Although lower Marsh Creek, in eastern Contra Costa County, CA, is heavily impacted by human activities on adjacent and upstream lands, scientists and residents have observed fall-run Chinook salmon in the channel. A grade control dam four miles from the mouth of the creek prevents Chinook from migrating to a more natural, unchannelized segment of lower Marsh Creek that may contain suitable spawning habitat. We assessed the quality of potential spawning habitat in a 1.2- mile reach of lower Marsh Creek. Through pebble counts and visual observations at six study sites between Concord Avenue and Marsh Creek Reservoir, we evaluated gravel quality to ascertain whether gravel sizes are within identified ranges for spawning, gravels are movable by spawning fish, fine sediment concentrations allow for egg incubation and fry emergence, and if gravel bars are large enough for spawning. Using a long profile, we investigated whether the gradient of the channel is acceptable for spawning. Using field measurements at our six study sites, along with USGS stream gauge records, we analyzed whether water velocities and depths during the fall months are suitable for salmon spawning.

Our results indicated that gravels in the study reach are at the smaller end of ranges reported in spawning studies and that gravels in the channel can be moved by spawning fish. Despite the presence of Marsh Creek Reservoir and Dam upstream of our study reach, gravels had a low level of embeddedness at half of six stations surveyed and moderate or high levels at the other half. Gravel bars are large enough for Chinook spawnin g. The average gradient of the channel bed is also acceptable for spawning. Water velocities and depths likely to support spawning are present about 2.5 percent of fall days (correlated to peak storm events). We concluded that the 1.2-mile stretch of lower Marsh Creek contains satisfactory habitat to support fall-run Chinook spawning. Based on these findings, we recommended removal of the grade control structure on lower Marsh Creek to allow salmon migration to the unchannelized section of lower Marsh Creek for spawning.