SR-57 Stormwater Mitigation Project

Caltrans proposes to provide long-term measures to reduce pollutant contributions to the San Gabriel River-Coyote Creek Watershed by construction of a detention basin within the northbound State Route 57 Tonner Canyon Off-Ramp loop at Post Mile 22.0 in unincorporated Orange County, California. The proposed project will construct a detention basin as a form of a treatment Best Management Practice that will connect and discharge to the existing drainage system. In addition, the proposed project will add a maintenance vehicle access road within the off-ramp, a Midwest guardrail system along the right-side of the off-ramp loop, and relocate and add additional light posts for the off-ramp.

Current project schedule:

  • Public Review period is from December 13, 2019 to January 24, 2020
  • Project Approval and Final Environmental Document is May 2020
  • Begin Construction 2022
  • End Construction 2024

The Draft Environmental Document is an Initial Study (with proposed Mitigated Negative Declaration) is available for review. The public review period is from Friday, December 13, 2019 to Friday, January 24, 2020. 

Please send inquiries to Alben Phung, Associate Environmental Planner at if you would like to:

  • review the Draft Environmental Document
  • submit a comment on the Draft Environmental Document

Map of project vicinity showing Tonner Canyon off-ramp loop