Is Lake Berryessa Headed for Another Ten-Year Drought or a Thousand-Year Flood?


By Peter Kilkus, President

Berryessa Environmental Management Systems (


California Droughts are Common


Long-term weather patterns are cyclic in nature but unpredictable in practice. The master chart below shows the history of Lake Berryessa since Monticello Dam was built in 1957. The lake has had two previous droughts, one in the late 70’s and a ten-year drought from 1986 to 1996.


Droughts exceeding three years are relatively rare in Northern California, the source of much of the State’s water supply. Historical multi-year droughts include: 1912-13, 1918-20, 1923-24, 1929-34, 1947-50, 1959-61, 1976-77, and 1987-92. The 1929-34 drought established the criteria commonly used in designing storage capacity and yield of large Northern California reservoirs.


California endured severe droughts in 1976 and 1977, which ended in 1978. The master chart below shows that Lake Berryessa levels did not even show the usual winter uptick during those years and had dropped to less than 50% of capacity by 1978.


The 1987-92 statewide drought was notable for its six-year duration and the statewide nature of its impacts. Statewide reservoir storage was about 40 percent of average by the third year of the drought, and did not return to average conditions until 1994. But Lake Berryessa had dramatically dropped to only 30% of capacity by then and did not fill up again until 1996. 1991 was the single driest year of the drought.


A 1994 study of relict tree stumps rooted in present day lakes, rivers, and marshes suggested that California sustained two epic drought periods, extending over more than three centuries. The first epic drought lasted more than two centuries before the year 1112; the second drought lasted more than 140 years before 1350. In this study, the researcher used drowned tree stumps rooted in Mono Lake, Tenaya Lake, West Walker River, and Osgood Swamp in the central Sierra. A conclusion that can be drawn from these investigations is that California is subject to droughts more severe and more prolonged than anything witnessed in the historical record.


Beginning in 2006 the 1986 pattern appears to be repeating. (See chart at the end of this report.) That’s the potential bad news. The good news is that the lake can and has risen rapidly during a single year: 30’ in 1963, 38’ in 1977, 30’ in 1980, 40’ in 1993, and 61’ in 1995.


On average, 75 percent of the State’s average annual precipitation of 23 inches falls between November and

March, with half of it occurring between December and February. A shortfall of a few major storms during the winter usually results in a dry year; conversely, a few extra storms or an extended stormy period usually produces a wet year. If there is an unusually persistent Pacific high-pressure zone over California during December through February it predisposes the year toward a dry year.


Although we are now in a three-year drought, an El NiĖo has been predicted for 2009-2010, which may mean a wet winter. (See the El NiĖo discussion at the end of this report.) It hasn’t happened yet, and we need significantly above average rainfall to fill the lake this year. So whether we continue to repeat the pattern of the 10-year old, 10-year drought remains to be seen.


Lake Berryessa, or Solano Water Project, is the main drinking water source for more than 400,000 people so a drought can have a serious impact. SCWA member agencies and their annual Lake Berryessa allocations in are-feet are: Solano Irigation District (SID) - 141,000 AF, Fairfield – 9,200 AF, Vacaville – 5,600 AF, Suisun City – 1,600 AF, Maine Prairie – 15,000 AF, and Vallejo – 14,750 AF for a total allocation of 187,150 acre-feet.


The Solano County Water Agency (SCWA) uses a wide variety of water management tools and options to maximize resource and minimize the need to import water. The SCWA and its member agencies have comprehensive urban and agricultural water conservation programs. They also have a Drought Contingency Plan which specifies that when storage in Lake Berryessa falls below 800,000 acre-feet as measured on December 1, they will implement reasonable water conservation measures, investigate potential emergency supplies and other reasonable measures which could reduce the depletion of storage in Lake Berryessa.


But Can Lake Berryessa Flood (Over the Dam)?


Not very likely. The lake level has never been higher than 446.7 feet (1983) since the dam was constructed. Since 1985 it has only reached 444 feet once in 1998. It is typically at or below 440 feet (Glory Hole). The master chart below shows that the lake has reached 440 feet twenty-four times in its 50-year existence. According to the Bureau of Reclamation, droughts are a significantly higher risk than floods since this is a reservoir which has some control over outflow rates but must provide water by law even if there is low rainfall.


There is only a 1 in 100 chance that there will be a flood level up to 450 feet. The top of the dam is at 455 feet. Highway 128 was designed to be the emergency spillway before the lake overtops the dam. Economic impacts due to potential floods are a very low probability and thus a low planning priority. The following
table, based on an analysis completed in 1986,
depicts the water elevation that, on the average, may be reached or exceeded for various
time periods.



Frequency (Years)

Probability (%)

Elevation (Feet)




















The Bureau of Reclamation did a Flood Elevation Probability Study in 1986. Although this is a bit of statistical game-playing, it is interesting to see the worst-case scenarios and the probability of their occurrence. Flood flows in this study were assumed to start at the initial elevation of 440 feet. All the frequency floods used in this study had a duration of 7 days. In other words, if the lake were already at 440 feet and then the various rainfall levels and flood flows (5-yr, 10-yr, 25-yr, 50-yr, 100-yr) began and lasted for 7 days, how bad could it get?


The impact of the 1984 Probable Maximum Flood flow produced a water surface elevation of 463.7 feet, or 7.7 feet ABOVE the dam for several days. This level was assumed to be the maximum reservoir elevation that could ever be reached during a flood. A related note in the study said, “Monticello Dam was assumed not to fail.”


Not to worry too much since this worst case scenario had a VERY low probability – somewhere between 10,000 and 1,000,000 years. Even the probability of the lake reaching the top of the dam at 456 feet was very low with a 5% confidence limit of 417 years recurrence and 95% confidence limit of 16,667 years recurrence. As stated in the study, “computed probabilities beyond a 100-year recurrence are not considered to be reliable; therefore, only confidence limits are provided. There is a 90% probability of a specific event occurring somewhere in the interval between confidence limits.”


A 90% probability sounds high, but would you place a “sure bet” that the lake would hit 463.7 feet, 7.7 feet above the dam, if you had to wait somewhere between 400 and 16,000 years to collect?





The above master chart needs a bit of explanation. Although consistently accurate rainfall data at Monticello Dam is not available before 1998 and the lake level measured in feet above mean sea level (msl) only exists after 1998, the Bureau of Reclamation has been keeping detailed data on lake capacity measured in acre-feet since the dam was constructed. The next chart is an example of that data in acre-feet of storage. This type of chart is available for 1958 to 2003 on the Lake Berryessa News web site at




Although the chart below is in acre-feet, each of the peaks and valleys has a text box with the date, level in feet above msl, and capacity in acre-feet. This allows a correlation between acre-feet and lake elevation and is how the elevation levels were added to the master chart above.





The early 1980s were mostly above average in precipitation in northern California and throughout the West, with the wettest year of the decade occurring in 1983 – also the highest level the lake ever reached, 446.7 feet, almost 7 feet above Glory Hole! The rainfall versus lake level chart below gives daily and annual rainfall totals versus lake levels for the 2000 to 2008 seasons.




Lake Level vs Acre-Feet of Storage


Local residents use a rule of thumb that the lake increases a foot in level for every inch of rainfall after the ground has been saturated by several initial inches of rain. This is not completely accurate since the relationship between the lake level and its storage capacity is not linear. The lake is shaped roughly like a bowl (with peaks and valleys and inlets and large flat areas), which means that the higher the water level gets the more rain is needed to raise it further. From the data below we can see that it takes about 25% more rain to go from 430’ to 440’ than it does to go from 390’ to 400’. The chart below showing rainfall versus level for the first three months of 1998 shows that the lake rose 16 feet with 26 inches of rain – or 7.4 inches of level per inch of rain.


Lake Level

Acre-Feet of Storage per Foot of Level Increase

350’ – 380’


380’ – 390’


390’ – 400’


400’ – 430’


430’ – 440’


440’ – 445’



The number of acre-feet rise with a 1-foot level rise implies the lake surface area is approximately 18,500 acres when full. The surface area when full at 440’ is 25% greater than the surface area at 400’ and 60% greater than at its low water mark of 362’ during the drought of 1986 to 1996.


The water at the dam is 275’ deep. The lowest level during the drought of 86-96 of 362’ (78 feet down) meant that the lake had dropped in level by 30%. But the capacity had dropped from 1,600,000 acre-feet to 400,000 acre-feet – a decrease of 75%.


A few years ago, SCWA hired a consultant to survey the bathymetry (bottom surface) of Lake Berryessa and determine if the lake had experienced significant sedimentation as well as create new capacity curves based upon the new survey data.  In general, the sedimentation was found to be fairly minimal, but the new capacity curves did show a reduction in Lake Berryessa capacity.


No One Is Planning to Raise Monticello Dam!


Every year or so someone starts a rumor about raising the dam – usually claiming the plan is to raise it ten feet. Where did this rumor start and why does it pop up every now and again? According to David Okita, general manager of the Solano County Water Agency, CalFed, a collaboration among 25 state and federal agencies (, did a “brainstorming” survey many years go of every potential future water project in northern California.


Although raising Monticello Dam made the original list of possible projects, after practical criteria such as cost, safety, flooding adjacent property, were applied to screen the list down to real opportunities, raising the dam was dropped from the list. It has never been discussed seriously since then. It is NOT in any plan and never will be.


The number one project on the list continues to be Sites, east of Maxwell. Those of you who are ATV/dirt bike/jet ski enthusiasts have probably gone to Stonyford or East Park Reservoir to ride. You’ve driven through Sites and seen the narrow canyon at the brown rock quarry where a dam similar to Monticello Dam could be built.


A simpler approach to storing more water in Lake Berryessa would be to raise Glory Hole itself by installing movable gates that could be raised and lowered as necessary to impound more water during the rainy season. Raising Glory Hole by 5 feet to 445’ would increase Lake Berryesssa storage by approximately 100,000 acre-feet without significantly affecting facilities along the shoreline. Although the highest the lake level has ever reached was 446.7 feet in 1983, and it has often exceeded 440 feet, according to David Okita of SCWA, a review of the historical data shows that the level does not exceed 440 feet often enough to justify the expense of installing this type of control device on Glory Hole, much less justifying raising the dam itself.


Rainfall Impacts


The water supply for Lake Berryessa is derived
from the 568 square mile drainage basin above the
dam. The elevation of the basin ranges from 182
feet at the dam to 4,722 feet at the upper end of
Putah Creek with most of the basin lying below
1,500 feet. There are four principal creeks that flow
into Lake Berryessa: Capell Creek, Pope Creek,
Eticuera Creek, and Putah Creek, the main
drainage of the basin.


How fast the lake rises is dependent on rainfall over the whole watershed. The lake is 23 miles long, 3 miles wide, with 165 miles of shoreline and is fed by the headwaters to the 576 square mile Putah Creek watershed. Rainfall levels vary significantly by location. Moskowite Corners is usually about 10% - 20% higher in rain totals than the nearby (7 miles) Berryessa Highlands. Below are some average rainfall totals for the region.


Calistoga, Angwin, and Napa provide an interesting precipitation comparison, but they are not within the Putah Creek Watershed. Middletown rainfall is a better comparison since it is really the headwaters to Putah Creek and flows directly into the lake.


Average annual rainfalls:


Monticello Dam (97-08): 28”

Markley Cove:  25.7”

Middletown (1938-1995): 48.6”

Angwin (1939-1995): 41”

Napa: 24”

St. Helena: 34.4”

Calistoga: 37.4”

Winters: 22.8”


Predicting the Future

Several climate prediction centers are forecasting El NiĖo conditions for the later part of 2009 and the early part of 2010. California does not always see an increase in precipitation during El NiĖo years. With the majority of the last ten years showing below normal precipitation in much of the state, researchers are hoping for a potential abundance of rainfall without flooding or coast-damaging storms.  


The National Weather Service issues 30 and 90-day forecasts. Academic institutions, such as the Scripps Institution of Oceanography in San Diego, have attempted experimental seasonal forecasts. The accuracy and level of detail of these efforts remains insufficient for water project operations. It is only recently, for example, that researchers have had sufficient understanding of global weather patterns and atmospheric/oceanic interactions to be able to identify conditions associated with the El NiĖo Southern Oscillation (ENSO) in the Pacific Ocean. That understanding has yet to be translated to forecasts of runoff, partly because ENSO events affect different parts of California differently.


Lake Berryessa had normal fluctuations in water levels from 1995 to 2006. A middling El NiĖo brought steady rains that filled Lake Berryessa to the brim in January, 2006 – the last year the lake spilled over Glory Hole. A previous El NiĖo brought seemingly nonstop rains in the winter of 1997-1998. See February, 1998 chart below – 18.9 inches in that one month – more than the total of 12.7 inches of rainfall in 2006-2007! I witnessed Lake Berryessa go up 9 feet in 12 days in 1998 from my deck at Steele Park Resort.


Despite the one big storm in October, which had little impact, rainfall has only been about 6 inches as of January 1, 2010. This is the same as during seven of the last thirteen years. And each of those thirteen years saw less than 25 inches in total. Can Glory Hole spill again in 2010?