Arroyo Seco Canyon Project — Areas 2 & 3
Fact Sheet

Project Description

The Arroyo Seco Canyon Project (ASCP) is an attempt by the Pasadena Water and Power Department (PWP) to increase the amount of water it diverts from the Arroyo Seco stream for municipal purposes. It includes two major elements:

  1. Dam and Diversion Replacement
  2. Expansion of the Spreading Basins at the mouth of the Arroyo in Hahamongna Watershed Park

The ASCP Draft Environmental Impact Report (DEIR) is available for agency and public comment until July 31, 2020.

Why is Hahamongna So Vital?

ASCP is located in Hahamongna Watershed Park near the mouth of the Arroyo Seco at the critical junction where the mountain stream enters the urban Arroyo on its way to the Los Angeles River. Hahamongna contains five unique habitat zones and is home to several endangered and threatened species. This is our region’s greatest natural treasure, vital for water resources, habitat, wildlife and nature-appreciation. It is the gateway to the San Gabriel Mountains and the connecting link to the Los Angeles River.

Fish Passage

Veteran fishers in the Arroyo Seco remember that PWP’s rickety diversion facilities in past years have frequently sucked up a lot of fish into their system. We agree that the antiquated board dam and pipes need to replaced with a more modern diversion system, but that system must include appropriate screens and allow passage for native fish and aquatic habitat, particularly during the Spring.

Environmental Flows

California law (Fish and Wildlife Code #5937) requires that dams and water facilities provide a minimum flow of water to enable species of special concern such as native trout to survive during critical periods. The DEIR fails to do so.

Possibility of Native Fish Restoration

While the Arroyo Seco was once a thriving steelhead stream and fishers’ haven, recent years have been hard on the fish. Dams and other human barriers have greatly inhibited their ability to move up and down the stream. After the Station Fire in 2009 and the extended drought since then, few native fish have been spotted in the Arroyo.

The Arroyo Seco Foundation (ASF) is working with agencies and stakeholders to improve conditions for native fish and other species in the Arroyo and in the Los Angeles River. In downtown Los Angeles at the confluence of the Arroyo Seco and the Los Angeles River there is a very exciting project sponsored by the California Wildlife Conservation Board that is examining how to create adequate conditions for native fish in the Los Angeles River. The Arroyo is key to that and to the National Oceanic and Atmospheric Administration’s southern steelhead recovery program.

Living Stream vs. Spreading Basins

The Arroyo Seco is an alluvial canyon filled with sand and gravel that has washed down from the mountains. This is the prime recharge area for the Raymond Basin aquifer that underlies Pasadena. Its natural character and composition, which make it ideal to percolate water into the groundwater basin as it has for several million years, make it far superior to biologically sterile, hydrophobic spreading basins that are plagued by evaporation, mosquitos, algae and soil compaction.

What Did the Judge Say?

In 2017 Superior Court Judge Richard L. Fruin chastised PWP for inadequacies in their initial environmental document. He ordered PWP to complete a full environmental impact report that considers alternatives and evaluates the impacts of increased stream diversion on downstream habitat in the Hahamongna basin.

Impacts on Downstream Habitat

The DEIR, now under review, fails to evaluate the cumulative impacts of their project and the County’s massive sediment excavation project that is just downstream of the proposed project.

Why No Public Meetings?

The DEIR for the project is now available for public review until July 31, 2020, but PWP has done little to let the public know about it or the impacts of the project. They held a sparsely-attended poorly publicized “scoping” meeting last November, but they do not intend to hold a public meeting, online or otherwise, to let the public know what is really in the DEIR.

What Happened to Area 1?

Area 1, apparently, has been eliminated from ASCP in this round of reviews. The previous ASCP included the removal of PWP’s abandoned headworks structure, extensive habitat removal and other environmental improvements. In his 2017 court order, Judge Fruin allowed PWP to complete these elements of the project, but PWP instead has cancelled them. PWP needs to remove its trashy, abandoned facilities and repair the damage that its operations have caused.

How Often Do the Spreading Basins Have Water in Them?

The current spreading basins have occupied the precious alluvial scrub zone at the mouth of the Arroyo since the late 1940s, but rarely are the 14 basins fully filled to capture stormwater. Veteran Arroyo watchers remember that the basins have only been fully utilized twice in the last thirty years.

What sense does it make to expand inefficient spreading basins that will be empty 98% of the time when the water will naturally sink through the alluvium into the groundwater basin beneath?

The Environmentally Superior Alternative

There is a better way to improve water resources while protecting the rare nature of Hahamongna. ASF recommends a revised ASCP program that includes:

  • a minimum environmental flow for fish and aquatic species of 3 cfs in the Spring;
  • no "engineered" stream material, and
  • using a living river and natural stream hydrology to allow the water to percolate into the groundwater basin rather than spreading basins.

What You Can Do

  1. Study the project documents to learn more about the project and the Arroyo Seco.
  2. Send your email address to to receive updates.
  3. Submit comments to the DEIR urging PWP to adopt the environmentally superior alternative backed by ASF and other stakeholders. Send your comments to by 5:30 pm on July 31, 2020.

Let the River Flow!