What The Heck Is A Thermal Curtain?


Bill Dennison, Supervisor

Plumas County; District 3


Sometimes the simplest questions are the most important. For months we have been discussing the pros and cons of a Thermal, or Temperature Curtain. Thus far, every aspect of the proposal is negative, including the fact that the $51 million taxpayer financed proposal will only reduce the water temperature down stream by about one degree Celsius.


The recent, reasonable question posed, “What is a thermal curtain and what is it’s size?” leads me to back up and put more emphasis on the actual Thermal Curtain structure and then reiterate what organization has requested  consideration of the structure and some of the obvious impacts to Lake Almanor, even though that has been described in earlier papers.

What is the Prattville Intake Thermal Curtain?

1) It is designed to divert the cold water outflow from the lower water level of Lake Almanor down to Butt Reservoir through a 12 ft. diameter pipe. The existing Intake draws water from the full depth of water in the lake, especially from the upper layer that develops in the lake during summer.

2) The Curtain would encompass an area of approximately 14 acres in front of the Prattville Intake.

3) It would measure 770 ft. wide and extend out from the shoreline 906 ft.

4) The Curtain would be suspended from cables that are held up by large floats and cement abutments.

Environmental/Use Impacts Caused By The Thermal Curtain.

1) No boating, or fishing would be permitted within the 14 acres of Thermal Curtain Area.

2) The large floats mounted along the cable will be aesthetically distracting.

3) The construction proposal requires the excavation of 42,000 Cu.Yds. of mud from the under-water levees constructed earlier to divert water to the Intake.

4) One of the proposed disposal sites is located in the Prattville area.

5) The Maidu Tribe is in the process of developing a resolution of protest of excavation at the Prattville Intake based on the location of Native American cultural and burial sites in that area.

6) The Curtain is designed to remove up to 50% of the Cold Water Pool from Lake Almanor.

7) A June 8, 2003 news release by University of Iowa described their modeling of the Prattville Intake Modification measure and quoted Professor A. Jacob Odgaard, one of the authors of the IIHR report as saying that “…a continuous withdrawal of only cold water would deplete the lakes cold water supply, resulting in damage to the lake habitat.”

8) The June 2004 PG&E study by Thomas R. Payne and Associates has stated other damages from the curtain, including:

a)      The virtual elimination of the passage of Wakasagi pond smelt passage that is important as a food source for the Butt Reservoir trophy trout.

b)      A low Dissolved Oxygen (DO) concentration in Butt Reservoir tailrace that is also detrimental to the fish habitat.

c)      The report stated; “Existing summertime temperature and DO conditions limit salmonoid habitat. During summer months, this limitation can be severe.”  “The predicted thermal curtain induced  reduction in salmonid habitat represent a substantial portion of that currently available when the existing conditions are severely limiting.”

d)      This could cause a “…38% decrease in the volume of salmonid habitat.”

9) Experienced Lake Almanor fishing guide Mark Jimenez has said that removing excessive cold water from the lake could trap fish at the mouth of Bailey Creek, as the hypolimnion water layer is lowered. In addition, he notes the probability that removal of cold water will cause a very high density of fish around the under-lake springs, causing additional stress to the fish.

A great deal can be learned about the potential impacts of the Thermal Curtain through discussions with Mr. Jimenez and Ron DeCoto, a long-time Department of Fish and Game Fish Biologist.

Background—for those who may have limited knowledge of the Water Temperature Issue.

Assumed Problem—

State Water Resources Control Board (SWRCB), under the California Water Basin Plan has designated the North Fork of the Feather River and the waters below, including the Rock Creek/Cresta water Reaches as Cold Freshwater Habitat. However, during some months, particularly August, the water temperatures are greater than the 20 degrees Celsius that has been specified by some fish biologists as the maximum temperature to meet Cold Freshwater fish needs. This 50 year-old ‘warm water” problem is said to be the result of the Lake Almanor, Butt Reservoir, Belden and other dams that have been constructed by PG&E. There are some who question the availability of any historic data to substantiate the downriver temperatures prior to construction of these facilities.

Proposed Solution(s)—

PG&E has been instructed by SWRCB, under the auspices of Project #1962 Rock Creek/Cresta license to develop reasonable alternatives for remedying this “water temperature problem”. The exact language of the License Condition #4 is very important: “In order to reasonably protect cold freshwater habitat, Licensee shall maintain daily water temperatures of 20 degrees Celsius, or less in the Rock Creek and Cresta Reaches, to the extent that Licensee can reasonably control such temperatures.” (Emphasis added to denote that the final analysis of the Thermal Curtain must be based on more than experimental models and must consider impacts that can result from removal of up to 50% of the Lake Almanor Cold Water Pool.)


To meet SWRCB requests, PG&E contracted with IIHR—Hydroscience & Engineering, College of Engineering, University of Iowa to determine the feasibility of modifying the Prattville Intake “….to selectively release colder water from Lake Almanor during the summer.”


This “notion” was established by a 1986 PG&E study by Woodward-Clyde Consultants that concluded that the water-temperature requirements for trout habitat could be met if water releases through Prattville could be lowered by about 3 to 4 degrees during the summer months. Studies now indicate that the curtain can only reduce the water temperatures at Rock Creek/Cresta by about 1 degree Celsius and that requires two more Thermal Curtains in Butt Reservoir for a total cost of $51 million. 


Modification of the Prattville Intake is only one proposed alternative and it is important that to date, PG&E has not provided any other alternative for 2105 Licensing Group (LG) review.


The Plumas County Lake Almanor 2105 Committee has requested that PG&E provide copies of other alternatives for decreasing the water temperatures that they have studied, including:

1)     Re-operation of the Caribou #1 and #1 hydroelectric facilities during August.

2)     Reduction of water releases through Butt Reservoir, with release of the same volumes (or less, if it meets the downriver temperatures) from Canyon Dam.

3)     Stream course restoration in tributary in Indian Creek, Spanish Creek, etc.


 Informational interviews should be conducted with Lake Almanor Fishing Guides to obtain information on fish location during critical months of July through mid-September and the impact that could result from a reduction in the Cold Water Pool.


This concludes another attempt to explain the physical structure of the proposed Thermal Curtain and provide another review of the total Water Temperature issue that will be of the utmost importance to Lake Almanor residents, Plumas County and NFFR fisheries in the years ahead.