Santiago Creek Dam Improvement Project
3 Documents in Project
Summary
SCH Number
2023050097
Lead Agency
Irvine Ranch Water District
Document Title
Santiago Creek Dam Improvement Project
Document Type
NOD - Notice of Determination
Received
Posted
11/13/2025
Document Description
The Project proposes to rehabilitate and replace the Santiago Creek Dam outlet tower and spillway facilities and to modify the embankment to permit operation of the facilities for a long-term water resource benefit. The Project includes construction of a new inlet/outlet works, including an inclined inlet/outlet structure, intake riser pipelines and platforms, bifurcation valve vault, emergency outlet pipeline, and emergency outlet valve vault. The existing spillway would be replaced with a new side-channel spillway, and a portion of the Irvine Lake pipeline would be upsized and relocated to below ground beneath the proposed spillway structure. The Project includes improvements to the dam crest, removing the dam face, installing a filter drain system within new embankment shell material, widening the dam crest, raising the elevation of the dam crest by up to one foot, and installing protective railings. Ancillary site improvements are also proposed for the Project, including a new inlet/outlet access roadway, spillway bridge, dam control building, and emergency access walkway and stairs. The Project also includes raising the spillway by six feet to approximately 797.9 feet, which is two feet higher than the existing maximum water storage elevation with the flashboards installed. Raising the spillway would allow the dam to impound water up to the 797.9-foot elevation contour year-round, which would allow for storage of an additional approximately 1,600 acre-feet of water.
Contact Information
Name
Fiona Nye
Agency Name
Irvine Ranch Water District
Job Title
Director of Water Resources
Contact Types
Lead/Public Agency / Project Applicant / Parties Undertaking Project
Phone
Email
Location
Coordinates
Cities
Irvine, Unincorporated area
Counties
Orange
Regions
Unincorporated
Other Location Info
The Project is located at Santiago Creek Dam at the northwest end of Irvine
Lake in unincorporated Orange County, California. Santiago Creek Dam impounds Irvine Lake. The Project is located south of State Route 261 and east of State Route 241 and Santiago Canyon Road.
Notice of Determination
Approving Agency
Irvine Ranch Water District
Approving Agency Role
Lead Agency
Approved On
County Clerk
Orange
Final Environmental Document Available at
Irvine Ranch Water District, 15600 Sand Canyon Avenue, Irvine, California 92618;
http://www.irwd.com/irvinelakeproject
Determinations
(1) The project will have a significant impact on the environment
Yes
(2a) An Environmental Impact Report was prepared for this project pursuant to the provisions of CEQA
Yes
(2b) A Mitigated or a Negative Declaration was prepared for this project pursuant to the provisions of CEQA
No
(2c) An other document type was prepared for this project pursuant to the provisions of CEQA
No
(3) Mitigated measures were made a condition of the approval of the project
Yes
(4) A mitigation reporting or monitoring plan was adopted for this project
Yes
(5) A Statement of Overriding Considerations was adopted for this project
Yes
(6) Findings were made pursuant to the provisions of CEQA
Yes
Attachments
Draft Environmental Document [Draft IS, NOI_NOA_Public notices, OPR Summary Form, Appx,]
Notice of Determination
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