Arroyo Seco Canyon Project

At a Glance

Pasadena’s water supply comes from both local sources and imported supplies. Imported water from Northern California via the State Water Project and from the Colorado River via the Colorado River Aqueduct, however, has become less reliable and the State is mandating that we reduce our dependence on these supplies. On the other hand, increasing conservation and our dependence on local supplies makes Pasadena more resilient to emergencies and better stewards of the environment. Storm water capture, specifically, is an increasingly important part of the region’s water picture. By utilizing the City’s long-standing water rights, the Arroyo Seco Canyon Project will capture runoff from large storms before this water is lost to the ocean, and restore a vital tool for groundwater recharge.

Pasadena’s water future rests on the ability to sustainably rely on local supplies, and the Arroyo Seco Canyon Project will be an important asset in PWP’s local resource portfolio.

Project Benefits

PWP has existing diversion facilities in the Arroyo Seco Canyon that over time have fallen into disrepair.

Barriers separate a road from damage to PWP's diversion and intake structure in the Arroyo Seco.

PWP's existing diversion and intake structure in the Arroyo Seco, built in the 1920's and 30's, continues to capture the City's water rights (top left). The facilities were damaged from storms that followed the 2009 Station Fire (top right).

The Arroyo Seco Canyon Project will renovate these facilities for the future in a way that integrates water management with the management of other resources in the Hahamongna Watershed. Among the project’s many benefits, it will:

  • Capture excess runoff in the Arroyo Seco from large storms;
  • Recharge the groundwater basin with water that would otherwise flow out to the ocean;
  • Increase overall water supply reliability by bolstering local water resources and reducing dependence on imported supplies from Northern California and the Colorado River;
  • Improve conditions for future native or planted fish by renovating the existing diversion and intake structure, including adding a new operable gate, new fish screens, and a new channel section specifically designed for fish passage; and
  • Add acres of new natural habitat to a space that was formerly a paved parking lot.

The project will restore a section of the stream below the new diversion weir with Engineered Streambed Material, specifically designed for fish passage, similar to the example shown above.

The project will enhance the natural habitat and the recreational experience for visitors of the local trail system as part of the expansion of percolation basins in an area that was formerly a paved parking lot.

Current Status

The City is designing improvements to the diversion and intake structure and the expansion of infiltration basins to increase groundwater replenishment.  PWP and its team of expert consultants in the fields of hydraulics, hydrology, geology, and biology have completed a number of studies that have informed the Project’s trajectory for sustainable, ecologically engineered upgrades to the City’s infrastructure.

These studies include:

  • A Hydraulics, Sediment Transport and Groundwater Analysis that is the basis for the Biological Impacts Memorandum (completed in 2018) which concludes that reduced downstream flows from the Project are not expected to result in any measurable effect on downstream riparian habitat.
  • Intake/Diversion Basis of Design Report (completed in 2020), including the Fisheries Review Letter by Dr. Camm Swift with an evaluation of fish presence in the Arroyo Seco, that shows the project is design for fish even though fish are not currently present.
  • A Geotechnical Feasibility Study Report completed in 2013 that supports the design of project features including stabilization of the Gabrielino Trail and the expansion of spreading basins in the former JPL parking lot.
  • Biological Resources Technical Report (2020), Cultural Resources Technical Report (2020), and Transportation and Air Quality analyses (2020) to support the analysis in the project’s Environmental Impact Report.

Environmental Impact Report

Preparation of the project’s Environmental Impact Report (EIR) began in 2019. The Draft EIR was compiled and made available in accordance with CEQA for a 46-day public review and comment period, ending July 31, 2020.

[su_tabs][su_tab title=Notice of Availability" disabled="no" anchor="" url="" target="blank" class=""]Notice of Availability (click for download)[/su_tab] [su_tab title="DEIR and Appendices" disabled="no" anchor="" url="" target="blank" class=""]
Draft EIR

App. A-1: NOP Initial Study
App. A-2: Initial Study Appendices
App. A-3: NOP Initial Study Comment Letters
App. A-4: Final ASCP IS-MND
App. B-1: Area 2 BOD Report
App. B-2: ASCP Area 3 Plans, 2015
App. C: Cal EE Mod Data
App. D: Biological Resources Tech Report
App. E-1: Cultural Resources Tech Report
App. E-2: Paleontology Records
App. F: Hydraulics Sediment Transport Groundwater Analysis
App. G: Field Measurement Noise Data
App. H: Transportation Data
App. I-1: Tribal Cultural Resources AB-52[/su_tab] [su_tab title="Draft EIR Sections" disabled="no" anchor="" url="" target="blank" class=""]

Section 0 - Title Page, Acronyms, TOC, Exec. Summary
Section 1 - Introduction
Section 2 - Environmental Setting
Section 3 - Project Description
Section 4.0 - Environmental Impact Analysis
Section 4.1 - Air Quality
Section 4.2 - Biological Resource
Section 4.3 - Cultural Resources
Section 4.4 - Greenhouse Gas Emissions
Section 4.5 - Hydrology and Water Quality
Section 4.6 - Noise
Section 4.7 - Recreation
Section 4.8 - Transportation
Section 4.9 - Tribal Cultural Resources
Section 5 - Mandatory CEQA Topics
Section 6 - Alternatives
Section 7 - List of Preparers[/su_tab][/su_tabs]

The Final EIR is now available and was approved by City Council on July 19, 2021.

[su_tabs vertical="yes"][/su_tab] [su_tab title="Final EIR (click here)" disabled="no" anchor="" url="" target="blank" class=""]Final EIR (click for download)[/su_tab] [su_tab title="Attachments (click here)" disabled="no" anchor="" url="" target="blank" class=""]

Attachment A - Draft EIR (see above)
Attachment B - Draft EIR Appendices (see above)
Attachment C-Update Study
Attachment D-Attachments to Comment Letter 5[/su_tab][/su_tabs]

Facts Matter

There has been a lot of information circulating about the Arroyo Seco Canyon Project. Below we've compiled the facts for our customers.

Claim: This project is harmful to the groundwater basin.

Fact: No, this is a storm water capture and groundwater recharge project that will be beneficial by keeping more water in the basin. During large storms, millions of gallons (thousands of acre-feet) of water flow into the stream only to pool behind Devil’s Gate Dam and be released by LA County for flood control. Water that is released through the dam is lost to the ocean. The project will allow more of this storm water to be captured and set aside in spreading basins for percolation into the underlying groundwater basin. PWP will pump only a portion of this water for supply. The remaining water, which would otherwise have been lost as outflow, helps replenish the groundwater basin.

Claim: The project is detrimental to fish.

Fact: Actually, the project will improve conditions for fish. Native fish have not been present in the Arroyo Seco above Devil’s Gate Dam for some time*. However, in late 2020 the California Department of Fish and Wildlife rescued fish from drying and post-Bobcat Fire conditions in the West Fork San Gabriel River and rainbow trout were released in the Arroyo Seco.  While the project EIR reflects the negative presence of fish in the Arroyo Seco, the project has always recognized the possibility for fish to reoccur and is being designed for and will be operated for fish. The project will remove PWP’s existing fixed dam that is one of the barriers to fish passage in the Arroyo Seco, and replace the diversion structure with a new operable gate. The project will also add a new section of roughened channel specifically designed for upstream fish passage, and add new screens to the intake for fish protection.

Claim: The project will take all the water out of the stream and be harmful to the habitat.

Fact: The majority of stream water will remain in the Arroyo Seco. The City already diverts all low flows during the dry months of the year, and a portion of flows during the wet, winter months. In fact, the City has been diverting water from the Arroyo Seco for over 100 years. But even when the City diverts its water rights, there is still water downstream. This is because of continued contributions from subsurface stream flow, surface runoff, tributary flow from Millard Creek, and a number of large storm drains that outlet to the Arroyo Seco. This is the water that supports habitat downstream of the diversion, and this water will not be affected by the project. Additionally, even after the project renovates the diversion, the majority of stream water will remain in the Arroyo Seco. Keep in mind that the project increases diversions from large storm events. Unfortunately, even the project will only be able to capture a small portion of storm water before it is lost to the ocean.

Claim: Spreading basins are ineffective at percolating stream water.

Fact: Spreading basins are widely utilized for groundwater replenishment.  Los Angeles County and the City of Los Angeles, for example, direct millions of gallons of storm water each year into spreading basins for infiltration. The concept behind spreading basins is that it provides an additional area for percolation to the underlying aquifer in addition to percolation that occurs naturally. When water from large storms flows down the Arroyo Seco, some of it will percolate in the natural streambed but the rest collects behind Devil’s Gate Dam where it is released by LA County for flood control. PWP’s project will capture an additional 1,000 acre-feet per year of storm water runoff before it reaches the dam, and set it aside in spreading basins so that it is not wasted.

*According to surveys and observations made by NOAA Steelhead Recovery coordinator Mark Capelli In August, 2018 and by California Fish and Wildlife Fishery (CDFW) Biologist John O’Brien through 2019 correspondence with Fisheries Specialist, Dr. Camm Swift.  Additionally, as stated in the “Translocation of Rainbow Trout to the Arroyo Seco from the Bobcat Fire Burn Area” Memorandum by CDFW, while the Arroyo Seco has historically supported a rainbow trout population, “…fish have not been observed during CDFW reconnaissance level and electrofishing surveys.”