Arroyo Seco Foundation
ASF Logo

Arroyo Seco
Canyon Project

Pasadena Water and Power Logo

Conceptual Design Report Available

Click HERE to download the full report

The Arroyo Seco Canyon Project (ASCP) is an innovative program to improve water resources, habitat, and recreational opportunities in the area between Hahamongna Watershed Park in Pasadena and the Angeles National Forest. Projects like this that contribute to long-term sustainability become all the more vital as our region faces severe drought conditions.

ASCP is a partnership between the Pasadena Water and Power Department and the Arroyo Seco Foundation (ASF) funded by the California Integrated Regional Water Management Program and Pasadena Water and Power Department. ASF provided the initial conceptual design and secured a $3.3 million grant from the State of California for the project.

Project benefits include:

  • Increasing local water supply
  • Enhancing water quality
  • Restoring aquatic and riparian habitat
  • Improving passive recreational opportunities

Project elements include:

  • Rehabilitation of the intake structure. The Arroyo Seco water intake structure diverts water from the Arroyo Seco stream for public use. The upgraded facility will better manage sediments and improve the ability to reliably supply clean fresh drinking water to Pasadena residents.
  • Installation of a public restroom. A restroom facility will be built next to the new parking area at the entry to the highly used Gabrielino Trail. This restroom will improve the water quality of the Arroyo Seco Stream by decreasing human bacteria in canyon water flows and will enhance the recreational experience in the canyon.
  • Removal of unused water facilities. The removal of the Headworks Facility will allow for naturalization of the stream and improve aquatic connectivity.
  • Habitat restoration. The area will be restored with ecologically appropriate woodland, plant, and riverine habitat to encourage birds, insects, butterflies, fish, and frogs to thrive. Invasive species will be removed through clearing of non-native plants.
  • Improving passive recreational opportunities. Installation of interpretive signs, picnic tables, a drinking fountain, and a horse trough will enhance recreational use.

Project Updates

September 2014

Hahamongna Nursery Now Open for Business

Oaks and sycamores, cottonwoods and willow - the Arroyo needs thousands of local native plants to enhance local habitat. Together with Pasadena Water & Power, ASF is developing a native plant nursery for the Arroyo Canyon Project. Now that nursery is open. This month we will begin training core volunteers to become our very important nursery leaders while finalizing the site setup with irrigation, shade structures, and working areas. Please contact Rebecca Shields Moose for details about how you can get involved at 323-426-1156 or

May 2013

The Arroyo Seco Foundation would like to say thank you to all of the wonderful Hahamongna stakeholders who have been so dedicated to keeping Hahamongna natural and have been actively participating in the public process of the Arroyo Seco Canyon Project (ASCP) design.

The Arroyo Seco Foundation and volunteers conducted a headcount on the Gabrielino Trail up the Arroyo Seco Canyon during this past Memorial Day Weekend. The goal of this headcount was to gain an understanding of the usage of this trail during a peak weekend. The information gathered from this count will be used to influence the design of ASCP. For more information and the results of the headcount, click here.

April 2013

We are pleased and excited to announce that ASF has partnered with La Loma Development Company, a sustainable design/build company, to grow native trees and shrubs from our immediate region of the Arroyo Seco Watershed at La Loma's grounds in Northwest Pasadena. The trees grown on La Loma's grounds will be used for tree plantings and restoration volunteer projects throughout the entire Arroyo Seco, including the Arroyo Seco Canyon Project. Through this partnership, La Loma will join the Arroyo Seco Foundation in advocacy for a harmonious approach to watershed management and habitat restoration for the ecosystem around the Arroyo Seco.

ASCP Conceptual Design

Click HERE to download the full Conceptual Design Report

Habitat Restoration

An area of human disturbance to the natural environment over the last few decades, this restoration plan will improve habitat for wildlife and the Arroyo Seco Ecosystem.

A stream restoration plan was developed for about 1,000 lineal fee of the Arroyo Seco stream in the headworks area. The conceptual plan view for the proposed stream restoration elements is depicted on Figure 5.3

A cross section view of the Arroyo Seco River through the restored area depicts the dynamic stream course interacting with a variety of habitat zones given different runoff magnitudes. The recreational users of the area will be able to immerse themselves in the natural area during the majority of the year, while high flows will replenish the habitat during winter rains.

Diversion Alternatives

Using an "Inflatable Dam" the surface water diversion from the Arroyo Seco for municipal water supply will be far more environmentally friendly allowing for aquatic wildlife migration and natural stream hydrology. The system will also be more efficient, allowing Pasadena to take full advantage of their water rights by diverting high flows that would otherwise runoff into the Pacific Ocean.

Parking Lot and Spreading Basins

With the Jet Propulsion Laboratory moving their employee parking to an on site, multi-level, facility the existing parking lot will be replaced by parking spaces for recreational users of the park, and with spreading basins for groundwater recharge.

Parking lot preferred alternative - Under this alternative configuration, a reduced size parking lot will be located on the northern end of the existing JPL East Parking Lot. This parking lot will include approximately 50-75 spots and accommodate space for trees to provide shading. This alternative also incorporates the vehicle routing and guard check needs for JPL employees that use the access road to enter JPL through the east-side entrance.

Spreading Basins - The existing spreading basins do not have enough capacity to take advantage of PWP's surface water rights. Additional spreading basin capacity can maximize the use of the City's surface water rights within the available space of the JPL parking lot.


The new restroom will serve recreational users of the Gabrielino Trail. This will improve water quality, and will be a great feature of convenience enhancing the overall recreation experience for park users.

As a result of stakeholder input, geotechnical issues in the area and structural issues of nearby bridges, the restroom will be located at the public parking area. This toilet will tie into existing disposal lines.

Click HERE to view more figures and tables from the Conceptual Design Report

Project Calendar

IS/MND Release

October 2014 - Release of the draft Initial Study / Mitigated Negative Declaration for the project. This will open the period for public comment prior to adoption of the IS/MND and pursuit of permits.

Public Comment Period

October - November 2014 - Period for official public comments on the draft Initial Study / Mitigated Negative Declaration for the project. Please contact us with any questions you may have, prior to or during this period.

Project Nursery Launch

October 2014 - Propagation will begin at the new nursery site in Hahamongna Watershed Park to produce plants for the restoration area and additional planting areas including the parking lot.

Construction Begins - Groundbreaking Ceremony

Spring 2015 - Groundbreaking of the Arroyo Seco Canyon Project. This festive event will feature tree plantings, walkthroughs and presentations by Pasadena officials and representatives of the Arroyo Seco Foundation. Initial construction is anticipated to begin around the spreading basins and the current JPL parking lot. Construction upstream will begin a few months later.

Project Map

Links & Documents

Pasadena Water & Power Department

Carollo Engineers

Save Hahamongna

Converse Consultants West - 1995
Final Feasibility Study Report, Proposed Percolation System, Leased East Parking Lot, Arroyo Seco Spreading Grounds
Pasadena, CA

Converse Consultants West - 1995
Hydrogeologic Investigation, Devil's Gate Water Collection Tunnel
Pasadena, CA
The purpose of this investigation was to evaluate geologic and hydrogeologic information in the tunnel area and determine the amount of groundwater that can percolate into the tunnel considering various hydrogeologic scenarios

Philip Williams & Associates - 2000
Analysis Flood Hazard, Sediment Management, and Water Feature Analyses, Hahamongna Watershed Park
Pasadena, CA

Arroyo Seco Watershed Restoration Feasibility Study - 2002<
The goal of the study was to develop a comprehensive, long-term plan to restore and enhance the Arroyo Seco. Major elements of the study considered flood and stream management, habitat restoration, water resources and recreational opportunities.

Habitat Restoration for the Arroyo Seco Watershed (ASWRFS Appendix F) - 2002
The Arroyo Seco watershed spans a diversity of habitat types and conditions. Restoration efforts must consider relatively intact, but threatened ecosystems within the Angeles National Forest as well as highly degraded habitats in urban areas.

Arroyo Seco Watershed Assessment - 2010
A comprehensive review of the state of the watershed and of projects to restore and enhance it. ASWA builds on the body of research, plans and reports that have been prepared in the last ten years to chart the course for an Arroyo Seco Greenway that will provide for restoration and better management of our local watershed.


Headworks Facility

Arroyo Seco Headworks Facility. Designed to trap sediment above the water intake facility. This structure is obsolete and serves no purpose at present. The Arroyo Seco Canyon Project will remove this facility and restore the natural habitat around this area.