Why Marin County's Reservoirs are Full This Year

Is it time to remove Nicasio Dam? I've been asking myself this question ever since I came across this great report from the 1960s on the effects of Nicasio Dam on Coho Salmon and Steelhead Trout. So I added up the unused portions of Kent, Nicasio, and Soulajule Reservoirs, since most have been kept pretty full since 1977 and all since 1992. We have a reservoir the size of Nicasio Reservoir sitting unused at the bottom of these three reservoirs. Add unused storage in Alpine Reservoir, and the numbers work out even better. If we allowed the full use of the reservoir capacities of Marin Municipal Water District (MMWD), instead of always keeping them more than half full, it effectively doubles the available water storage, and would make the presence of Nicasio Dam and Reservoir unnecessary. The following graphs from CDEC show the amount of water unused sitting on the bottom of each reservoir:

Alpine Reservoir levels since 1977, adapted from CDEC. 3,500 acre-feet unused.

Kent Reservoir levels since 1977, adapted from CDEC. 12,500 acre-feet unused.
Note that the reservoir capacity was expanded in 1983.

Nicasio Reservoir levels since 1977, adapted from CDEC. 8,000 acre-feet unused.

 

Soulajule Reservoir levels since 1979, adapted from CDEC. 4,500 acre-feet unused.
Note that the dam was built in 1979.

That means if Nicasio Dam had been taken down in 1990--or even before with some extra water conservation that year--it would have allowed salmon runs to be restored in Halleck Creek and Nicasio Creek over the last 24+ yrs), with no impact on Marin County's water supply. This ignores downstream water quality impacts from low reservoirs (temperature, sediment) and any challenges such as increased erosion and sedimentation within the reservoir from greater fluctuations in level. These impacts would have to be carefully studied and mitigated before such a strategy could be pursued. But it illustrates how we've wiped out the salmon population above Nicasio Dam just for insurance that we haven't needed in almost 40 years. Creative solutions like removing or reconfiguring dams could make a much bigger impact on salmon restoration than the small-scale solutions approved by MMWD.

Nicasio Reservoir spilling on December 21, 2014.

Since moving to Marin County, I haven't had time to study MMWD's water system in the detail that I'd like, but it does seem to be a tad overbuilt since 1977. When you manage a water district, you do want to err on the side of being over-prepared, but not to the point of unnecessarily extirpating endangered species from entire watersheds. Anyway, this is mainly just food for thought--if we can strategically consolidate our impacts so that entire sub-watersheds can be restored, we may have better success than spreading out our efforts evenly. On paper, State law requires full protection of fish in good condition everywhere dams are built, but since Fish and Game Code section 5937 is rarely enforced as effectively as it should be, we may need to acknowledge that the lack of political will might require a change in tactics for those actually working on the ground to restore our native fish and wildlife.

I also wonder about the sedimentation rates of the reservoirs (i.e. this insurance policy may be getting used up quickly!). That cuts both ways--once the reservoirs fill with sediment, you are happy to have a greater capacity, but on the other hand you've got a huge expense to dredge or sluice them, and having one less reservoir saves that future cost at that location. We've got a long way to go to be sustainable on a centuries time scale. It would be a shame to cause an extinction for just a few decades of use of a reservoir that fills with sediment and then needs to be taken down. It isn't too late to fix mistakes made long ago and remake our systems in a sustainable way for the future...


This news story, which has a great account of historic Sacramento Valley salmon from farmer Bob Hennigan, is mainly about Marin County's reservoirs being full during this drought. However, the story is very mis-focused on Marin County's affluence as a reason for its reservoirs being full. I guess that is all people think of when they think of Marin County--as I did before I got to know its economic diversity, which includes many people struggling to pay rent as they compete with second homeowners and workers commuting to high-wage jobs in San Francisco. I suppose the affluence allows MMWD to replace water mains before the pipes break--something all water districts should be doing, but don't. But this mis-focus distracts from the real reasons the reservoirs are full. As I point out above, the MMWD system seems to be overbuilt and managed conservatively. MMWD learned from the 1977 drought, a drought that hit Marin County harder than just about anywhere else. Not only did it build or expand reservoirs since 1977, but it also built a pipeline to Sonoma County to augment its local supply with water exported from the Russian River and Eel River.

Who else has overbuilt? Phil Matier in the video talked about San Francisco being told "think twice before you flush" as if Marin residents didn't need to. He must be unaware that right now, the Hetch Hetchy system (Hetch Hetchy, Eleanor, and Cherry Reservoirs) has enough water stored for .23 acre-feet per person (enough to last 4.5 years), while the MMWD system has only .21 acre-feet per person (enough for 1.9 years). And MMWD is being held to a 20% conservation standard while San Francisco is being asked to conserve just 8%--rightly so given SFPUC uses half the water per person as MMWD and has less outdoor irrigation, but Phil's comments ignore those details.

Marin was also a bullseye for many of the storms that hit the state in December--in other words, it was lucky--just look how low the reservoirs were last year.

Also, MMWD is one of 127 water agencies in the state that use less than 110 gallons per capita per day. Only 24 of those agencies are saving at least 15% since 2013, and Marin is one of those 24. So despite already using a low amount of water per person, people have stepped up and conserved more here than in most places in the state. San Francisco is saving 8%--also a good level of effort given its already-low water use.

But what is the biggest reason the reservoirs are full? I think it is because of the relatively small population relative to the annual production of its relatively-wet watersheds. The far-sighted conservation of large areas of the county as open space, and a low growth rate, has allowed its water demand to be low compared to its water supplies. If it hadn't been for conservationists who protected so much of the county starting in the 1960s, the population could easily be double that of today, as would its water use, and the water situation during this drought would be much different. It is an instructive example of how growth and conservation policies here can have multiple beneficial effects, and how destructive the growth policies in much of the rest of the state are, on so many levels.


REVISED 4/24/15 TO REFLECT WATER BOARD'S NEW USAGE TIER INFORMATION.

NOTE: As of  4/29/15 the Water Board's proposed usage tiers document was updated again.

DECEMBER 2021 UPDATE: A severe 2-year drought (driest 2 yrs in 140-yr record) caused these reservoirs to cumulatively drop about 5,000 acre-feet below the thresholds marked in red on the graphs above before an October 24th record rainstorm caused them to rise. 5,000 acre-feet of water could have been saved if everyone had conserved approximately 10 gallons per day over the past two years.