with the rootball facing the edge of the new channel. One of the rootballs will be placed at the minor bend in the channel to ensure flows are directed towards the weir structures.

Marin

San Francisco Bay Area, Unincorporated