The atmospheric river that drenched West Marin with record October rainfall downed trees and power lines, put a dramatic end to fire season, and brought rainfall totals for the month to more than a foot in some places. But despite its intensity, the storm won’t mean much for the drought unless it is followed by an especially wet winter, officials said.

According to the National Weather Service, more than 16.5 inches fell on Mount Tamalpais in 48 hours. At Lake Lagunitas, Marin Water has measured more than 18 inches of rain since July, which is 87 percent of the rainfall for all of last year. Nearly all this year’s rain fell in the last 10 days, and more than half fell in a 24-hour period starting Sunday. 

“The way this storm came through, we could not have designed it better to get that runoff,” Emma Detwiler, a spokeswoman for Marin Water, said. “The soil was really primed to let all the runoff into the reservoirs.”

The district’s average reservoir levels jumped from 33 to more than 50 percent capacity, but are still significantly lower than normal. The rain is a “step in the right direction,” Ms. Detwiler said, but it will need to come again, and again, to replenish the stores.

Nicasio Reservoir, the district’s largest reservoir by surface area, is visibly higher, at about 30 percent capacity. That’s up from 12 percent on Oct. 15. And in Chileno Valley, Laguna Lake, which had been reduced to a dry, cracked lakebed, was full after Sunday’s rain, restoring a key breeding habitat for wildlife like waterfowl and newts. 

In Inverness, the storm brought the total rainfall to about 12 inches for the month so far, shattering records going back almost a century. October 1947 came closest, with about half as much rain. 

But the ample rainfall didn’t put much of a dent in the town’s water emergency. Wade Holland, the customer services manager for the Inverness Public Utility District, said though early rain is certainly preferable to an October heatwave, it won’t help much if conditions are dry by spring. “Twelve inches in October may not be as good as four inches in May, in terms of getting through the summer,” Mr. Holland said. IPUD will keep its water usage restrictions in effect at least until spring.

The rain did relieve the pressure of fire season, likely putting an end to wildfire risk throughout Marin for the year. Mark Brown, executive officer of the Marin Wildfire Prevention Authority, said the rain “drops our fire risk dramatically” because it dampens light fuels like dead leaves. But live plants can only absorb so much moisture at once.

“We had a significant drought interrupted by one rain year, followed by another significant drought,” Mr. Brown said. “The heavier fuels never had a chance to recover from the first drought, and one winter is not enough time for those heavy fuels to catch up.” 

During Sunday’s storm, the National Weather Service measured wind gusts of up to 77 miles per hour near Woodacre and winds of more than 50 miles per hour throughout West Marin. The Marin County Fire Department responded to 163 calls related to downed trees on Sunday and Monday morning, and PG&E reported outages across the area. Hundreds of households were without power in the San Geronimo Valley on Sunday evening after a mobile transformer fell off its trailer and caught fire at the Woodacre substation.  

Despite the outages on Sunday, Lagunitas School was back open the following morning, and attendance was near normal, superintendent John Carroll said. The same was true at Bolinas-Stinson and West Marin Schools. 

Many of West Marin’s creeks that had been reduced by the drought to low trickles suddenly became raging torrents, threatening to inundate nearby properties. The level of Lagunitas Creek rose from one foot to 16 feet, its flow in cubic feet per second increasing by a factor of 1,000 at one point, according to North Marin Water District.

The district’s general manager, Drew McIntyre, said the district cancelled a request to Marin Water to release more water from its reservoirs into Lagunitas Creek after its flow broke records for October. 

Mr. McIntyre said it would take multiple months’ worth of rain to flush out the aquifer that supplies the Coast Guard wells, which have been subject to salinity intrusion, and that little rainfall made its way into Stafford Lake. The reservoir, which supplies the district’s Novato customers, remained relatively stable. “This is not going to undo a multiple-year drought,” he said.

Both water districts must carefully monitor flows in Lagunitas Creek to ensure adequate spawning habitat for salmon. But the record-breaking flows don’t guarantee success for all species of fish, said Michael Reichmuth, a fisheries biologist with the Point Reyes National Seashore.

“Given that we’ve never had a storm of this magnitude in October, one of the uncertainties is how the juvenile fish are faring,” Mr. Reichmuth said. When heavy rains come later in the year, young coho salmon are generally large enough to handle fast currents and swim out to sea. But when they arrive this early, he said, “we could have higher mortality of juvenile fish.”

At the same time, the storm could be a boon for Chinook salmon at the peak of their spawning season. Lagunitas Creek is generally deep enough to support the species, but the smaller Olema and Redwood Creeks are often too low in October. After the storm, both could be supporting spawning Chinook. 

“When you have this kind of a rain event, they have access to pretty much any of the creeks they want,” Mr. Reichmuth said. Surveyors began counting salmon in Olema Creek on Wednesday, after flows returned to a safe level. 

On Sunday, some creeks jumped their banks, submerging sections of roads and leaving a few drivers stranded in cars. Marin Fire responded to three water rescue calls and 20 vehicle accidents on Sunday. 

By Monday morning, most of the havoc was cleared up. Levee Road between Point Reyes Station and Inverness Park, which in places was covered by several inches of water overnight, was dry by the start of the business day. Bear Valley Road, which the National Park Service closed Sunday afternoon due to flooding, was quickly reopened, as was a stretch of Highway 1 between Olema and Point Reyes Station. Platform Bridge Road, which runs along Lagunitas Creek, remained closed until the afternoon. The parking lots, trails, and campground at Samuel P. Taylor State Park were closed until Wednesday because of hazardous creek conditions. 

Some West Marin businesses along creeks and shorelines were hit hard by flooding. Early Sunday morning, Olema Campground manager Gabriela Bell was awakened by the fire department responding to campers who had called to report flooding. Olema Creek, which was all but bone dry last week, had swamped their campsites. 

“The creek is still flowing through the campground,” she said on Monday. “Right now, we have no sewer and no power.” Ms. Bell said she was unsure of the campground’s flood insurance coverage, which she allowed to lapse when rates tripled after the campground filed another claim in 2016. Ms. Bell accused the Point Reyes National Seashore of inadequately managing the creek bed to prevent serious flooding.

“I hope something can be done to get all the fallen trees out of there, so the creek would actually flood in its creek bed, and not through the campground,” she said. “Would it take someone dying from the floodwater?”

The Inverness Store is just steps from Tomales Bay, so it’s particularly vulnerable to tidal flooding. Co-owner Nav Singh said she decided to call for help once the water level reached six inches in the backroom and she couldn’t take it out with buckets fast enough.

“We’re below sea level, so when the tide comes up, it’s pretty spooky,” Ms. Singh said. “It just comes from the cement floor. Concrete’s not waterproof.” 

Ms. Singh estimated the water damaged a few hundred dollars’ worth of packaged food, but it could have been worse. In 2006, the store lost most of its inventory to a major flood. 

“I knew the rain was coming, and now we’re really mindful about putting inventory up higher,” she said. 

The Parkside Cafe in Stinson Beach was flooded Sunday evening by a surging Easkoot Creek. 

“The creek just kind of took over everything,” manager Jessie Hyde said. On Monday, the cafe was back in business, but Ms. Hyde said they couldn’t have reopened without help from the fire department. 

An April 2018 storm damaged the cafe and destroyed a section of the Stinson Beach parking lot, despite a series of costly measures undertaken by the county and park service to prevent creek flooding. The measures effectively prevented damage to homes, but as Sunday’s storm showed, the area remains vulnerable to flash flooding. 

Julian Espinoza, a spokesman for the Golden Gate National Recreation Area, said the park service is planning a rehabilitation of the parking lot.

“We expect to make a broader announcement about that project, which would maintain sustainable visitor access to the beach and increase flood protection, in the coming months,” Mr. Espinoza said. He did not describe the planned flood protections.