Devil's Gate Dam Sediment Fact Sheet

Dam History

Devil’s Gate Dam in Hahamongna Watershed Park was the first dam built by the LA County Flood Control District in 1920

Sediment Level

Over the years sediment has built up behind the dam, but the level of sediment stabilized in the 1930s and has ranged from 2.5 to 4 million cubic yards (mcy) since then. In 2014 County Flood proposed reducing the level to 1.5 mcy, a level not seen since shortly after the dam was built.

Hahamongna Watershed Park

Pasadena established Hahamongna Watershed Park in 1993 to recognize the unique environmental values and resources found in that rare alluvial canyon at the base of the San Gabriel Mountains. Pasadena has consistently urged the County to conduct an ongoing sediment management program rather than waiting for massive removal programs.

History of Sediment Management

County Flood Control has not removed any significant quantity of sediment from the basin since 1994, twenty six years ago, when it removed 190,000 cubic yards, only 11% of the amount the County now wants to remove.

Station Fire

The Station Fire in 2009 and subsequent floods that year and in 2010 added more than a million additional cubic yards to the 2.7 million cubic yards that had previously accumulated over the years.

Original Plan

After the Station Fire, the Flood Control District proposed removing 1.67 million cubic yards from the dam basin in 2010 on an emergency basis, but the plan was rejected by the Regional Water Quality Control Board as too large and disruptive. In April 2011 the County Board of Supervisors instructed the Flood Control District to conduct a full environmental impact report on their sediment removal program for Devil’s Gate Dam.

Community Support

A series of public meetings demonstrated overwhelming community support for a slow and sustainable sediment management program that would reduce impacts on the surrounding neighborhoods and protect the precious habitat in Hahamongna. The City of Pasadena established a Sediment Working Group that also took the "Go Slow" approach. Their recommendations were unanimously approved by the City Council twice in 2014. Those recommendations included 1) removing less soil and habitat, 2) taking a longer period of time to reduce neighborhood impacts, and 3) an ongoing sediment maintenance program, and 4) a surgical approach to habitat protection when sediment is removed.

Big Dig Approval

In October 2014 the Flood Control District released their Big Dig program, which was approved by the County Board of Supervisors in November 2014 by a 4-1 vote.


The cost of the sediment removal program is now estimated to be as high as $100 million dollars. The trucking contract alone is for $73 million. A publicty contract has also been granted for $3.6 million. And those numbers do not include the mitigation program, which will also be very expensive.

Amount To be Removed

The Big Dig plan, approved by the County Supervisors in 2014 would have removed 2.4 million cubic yard of sand and sediment over a 3-5 year period. Critics charge such an unprecedented massive removal of sediment can only be explained if Arroyo water is to be diverted through a pipeline five miles east to Eaton Canyon, a project not mentioned in the County's Environmental Impact Report.

Air Pollution

The County claimed their Big Dig program would use "low emission" trucks, but they intend to use 425 diesel trucks per day, the same trucks that emit the deadly Black Carbon so injurious to public health. The revisions state that the Flood Control District will use trucks that meet the 2010 standard, but these trucks will be affected by steep grades as well as long waits for loading and traffic congestion that will have a significant impact on their emissions. In addition, the FEIR admits that the the project will have enormous impacts on sensitive receptors: "The Project is located adjacent to residential areas and 10 schools within one half mile; i.e. Crestview Preparatory, Franklin Elementary, Hillside, Jackson Elementary, La Cañada High, Nanny’s Nursery, Odyssey Charter, Pasadena Unified, Sycamore, and Woodbury Preschool Village."

Continuing Opposition

There has been tremendous opposition to the County’ Big Dig program from local communities, the City of Pasadena and environmental advocates.

Failure to Obtain Permits

The County Flood Control District has had a very difficult time securing the permits needed for their project because of the inadequacy of their environmental document and mitigation plan. This has delayed the Big Dig implementation, which was originally set to begin in the Fall of 2015.

No Big Dig Lawsuit

In December 2014 the Arroyo Seco Foundation and the Pasadena Audubon Society filed a lawsuit challenging the Big Dig Program on environmental grounds. Mitchell M. Tsai and several other attorneys represented the petitioners. At a hearing on February 14, 2017 Judge James Chalfant ruled that the Flood Control Districts environmental impact report for the projec was critically deficient regarding air pollution and environmental mitigation. He rejected the concern that the cumulative impacts of the project were not adequatly documented. He also ruled that the County Supervisor will have to reconsider the project for approval.

Endangered Species

Federally endangered species such as the Least Bell’s Vireo as well as several species on the California State Species of Special Concern list have been sighted within the area proposed by the Flood Control District to be demolished, yet federal incidental take permits for these species have not been applied for.

Mitigation Program

The Flood Control District failed to specify and analyze a mitigation program as part of their EIR for the project. In 2016 they proposed trying to restore riparian habitat in upland areas, but the environmental regulators aren't going for it. Finally in August 2018, almost four years after program approval, they received approval for a habitat mitigation plan that include remote riparian restoration in the Antelope Valley near Lancaster.

The County's Devastating Program

ASF and PAS feared LA County's Big Dig program would be devastating to our communities and to our region's most important environmental treasure because of:

  1. The County’s failure to develop an ongoing sediment management program, failing to remove any significant sediment from the dam for more than 20 years;
  2. The permanent destruction of more than 50 acres of rare streamzone habitat in one of Southern California's most precious alluvial canyons;
  3. The noise, dust, air pollution, traffic congestion and negative health impacts the project would cause;
  4. The County’s refusal to consider a more sustainable sediment management program that takes into account community concerns about traffic, noise, habitat destruction, etc.

Judge James Chalfant's Order

On, March 23, 2017, The Honorable James C. Chalfant ruled that the final environmental impact report (FEIR) for the Devil's Gate Sediment Removal Project was critically flawed regarding air quality, mitigation measures and cumulative impacts of the project. He ordered the County Flood Control District to revise the elements of the FEIR dealing with those issues to ensure that they are fully compliant with CEQA. He ordered that the Flood Control District's revision should then be made available for a forty-five day review period. After the district has reviewed the public and agency comments regarding the revisions, the program and the EIR must be presented to the County Supervisors and they must certify that the entire EIR is fully compliant with CEQA. The judge then conducted a further review of the program and EIR.


On July 24, 2017 the Flood Control District issued revisions to their Final EIR. The revisions deal with three major issues:

  1. Air QualityIn the FEIR the Flood Control District promised to use only diesel trucks that meet the 2006 EPA standard. In the revisions the District agreed to only use diesel trucks that meet the 2010 standard but did not consider the use of low-emission trucks.
  2. Mitigation
  3. Judge Chalfant ordered the Flood Control Distict to prove that a 1:1 mitigation ratio for replacing critical habitat in the Hahamongna basin would fully protect the habitat and the species that rely on it. The Flood Control District revisions do not document the matter with scientific or technical analysis but simply state that the 1:1 standard has been used on other projects.
  4. Cumulative Impacts
  5. The District again fails to adequately evaluate what the biological impacts of related projects, such as the Trans-Altadena Pipeline, which would export water from Hahamngna and transfer it to the County's spreading basins five miles east in Eaton Canyon.

Response to Comments on Revisions

In late July the Flood Control District posted revisions to key parts of the EIR and submitted the revised report to the public for a review period that ended on September 18, 2017. The Flood Control District then responded to the comments that were made by inviduals and agencies during the review period. You can view the responses here: Responses to Comments on RFEIR

Supervisors Scale Back Big Dig

The Los Angeles County Flood Control District went back to the the Board of Supervisors on Tuesday, November 7, 2017 to request another approval of their Big Dig program for Hahamongna Watershed Park. The Supervisors were required to certify that the environment impact report for the project is fully compliant with the California Environmental Quality Act (CEQA).

At the Board of Supervisors' Meeting of November 7th, Supervisor Kathryn Barger introduced a major amendment to the Flood Control District plan that reduced the amount of sediment to be removed from 2.4 million cubic yards to 1.7 mcy and added ecosystem restoration and water conservation activity to the project. The amendment was unanimously adopted by the Supervisors, but numerous concerns about the number of trucks and environmental mitigation matters remained unresolved.

Second Lawsuit

The Arroyo Seco Foundation and Pasadena Audubon Society, the petitioners in the No Big Dig lawsuit, were pleased that the amount of sediment to be removed from the Hahamongna basin would be reduced 30% as the result of Supervisor Barger's motion, but when the two groups sat down with Flood Control officals to discuss the remaining concerns, it became clear that the Supervisors' motion did not reduce the negative impacts of the project proportionately. There would still be 425 trucks per day from dirty diesel trucks, massive traffic congestion and air pollution. And sadly the Flood Control engineers refused to budge on their intention to create permanent 50 acre weed patch and pit in the middle of the Hahamongna streamzone, destroying all valuable habitat there every year in perpetuity.

Even though the environmental groups had received a favorable ruling from Judge Chalfant, all of the proposals from ASF and PAS to reduce the negative impacts of the project were flatly rejected. The two groups then decided to file an appeal of the first lawsuit and to file a second lawsuit to address issues that had not been determined in the first lawsuit.

Here Come the Trucks

In the fall of 2018, the Flood Control District announced that they would begin their massive program in October, but now there was a delay. Their initial task was to destroy 70 acres of prime riparian and alluvial scrub habitat and truck it off to Scholl Canyon. Then they will begin their excavation of sediment next spring. After that four hundred and twenty-five trucks each work day would descend into the Hahamongna basin to be filled with sand, soil and rocks, which would be hauledd off to landfills in Irwindale and Sun Valley.

Destruction Begins

At the end of November, 2018 contractors for the Flood Control District began their destruction of all habitat in a 50 acre zone immediately north of Devil's Gate. Within a few weeks they had decimated the willow forest and related high value riparian habitat and turned it into sawdust, which they then trucked off to nearby Scholl Canyon landfill. By the end of December the Hahamongna basin had a barren 50 acre devastation zone in the middle. This marks the area the Flood Control District intends to permanently denude even after their excavation program is complete in about four years. They now state that their sediment excavation and trucking program will begin in April, 2019.

Excavation Begins

In May, 2019 after numerous delays, the County's bulldozers, excavators and trucks dug into the Hahamongna basin. By the Fall, they had removed approximately 450,000 cubic yards of sediment from the basin, leaving a barren 50 acre pit in the Arroyo Seco streamzone behind Devil's Gate Dam.

Favorable Tentative Ruling

In June 2019 Judge Chalfant released a tentative favorable ruling on the second lawsuit filed by ASF and PAS in 2017.

The Honorable James C. Chalfant of Los Angeles Superior Court issued a 22-page tentative decision in favor of the Arroyo Seco Foundation (ASF) and Pasadena Audubon Society (PAS), finding that the Flood Control District's environmental impact report was inadequate in several key areas. Rather than contesting the judge's decisive ruling, Senior Deputy County Counsel Michael Simon asked for a postponement of the trial to allow the County to get together with ASF and PAS to develop a comprehensive settlement agreement. ASF and PAS agreed to the postponment and have been engaged in settlement discussions since then.

Supervisors Consider Settlement

After lengthy discussions, ASF and PAS have worked out a settlement agreement with County officials that will reduce the negative impacts of the project and provide for a more environmentally-sensitive approach to the future of Hahamongna. At their meeting of July 7, 2020, the County Supervisors will consider approving the settlement agreement, which is the recommendation of Supervisor Kathryn Barger.

Hahamongna is a unique Southern California treasure, a rich legacy for future generations. Let's not let excavators, bulldozers and outdated engineering approaches destroy it.