Devil's Gate Dam Sediment Fact Sheet

Dam History

Devilís 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. County Flood now wants to reduce 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 three years ago, when it removed 190,000 cubic yards, only 8% 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 Devilís 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 Work Group that also took the "Go Slow" approach. Their recommendations were unanimously approved by the City Council twice in 2014.

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.

Amount To be Removed

The Big Dig plan, approved by the County Supervisors would remove 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 claims their Big Dig program will 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.

Continuing Opposition

There has been tremendous opposition to the Countyí 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. 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. At a hearing on March 23rd, Judge Chalfant will consider whether the cumulative impacts of the project have been adequatly documented. He will also rule on whether the County Supervisor will have to reconsider the project for approval.

Endangered Species

Federally endangered species such as the Least Bellís 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. During the summer they proposed trying to restore riparian habitat in upland areas, but the environmental regulators aren't going for it. Now two years after program approval, they still don't have the permits necessary to begin their sediment removal program.

The County's Devastating Program

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 Countyís 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 Countyís refusal to consider a more sustainable sediment management program that takes into account community concerns about traffic, noise, habitat destruction, etc.

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 Let's not let excavators, bulldozers and outdated engineering approaches destory it.