John Vonderlin: The Water that might have been (Part I)

Story by John Vonderlin

Email John: [email protected]


Hi June,
I posted earlier about the “Coastal San Mateo County Investigation” document that I had found on Archive,org’s Way Back Machine. Published in March, 1966, by the California State Water Resources Board, it is a thorough analysis of Coastside water resources, agriculture and the plans to manage and develop them at that time. Given that to the best of my knowledge, none of the proposed developments ever occurred, it might be viewed as “The Last Great Hurrah” for those that envisioned a sprawling urban and residential landscape from San Francisco to Santa Cruz.
On an introductory page this paragraph sums up their intentions pretty well:
“The results of our studies indicate that improved

highway transportation

facilities to be constructed during


coming decade will allow the coastal area of San Mateo


to participate in the rapid growth of the San Francisco Bay Area. This growth will be accompanied by increasing demands for water supplies and fresh water recreation opportunities. The studies also revealed that the water requirements of this area could be met by development of local streams. A number of damsites were evaluated and six staged development plans were


and tested for economic justification. An economically justified staged development plan which offers the opportunity to maximize the net benefits accruing to the coastal area is recommended for feasibility study by the local people.”

In a summary of their findings they noted as justification for their plans:


population growth and increasing demands for developable land in the San Francisco Bay Area will exert

great urbanizing pressures on the coastal area of San Mateo County south of Montara Mountain. This area is the last large, close-in district on the peninsula suitable for development. Urban growth will bring increasing demands for municipal and industrial water supplies.”

They continue:


of population for the coastal area indicate an increase from 7,635 persons in 196O to 118,000 in 2020 and to about 238,000 under conditions of maximum development. This great increase will result from the development of the coastal area as a “bedroom area”. This growth will be made possible by the completion of modern standard highway transportation facilities which will allow the working force of the coastal area to commute

to other parts of the San Francisco Bay Area.”


Their general conclusion of the studies was:
The runoff of the streams of the coastal area was


to determine if the supplemental water requirements could be supplied by local development. It was found that there were adequate supplies of good quality water for all beneficial uses.


determined that available, physical worksadequate local runoff was to conserve this runoff were evaluated. A total of 43 dam and reservoir sites were studied. Twelve of these sites were given detailed study and were considered, together with appropriate diversion and conveyance facilities, to be possible units of staged development plans to meet the supplemental water requirements of the area. Six alternative staged development plans were formulated. The units of each plan were staged to the year 2020 and the additional units required to meet the estimated maximum supplemental

requirements were defined. Each

plan was tested for economic justification for a 50-year period of analysis from 1970 to 2020.”

I’ve attached the map showing the 43 damsites that had been considered. For various reasons, all but 13 were eliminated. A better alternative in the same watershed or the costs of impounding and delivering the water were the primary reasons for elimination. The 13 semi-finalists were:



2. Purisima No. 1


Purisima No. 2




San Gregorio No.2




Loma Mar No. 2




Juanita No. 2


Bean Hollow


Pigeon Point


Gazos No. 1



With continued study, the list was pruned further. One of the finalists was Denniston Reservoir. Now, this isn’t the present day Denniston Reservoir, pictured in the attached 2009 Google photo, or a 2006 U.S.G.S map or a 1982 photo from the Coastside Count Water District website, but a Denniston Reservoir on steroids. The proposed reservoir is pictured in the ScreenShots I excerpted from the document. It was described thusly:
Denniston Dam would be located in the southwest

quarter of Section 2, Township 5 South, Range 6 West, MDB & M, approximately one mile north of El Granada. A mathematical operation study was made for a 4,500 acre-foot reservoir. Small mandatory releases were allowed


existing water use and a pool of 250 acre-feet was reserved for sediment deposition and fish. The net yield was 1,300 acre-feet per year.


development in the reservoir area is now


to a rifle and pistol range operated by the Brisbane Rod and Gun Club. Poor access, rather steep slopes, limited area available for development, and a lack of trees, make the overall reservoir recreation potential relatively poor.


and Game resources of the watershed are not


to any degree. The reservoir would cause the loss

of some

quail habitat, but would not adversely affect other species of wildlife or the small resident trout population. With proper management a self-sustaining population of warmwater game fish could be supported. A mathematical operation study was made for a reservoir


a capacity of l6,800 acre-feet. Allowing a sediment,

recreation, and fish

pool of 1,000 acre-feet, a net yield of 3,900 acre-feet per year could be obtained. The reservoir recreation potential is only fair because of the open cover pattern and the scanty tree cover. There are some trees in the canyons, but most of the drier

slopes lack even

brush cover. Access is good and most recreation uses could be accommodated, but the size of the reservoir would limit the amount of use.”

Now with an expected production of just about enough for 1,500 homes, this reservoir was slated to be only a small part of “The Big Picture.” In the document they detail six proposed plans, involving different combinations of reservoirs, that they were considering. I’d like to share their favored plan and some its components in future postings. I think you’ll be startled by the size and scope of their plans and the changes they would have wrought to the vista as we know it today, if they had ever been brought to fruition. Enjoy. John