A hydrologic and hydraulic study that identifies restoration opportunities in the Rose Creek Watershed has recently been completed by San Diego Earthworks, thanks to funding from the California Coastal Conservancy. Through extensive field work, data collection, mapping and modeling, The Rose Creek Watershed Wetland, Riparian and Water Quality Restoration Opportunities Analysis recommends a variety of specific restoration projects to improve one of our region’s most beautiful recreational and environmental assets. Three iterative restoration scenarios were modeled to identify 19 sites that would be suitable for restoration based on landform, adjacency to the creeks, and non-native or lower quality existing habitats. The goal was to identify and analyze specific opportunities to restore wetland and riparian functions, including floodplain reconnection, habitat enhancement and hydrology and water quality improvement.

The Rose Creek Watershed in western central San Diego County drains over 23,000 acres into Rose Creek, which is the major freshwater source for Mission Bay. Although the watershed’s headwaters originate on the relatively undeveloped land of Marine Corps Air Station Miramar, it becomes increasingly urbanized as it moves west towards Mission Bay. The watershed transects the communities of University City, Clairemont, and Pacific Beach, as well as Rose Canyon Open Space Park and Marian Bear Memorial Park (a.k.a. San Clemente Canyon). The watershed’s hydrologic and hydraulic functions are severely degraded, resulting in environmental impacts to stream channels, ecological integrity, and water quality.

“Because the majority of the watershed was fully developed by the mid 1980’s, few, if any best management practices (BMPs) were implemented to help mitigate for the increase in impervious surfaces,” explains the report. “The lack of BMPs has allowed for not only increases in flows, but has also caused a substantial increase in the pollutants entering Rose and San Clemente Creeks compared to historic conditions.”

A fully functioning watershed stores rainwater by spreading out the water flow across a floodplain. This decreases the water’s speed, reduces flood peaks, and distributes the water throughout the environment over a longer period of time. When impervious surfaces such as roads and sidewalks are added as a result of urbanization, the natural flow of rainwater is modified and diverted. The increased amount and speed of the rainwater flow causes “channel incision,” meaning the creek deepens into the canyon valley bottom while creating steep slopes on either side. This process causes problems for riparian and wetland ecosystems, such as restricting water flow to the root systems of native trees, making the water more “muddy” (which affects water quality and aquatic plant growth), and increasing downstream flooding during storms.

A clear example of channel incision.

The sycamore tree is emblematic of San Clemente Canyon, and many grow in Rose Canyon as well. Unfortunately, it may be disappearing due to the impacts of channel incision. The presence of hundreds of these trees scattered throughout the canyons is evidence that this valley was formerly a floodplain that received regular flooding. Urbanization of the surrounding area has resulted in significant changes to the natural floodplain. For example, some sycamores show signs of stress, and few new saplings or seedlings have been observed. Although City of San Diego biologists suggest that a fungus may be contributing to the trees’ demise, the five feet of channel incision that has occurred throughout the floodplains which is restricting water flow to these trees is also a critical piece of the puzzle.

A sycamore tree that has been damaged by the degrading floodplain structure.

Restoring the water flow in the Rose Creek Watershed poses a challenge due to existing habitats, gas lines, concrete-lined channels, existing structures, railway lines, and potential future uses. However, active management through restoration can play a significant role in the preservation of this recreational amenity that connects inland communities with Mission Bay. Restoring incised channels to better replicate their original floodplain structure would allow natural processes, such as the growth and recruitment of new sycamore trees, to occur and sustain themselves over time.

The report suggests a variety of specific restoration strategies and techniques such as the creation of step pools to reverse the effects of channel incision and enhance the watershed’s natural structure. For example, Rose Canyon Site 3 is severely incised, with no access to a floodplain where water can be captured and stored. This has resulted in a dominance of non-native grasses. Step pools and site grading are recommended techniques to lower the adjacent topography.

In-stream step pools and are a recommended restoration technique that can rebuild channel elevation in the Rose Creek Watershed.

The report was prepared by cbec, inc., eco-engineering, Trestles Environmental Corporation, and KTU+A for San Diego EarthWorks. The project was identified as an Action Item in The Rose Creek Watershed Opportunities Assessment, a long-term planning document that identifies ways to improve the watershed’s biological, cultural and recreational assets. The report can be reviewed online and the input files can be made available. Contact Kelly Makley, the Rose Creek Watershed Coordinator, at kelly@rosecreekwatershed.org for more information.