Arroyo Seco Canyon Project

What is the Arroyo Seco Canyon Project (ASCP)

ASCP is a plan proposed by the Pasadena Water & Power Department (PWP) to divert more water from the Arroyo Seco stream and to expand spreading facilities at the mouth of the Arroyo Seco near JPL so that PWP can pump more water from the Raymond Basin that underlies the Pasadena region.

ASCP will be presented to the Pasadena City Council in early 2021 for approval of the program and the Environmental Impact Report (EIR).

Why is Hahamongna So Special?

Hahamongna is that 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.

What Is A Spreading Basin?

A spreading basin is an engineered zone intended to allow diverted water to percolate into the groundwater beneath.

But is this the best way to conserve water for the future? We believe that in an alluvial canyon like the Arroyo Seco a natural stream zone — a living river — would be a more efficient and environmentally appropriate way to conserve water.

This nature-based solution would also be better for fish and birds and habitat and people.

What's Wrong With ASCP???


What's Wrong Slides - Tim Brick

Biology Impacts - Angel Pinedo

Declining Raymond Basin - Ken Kules


Arroyo Currents video presentation on 12/17/2020

Does Hahamongna Really Need More Spreading Basins?

Let the River Flow

Heal the Raymond Basin

Pasadena City Council Hearing
Mon, July 12, 4:30 pm

Tell the Council What You Think


When Was the Last Time You Saw the Spreading Basins Full?

Have you noticed the extensive series of barren pits that line the east side of the Hahamongna basin? Pasadena Water & Power wants to expand their "spreading" facilities to convert the old JPL parking lot into six additional new spreading basins, even though their current 14 basins are rarely used.

Now that LA County Flood Control District has mapped out the 50 acres at the south of the basin near Devil's Gate Dam they will clear of habitat each year, if PWP moves ahead with its Arroyo Seco Canyon Project, it doesn't leave much room for nature in our region's greatest natural treasure, does it?

Why No Public Meeting?

People have lots of important questions about ASCP, but PWP does not intend to hold any public meetings, online or otherwise, to discuss and explain the Canyon Project. A small, poorly-attended scoping meeting was held more than a year and a half ago, but not since the environmental impacts have been revealed.

There's a Better Way

An Environmentally Superior Alternative can conserve water and protect the habitat and natural resources of Hahamongna. This alternative can provide for fish passage and use the Arroyo Seco stream to spread the water so it can percolate into the Raymond Basin beneath Pasadena.

Arroyo Seco Foundation, PO Box 91622, Pasadena, CA 91109-1622 | (323) 405-7326 |