SAN RAFAEL (CBS/BCN) — A heavy downpour overnight helped the Marin Municipal Water District’s seven water-supply reservoirs reach 100 percent capacity for just the third time in the past 20 years during December, district officials said.

Water began flowing today over the spillway at Kent Lake, the district’s largest reservoir and the last to fill, according to the district.

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In the previous two decades, the reservoirs have filled to capacity only twice during the month of December – in 1996 and 2005. Historically the reservoirs average about 70 percent capacity this time of year.

About 3.5 inches of rain were recorded overnight at Lake Lagunitas, which has received 29 inches of rain this year, nearly 160 percent of its yearly average.

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At this time in 2009, the lake had received about 17 inches of rain, and the reservoirs were at 57 percent capacity, according to the district.

The district’s seven reservoirs have a combined capacity of 79,566 acre-feet. One acre-foot equals 325,851 gallons.

The reservoirs are the primary source of drinking water for 195,000 residents in central and southern Marin County, according to the district.

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