Thermal Curtain Overview

The Temperature Issue

In April 2004, the Project 2105 Relicensing Settlement Agreement was completed and signed by PG&E, Plumas County, and other interested parties.  The Settlement Agreement resolved several issues including Lake Almanor water elevations, fish releases, water quality, and recreational facilities but did not resolve the issue of water temperatures in downstream reaches of the North Fork Feather River (NFFR).  The water temperature of California rivers and stream is regulated as a water quality issue by the California State Water Resources Control Board (SWRCB).


§401 of the Clean Water Act allows individual states the opportunity to grant or deny §401 certification for water quality issues for federally regulated projects and prohibits federal agencies from issuing a license until such §401 certification is issued by the state.  In California, the SWRCB issues §401 certifications for FERC projects such as Project 2105.


The SWRCB has determined through its basin planning process that the North Fork Feather River was, prior to the construction of power facilities (Almanor Dam, Butt Valley Dam, etc.) that alter the flow of water, a “Cold Freshwater Habitat” which would support a cold water habitat.  A Cold Freshwater Habitat designation requires water temperature not exceed watershed-specific temperatures determined by the SWRCB.  For the NFFR, SWRCB has determined that water temperature to be 20 deg C (68 deg F).


In the reach downstream of Poe reservoir, the summer water temperature occasionally exceeds 20 deg C during some late summer months, particularly in dry years.  While there is ample evidence that those lower reaches, some 40 miles downstream of Lake Almanor, historically exceeded 20 deg C, the SWRCB has held fast to its Cold Freshwater Habitat designation.


Because the lower reaches of the NFFR do not strictly meet the SWRCB's requirements for water temperature, the SWRCB will not issue a §401 certification for Project 2105 without a plan to mitigate the water temperatures.


In its November 2014 “Upper North Fork Feather River Hydroelectric Project Draft Environmental Impact Report” prepared by North State Resources, Inc., SWRCB has determined that the construction of Thermal Curtains in both Lake Almanor and Butt Valley reservoirs could reduce the water temperatures in the NFFR to an acceptable level.  As an alternative to Thermal Curtains, the SWRCB has determined that large cold water releases directly through the outlet at the dam could also reduce water temperatures.  The recommended releases are 250 cfs between June 15 and September 15 and would be done in lieu of power releases through the power intake at Prattville for use through Butt Valley and Caribou 1 and 2 powerhouses.


In SWRCB's February 11, 2015 notice of its presentation to the public in Chester, the SWRCB appears to recommend that the Thermal Curtains not be constructed until a test is done to determine if releasing water through the dam outlet is effective in reducing NFFR water temperatures.  SWRCB does not state how long such a test would last or what criteria would be used to determine its effectiveness.


How do Thermal Curtains work?

The reservoir outlets at Prattville and near Butt Valley dam mostly draw water from near the top of the reservoirs where the water temperature tends to be warmer due to a thermocline, particularly in the summer months.  The SWRCB believes that if cooler water is drawn from deeper in the lake, water temperatures downriver will be reduced.


Thermal curtains are designed to do this by blocking water near the surface with a fabric curtain and forcing cooler water near the lake bottom to be drawn into the intake.  The proposed curtains, constructed out of Hypalon, a kind of synthetic rubber fabric made of chlorinated and sulfonated polyethylene, are suspended from floating tanks on the lake surfaces and hang vertically in each lake.  In order to leave space for water to pass underneath, the curtain is tethered to the lake bottom with 5 foot long cables.


2105 Working Group Concerns with Thermal Curtains

The 2105 Working Group is concerned that the construction of thermal curtains could result in:

  • harm to the Lake Almanor and Butt Valley Reservoir fisheries.

  • temperature changes in Lake Almanor that may cause detrimental effects on food chain availability, dissolved oxygen concentrations, and thermocline elevations.
  • negative visual impacts on the water surfaces of each reservoir.
  • detrimental effects on fish spawning and fish passage between Lake Almanor and Butt Valley Reservoir.
  • possible decrease in lake levels if additional water releases are needed to help control NFFR water temperatures.
  • an 18 month delay in obtaining the final Project 2105 FERC license in order to complete the CEQA and NEPA process to assess the construction impact.

Because of possible detrimental environmental impacts, large construction costs, visual impacts and negative impacts on boating, the 2105 Working Group is opposed to constructing Thermal Curtains.  The Working Group will continue to participate in the 2105 Licensing Group to work toward a reasonable alternative in controlling the water temperatures downstream of Lake Almanor.


Whiskeytown Thermal Curtain

Thermal curtains have been used successfully by the US Bureau of Reclamation at Whiskeytown and Lewiston lakes located on the Trinity River and Clear Creek, respectively.  Whiskey Reservoir is located west of Redding on state highway 299.  Lewiston Lake is located on the Trinity River a couple of miles north of Lewiston, Ca.  Both projects are part of the USBR's Central Valley Project.



Whiskeytown curtain   Produced by Geosynthetics Magazine documenting the installation of the Whiskeytown curtain


Whiskeytown You Tube video showing the installation of the Whiskeytown thermal curtain.



Other Documents

SWRCB Region 5 Basin Plan


SWRCB Beneficial Uses Definitions


Clean Water Act Section 401 Handbook


What the Heck is a Thermal Curtain? by Bill Dennison, former District 3 Plumas County Supervisor


Scoping Options for Mitigating Cold Water Discharges from Dams


Application of Flexible Curtains to Control Mixing and Enable Selective Withdraw in Reservoirs


Water Temperature Modeling Review Central Valley September 2000