Hahamongna: Lawsuit Settlement Reached for More Environmentally-Friendly Flood Protection


The City of Pasadena’s Hahamongna Watershed Park is a natural treasure well known to area birders. Owing to the variety of habitat, over 240 bird species have been seen at Hahamongna, and around 55 species nest annually. An oak woodland surrounds the main multi-use park areas on the west side of the Hahamongna basin, an extensive riparian forest thrives in the southern half of the basin upstream from the Devil’s Gate Dam, and the Arroyo Seco flows seasonally, from the San Gabriel Mountains, through the central wash toward the dam.

Hahamonga Watershed Park: A national treasure

As a result of the Station Fire in 2009, which extensively burned the upper Arroyo Seco watershed, significant rain in the following two winters deposited 1.1 million cubic yards (MCY) of sediment in the Hahamongna basin. This decreased the capacity of the dam to provide flood protection for downstream communities in future storms. In order to regain dam capacity to handle even more extreme fire/flood events, the L.A. County Flood Control District (LACFCD) planned in the years following a massive sediment excavation and removal project (with some alternatives designed to excavate as much as 4.0 MCY), leading to community outcry against the negative aspects of the “Big Dig”. The Environmental Impact Report for the LACFCD sediment removal project, released in 2014, was successfully challenged in court in a lawsuit brought by Arroyo Seco Foundation and Pasadena Audubon Society, with support from the wider community. The court decision led to revisions to the EIR in 2017, improving some aspects of the project, and contributed to the L.A. County Board of Supervisors decision to reduce the project size from 2.4 MCY to 1.7 MCY. However, ASF and PAS challenged further aspects of the project in a second lawsuit filed in 2017.

The second lawsuit resulted in a tentative decision in June 2019 in favor of ASF/PAS on several issues, which launched settlement negotiations with L.A. County. Meanwhile, the LACFCD project had started in late 2018 with the bulldozing of 50 acres of riparian habitat – a devastating reality to area residents, birders, and other members of the community – followed by the start of sediment hauling in May 2019.


Settlement Agreement, July 2020

The lawsuit settlement negotiations continued for a year, finally resulting in an agreement between L.A. County and ASF/PAS which was approved by the Board of Supervisors on July 7, 2020. The agreement is a better compromise between flood safety and restoration and protection of the environment. The sediment removal project still targets the reduced amount of 1.7 MCY, but several measures will be implemented to reduce the impact on air quality, wildlife habitat, and appearance of the park.

The settlement reduces the footprint of the project by 20 acres, almost 30% of the design area. 14 acres at the north end of the project area, which were to be bulldozed in this second year of the project, are instead preserved. Over 6 stream-adjacent acres at the south end are now removed from the annual maintenance zone, and riparian habitat will be allowed to grow there. These concessions should bring substantial benefit to breeding and migratory birds as well as animals such as mountain lions which use the Hahamongna basin as a corridor between the San Gabriels, San Rafael Hills, and potentially the Verdugo Mountains.

Settlement reduces project footprint by 20 acres

On top of the reductions in project truck emissions resulting from the first lawsuit judgment, additional measures are introduced by the settlement of the second lawsuit to ensure that trucks are meeting standards limiting harmful emissions, and to incentivize use of clean-air trucks.

The habitat restoration program being implemented by L.A. Co. as mitigation for the sediment project will be improved by using a substantially larger fraction of locally-sourced plants. The on-site Hahamongna Native Plant Nursery will be ramping up capacity to propagate Hahamongna/Arroyo Seco plants for this effort.

As the restoration takes shape, Pasadena Audubon will be looking for opportunity to install up to three bird blinds to enhance opportunities to view nature up close in Hahamongna.

The LACFCD project has led to some undesirable aesthetic impacts along the access trails at the south end of Hahamongna. The settlement includes commitments from L.A. Co. to restore the natural ambience at those locations by the end of the main sediment removal project in 2022.

Other terms of the settlement touch on long-term planning for the larger Arroyo Seco region and aim at a spirit of cooperation between L.A. Flood Control District and local environmental organizations including Pasadena Audubon.


Hahamongna, Birds, and You

Hahamongna Watershed Park remains open for birding and other recreation during the sediment removal project. On weekdays during much of the year between now and 2022, the trails at the south end are closed for the sediment excavation and hauling. Birders may see crews on site performing the habitat restoration: a major, multi-year undertaking for which we will have to exercise some patience for seeing the benefits.

Visitors to Hahamongna have an opportunity to play an important role in ensuring that the implementation of the sediment removal project is following all of the rules designed to minimize its harmful impact.

Bird walks in Hahamongna organized by Pasadena Audubon are on hold in mid 2020 as a result of the COVID-19 crisis, but will resume when it is safe to do so. In the meantime, reports of bird sightings by individuals to eBird or the pasadenaaudubon groups.io email group are much appreciated!

The habitat restoration program in Hahamongna is designed to benefit most the riparian-specialist bird species in the long term. We are pleased to report that, already in 2020, a pair of endangered Least Bell’s Vireos nested in Hahamongna – the first ever well-documented on-site nest to our knowledge. We are hopeful that this species will return annually, and Hahamongna will contribute to the long-term preservation of these threatened birds.

hahamonga is a unique haven for birds

To preserve Hahamongna as a refuge for nature, our community will need to remain vigilant and active. (Remember the proposed construction of two new soccer fields that were to be built where Least Bell’s Vireos now thrive)? Pasadena Audubon will keep defending birds and habitat at Hahamongna for the long term; our newsletter and general meetings are great ways to keep up with what’s happening. To find out how to get more involved, please contact the Pasadena Audubon board (see our Newsletter elsewhere on this site.)