Rendering by Cities of Pasadena and South Pasadena.
In October 2024, I volunteered with the Pasadena Complete Streets Coalition for a Walktober event organized by Day One for Councilmember Hampton of Pasadena in Hahamongna.
By Pilar Reynaldo
The walk included members of Pasadena Water and Power, Ursula Schmidt and staff, as well as watershed coordinators with the Safe Clean Water Program (Measure W). We learned about Pasadena’s historic water rights, the history of Hahamongna, and the Devil’s Gate Dam. The watershed coordinators unveiled two future watershed projects, both proposed for the Lower Arroyo—a space I know and love.
After the Walktober event, I dove into learning everything I could about the joint environmental project led by the City of Pasadena in partnership with South Pasadena.
The Arroyo Seco is one of our region’s most cherished landscapes, a place where people walk, run, horseback ride, and reconnect with nature. Unfortunately, the water flowing through the channel carries bacteria and heavy metal levels that exceed state health standards before continuing downstream to the Los Angeles River and the ocean.
The Arroyo Seco Water Reuse Project is designed to address these problems by removing pollution from urban runoff—capturing stormwater to recharge groundwater, restoring habitat, and creating new parkland in spaces currently fenced off or inaccessible. The projects intend to comply with the requirements set by the Los Angeles Regional Water Quality Control Board to reduce bacteria levels in waterways designated for recreational use.
Testing along the Arroyo Seco Channel during dry weather conditions has repeatedly found bacteria levels above allowable limits where San Rafael Creek flows into the Arroyo Seco Channel. The project would ultimately treat runoff from a drainage area of approximately 5,005 acres, using what the U.S. Environmental Protection Agency defines as nature-based solutions consisting of constructed wetlands.
The wetlands are designed to mimic natural ecological processes. Wetland plants, soil, and microbial activity will help break down contaminants as water infiltrates for possible groundwater recharge. Water that does not infiltrate will flow through an underground filter to be treated before returning to the Arroyo Seco Channel and, ultimately, the Los Angeles River.
This map shows the storm drain systems that flow directly into the Arroyo:
(Map by Noel McCarthy)
Historic waterways already altered
Most of the watershed has already been modified by decades of infrastructure, including the channelization of the Arroyo Seco itself and the lower portion of San Rafael Creek in the early to mid-1900s. Natural flows in San Rafael Creek were altered as early as 1876, when Johnston Lake was formed to capture springs and upstream inflows. The proposed project would not significantly alter the creek’s hydrology; instead, it would enhance the existing non-functioning wetland into which the water flows to improve water quality.
Golf course irrigation is not the project’s purpose
Officials have emphasized that the plan is not increasing or modifying the current or historic water diversion levels that have provided irrigation to the nearby Arroyo Seco Golf Course since 1955. This man-made diversion dates back to early local settlements, when a dike system was first constructed to divert water from the Arroyo Seco for use by tribal communities, and was later reconstructed in the 1940s as an underground piping system.
Environmental review and tribal consultation
The project is currently undergoing environmental review and has completed tribal consultation with local tribes per AB 52, California’s tribal consultation law. Consultation began in April 2023 during preparation of a Mitigated Negative Declaration and was conducted again in April 2025 as part of the preparation of an Environmental Impact Report. Requested mitigation measures from the responding tribe have been incorporated into the environmental documentation and requirements.
Tree removal and replanting
Construction will require the removal of 136 trees across the two project locations, 82 of which are invasive Mexican fan palms considered a fire hazard. The cities intend to plant 216 trees, as well as over 1,100 native plants and shrubs at the San Rafael site and more than 2,100 at the San Pascual site.
Despite what opposition has said, the majority of existing keystone species (oaks and sycamores) will not be removed. The final number of trees after project completion will be 282—81 more than currently exist. The City of Pasadena is propagating select species for both sites. Fifty-two native oaks are being grown for planting, including coast live oak, Engelmann oak, and valley oak. The City has also purchased boxed native western sycamores from a local nursery for both sites; some are already 10 to 15 feet tall.
Sycamores have an average growth rate of 2–3 feet per year, meaning they could grow an additional 10 to 15 feet within the first five years after construction. They can reach heights of up to 70 feet within the first 20 years.
Public access to remain open
Project planners confirm the project will not reduce access to passive recreational areas; in fact, access will be greatly increased. Much of the area currently functions as a neglected drainage corridor choked with invasive species rather than usable open space.
At San Rafael, increased passive recreational opportunities will be provided west of the creek in an area currently overgrown, fenced off, and inaccessible. At the San Pascual site, fencing will be limited to split-rail fencing along the Arroyo Seco trail between the project site and Stoney Drive—existing fencing that is in need of repair. Additional split-rail fencing will be installed around infiltration basins to protect public users and equestrian riders. The rest of the site will be open and far more accessible than it is now.
Some residents have understandably worried that the project could affect San Pascual Park. It does not. The project area sits on the opposite side of the Arroyo Seco Channel and does not impact the park’s trees, playgrounds, or open space.
This work is being funded by Pasadena and South Pasadena through regional water-quality programs—not by the City of Los Angeles.
In a perfect world, the Arroyo Seco would never have been channelized and filled with concrete. Many of us wish that leaders at the time had chosen the Olmsted brothers’ solution after the great flood of 1938 instead of the Army Corps’ concrete channelization of the Los Angeles River. But that didn’t happen—and since then, the floodplain we know as Los Angeles has been built out with housing and infrastructure.
The reality is that purchasing and removing all the development built in the floodplain since channelization is financially unfeasible now and for the foreseeable future. This proposed model—creating wetlands that parallel the flood control channel—is a practical approach that can and should be replicated where space and funding allow.
Completing these projects does not eliminate the possibility of a future in which parts of the channel are restored to a more natural state if the will and funding align. In the meantime, these projects will help ensure that water flowing into the river and eventually the ocean is not contaminated with harmful toxins, while restoring native habitat. That benefits not only those of us who spend time in the Arroyo, but also migratory and pollinating species.
As decisions move forward, leaders and agencies in Pasadena, South Pasadena, and Los Angeles—including City Council offices, water agencies, and parks departments—will be listening. If you value cleaner water, restored habitat, and more accessible green space in the Arroyo, now is a good time to share your support.
These projects are practical steps toward a cleaner, greener, and more welcoming Arroyo for everyone.
Pilar Reynaldo is an avid Arroyo Seco walker; community advocate, master gardener, lover of nature. She advocates for greener, safer and more pedestrian friendly – safer – streets.
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