Hahamongna is the rare spot in the Arroyo Seco at the foot of the San Gabriel Mountains where the mountainous watershed meets the urban plain. Periodically floods roar into this basin. Bounded on the north by the mountains and Jet Propulsion Laboratory and on the south by Devil's Gate Dam, Hahamongna contains five unique habitat zones that only exist in alluvial canyons near the mountains. Most sites like this in Southern California have been destroyed.

Don't let Hahamongna go the way of other lost environmental treasures in Southern California.

Here is the County's Hahamongna Presenation


Presented to the Hahamongna Watershed Park Advisory Committee on November 30, 2010.

The Meaning of Hahamongna

The original settlers of the region were sometimes called the Hahamongna Indians. The word means "Flowing Waters, Fruitful Valley" in the native Tongva language.

Reports on the County Sediment Removal Plan

Mary Barrie of Friends of Hahamongna provided an excellent summary of the presentation and discussion on Arroyo_Seco_News: Postfire sediment removal in Hahamongna.

Lori Paul added to the discussion there with another interesting post: FYI: Massive Post-Station Fire Excavation in Hahamongna Watershed Park.

HWPAC Resolution

The County's Plan

  • Remove 1,671,000 cubic yards of sediment

  • Construction to begin September 2011

  • Estimated 3 years to complete

  • 300-400 truck loads per day

  • Work Season: May - December

  • Working Days: Monday – Friday

  • Working Hours: 7:30am – 5:00pm

  • Sediment will be hauled to Manning Pit SPS and Azusa Land Reclamation

  • Excavation Area: 50 acres

  • Permanent Road

  • Willow Tree Removal (permanent): 15 acres

  • Native plants include Alder Woodland, Alluvial Fan Sage, Black Willow Series, and Coastal Live Oak Woodland

  • Preliminary Cost: $35 million

2010: County Announces Habitat Removal Program at Hahamongna as "Emergency"

December 3, 2010 - Officials of the County of Los Angeles Department of Public Works unveiled their proposal to remove 1.6 million cubic feet of sediment from the Hahamongna basin and permanently scar fifty acres of prime habitat just north of Devil's Gate Dam to a meeting of the Hahamongna Watershed Park Advisory Committee Tuesday night.

The County intends to proceed with the plan beginning next September without conducting any environment review of the projects impacts or of more sensible and sensitive alternatives.

It's hard to overstate how big and potentially devastating the County's Devil's Gate sediment removal project is. To give some perspective, the massive rehabilitation of the dam that occurred in the 1990s cost $9 million, while the sediment project is now estimated to cost $35 million, almost four times more. The fifty acres that will be permanently scarred according to the County plan represent the most precious and environmentally-sensitive zone in our region's most important natural resource.

The basic problem is that the County has failed to have an effective reservoir management program in place for many decades. While a million cubic yards have flowed into the basin after the Station fire, most of the other five million cubic yards have been there since the great flood of 1938. There has been no sediment removal in the basin since 1994, and then it was a project only one tenth the size of the current proposal. Waiting for an emergency is not an effective management strategy, and the cost and consequences of the current proposal make that clear.

The dam rehabilitation of 1996-97 dramatically changed basin management. A key element of that project was to reconfigure and expand the size of the spillway to make it four times larger than before. The intent was to accommodate a smaller reservoir pool than the original 1920 dam design and to accelerate early release of flood water. Instead of removing millions of cubic yards of sediment in the 90s, the County engineers chose to expand the spillway. The County's current proposal seems to call the wisdom of that design into question.

There are other ways to manage the basin in a more environmentally sensitive manner, but the County does not seem to have taken them seriously. An ongoing program of sediment removal and using flood flows to remove sediment can effectively manage the flood basin and preserve habitat. An environmental review would have forced that consideration of alternatives, but the County's emergency declaration will deny us that.

Pasadena, La Caada Flintridge and Altadena all have a great deal at stake and need to take the lead in insisting that the County develop a more environmentally sensitive plan. This should be a widely discussed issue in the upcoming municipal elections in Pasadena and La Caada . Only public pressure and regulatory hammers can ensure a more sensible and sensitive management plan.

Imagine if we had $35 million to restore the Arroyo's ecosystem rather than destroy it!