top of page

Interlake Tunnel

What is an Interlake Tunnel and What does it mean for you?

Monterey County Water Resources Agency (MCWRA) is proposing building a tunnel to divert water from Lake Nacimiento into Lake San Antonio.

The Nacimiento Regional Water Management Advisory Committee (NRWMAC) is concerned how the tunnel might affect Lake Nacimiento lake levels, recreation activities, property values, and water availability.

This document briefly explains the tunnel project, potential benefits, and questions landowners and lake enthusiasts may have.


Here is what has been said by
Monterey County Water Resource Agency

Interlake Tunnel Project

  Technical Memorandum


The proposed Interlake Tunnel is a gravity flow water conveyance tunnel approximately 12,000 feet long connecting Nacimiento and San Antonio reservoirs in San Luis Obispo and Monterey Counties respectively. Conceptual design of the tunnel project envisions a reinforced concrete lined tunnel with an inside finished diameter of 10 feet and a slope from Nacimiento to San Antonio of 0.4%. The tunnel will have an invert elevation in Nacimiento Reservoir at approximately 745’ MSL* with an inlet structure equipped with debris racks and stop logs to facilitate tunnel maintenance. The outlet structures in San Antonio will include a valve facility housing and a spherical valve for operation of the tunnel. The outlet structure will terminate in San Antonio reservoir with an energy dissipation structure.


*MSL refers to Mean Sea Level


The estimate cost of this project as of July 24 2019 was $189 Million.

(Estimated $165 Million to be paid by farmers.)

NRWMAC's Concerns for the Tunnel Project

Physical Design


What design requirements or thoughts drive the size, location, elevation, shape, of the tunnel, the inlet and outlet works?


What is the priority of these requirements?


What are the design drivers for?

  1. Location of the inverts (tunnel inlet and exit)?

  2. Elevation of the inverts?

  3. Size of the tunnel?

  4. Size and Shape of other features?

Operational Design


Is there a prescription or rule for the transfer of water from one lake to the other? Any information on timing, amount of transfer, flow rate, or other parameters? What logic was used to model the flows between the lakes?

A concern of NRWMAC is why would MCWRA want the tunnel so deep? If the objective is to capture the 27,000 to 35,000 acre-feet of potential annual spill, it can be accomplished with the tunnel at the higher elevation. There appears to be no reason for transferring water to San Antonio when Nacimiento is only at 35% capacity (elevation 745).


Operationally, an agreement needs to be in place that clearly outlines the rules of operation of the lakes after the tunnel is installed. NRWMAC must be certain that new operational rules protect Nacimiento from unnecessary transfers.

NRWMAC fully supports the Interlake Tunnel Project as long as the transfers and releases benefit recreation.

More Questions Than Answers

  • Why not consider the Jerrett Dam Project? It has the potential for 135,000 acre-feet of additional storage.

  • Will this project require a vote by SLO County voters as well as Monterey County voters for approval and funding?

  • Are there Tunnel alternatives? What are the costs?

  • Will Lake Nacimiento be drained for the years of construction? What are the construction impacts to the water, Canyon road and lake water elevation?

  • While MCWRA said they would gladly turn over the modeling work when completed, the modeling work and back-up data still has not been made available. Where is the modeling data?

  • Is it possible to modify other existing dams in the region to increase capacity without the Interlake Tunnel?

  • What are the impacts to the local economy, boating and camping?

  • What are the current financing sources? Who is paying for this?

  • It has been reported the Tunnel is now estimated at a completion cost of $189.5 million, and at current MC budget levels that means it is estimated that $165 million will need to be provided by Salinas valley property owners via an assessment fee?

  • MCRWA and Fish & Game cannot agree on the screen size for White Bass restriction. How will they keep the White Bass at bay?

How You Can Help

  • Send us your questions, concerns, and comments by filling in the contact form on this page.

  • Share with friends, neighbors, and others through word of mouth or social media.

  • Donate to NRWMAC so that we can continue our efforts to retain enough water for lake property owners and lake enthusiasts on Lake Nacimiento. Click the Donate Now button.

bottom of page