Confined to a narrow flood control channel for more than 50 years, Colgan Creek is once again getting some room to roam.
Workers have been sculpting a curvaceous new course for the creek in an ambitious bid to restore the city’s most polluted waterway to a more natural state.
Along a half-mile stretch of Bellevue Avenue near Elsie Allen High School, bulldozers and excavators have been removing hundreds of truckloads of dirt as they reshape and widen the previously flat, straight channel.
“It’s nice to see a project you’ve been working on so long move forward,” said Greg Dwyer, the city’s civil engineer on the project.
Built by the U.S. Army Corps of Engineers around the 1950s, the original channel along Bellevue was about 70 feet wide with steep walls, a largely flat bottom and little vegetation to shade the water.
The new channel profile will be up to 100 feet wider in places with gently sloping banks planted with a variety of native trees, shrubs and grasses. The wider channel will greatly improve the capacity of the creek to carry floodwaters.
Now instead of running in a straight shot, the creek will be allowed take a detour onto farmland the city acquired from an adjacent property owner for the project. Creating a watercourse that curves is important because it mimics the way area creeks would have naturally meandered as they made their way around obstacles on their journey toward the Laguna de Santa Rosa, Dwyer explained.
Such creeks had varied riparian habitat, alternating between shallow riffles, narrow channels of faster moving water, and pools offering safe, cool habitat for fish.
Replicating that variation in water speed in Colgan Creek is a challenge because the creek has low flows most of the year and only drops a few feet in elevation over the half-mile stretch from Dutton Meadow to where the creek crosses Bellevue, Dwyer explained.
The solution was to design a creek so that during low-flow periods, it follows a narrow, curving course within the wider channel, one that passes over and around obstacles such as logs and boulders anchored to the creekbed, Dwyer explained. Outside this deep channel, called a thalweg, the remainder of the new channel will act as a floodplain, giving the excess water an escape route.
The project is an important one for helping improve the health of the Laguna, said David Bannister, executive director of the Laguna de Santa Rosa Foundation.
“If we can restore the creeks that have been turned into flood control channels back into healthy creeks, then there are multiple benefits for everybody,” Bannister said.
The work underway now through October is a $1.6 million piece of a 1.3-mile long three-phase project estimated to cost about $16 million.
Three city elementary schools and high schools are planning to incorporate the restoration efforts into their curriculum, said Brian Hines, project manager for the Colgan Creek Watershed Education Project, a Trout Unlimited program.
Students will learn how to test and track several measurements of water quality including temperature and dissolved oxygen levels, as the restoration progresses, Hines said.
“The great thing about this project is it’s just a great laboratory for watershed education right in front of the high school,” Hines said.