FAIRBANKS -- The most effective flood insurance policy in Fairbanks can be found 17 miles east of the city, about 35 river miles along the meandering Chena River.
It takes the form of an unusual dam with four 30-ton gates that operate like giant garage doors, stemming the flow of high water when the river rises. The floodgates are one element in an extensive federal flood control project that cost a quarter-billion dollars by the time of its completion in 1979.
The Army Corps of Engineers activated the dam last Saturday for the 20th time since 1981, partially closing the floodgates to keep water out of downtown Fairbanks.
The water had dropped enough by Monday morning to stop impeding the river flow and the corps opened the gates. That allowed water that had backed up behind the dam to flow downstream. After heavy rains returned Wednesday, the corps considered lowering the gates a second time but the water levels did not rise to the occasion.
The upper Chena drains about 1,500 square miles above the dam, much of it hilly country where there is often more rain than in Fairbanks. Water levels are expected to continue dropping over the weekend.
In some parts of Alaska, the rainfall totals from Fairbanks would barely qualify as a damp day. But in a place that gets about as much precipitation every year as Tucson, 3 inches of rain is a deluge. About two to three times that much has fallen since early last week in the hills east of Fairbanks, swelling the Chena River in the process.
And so what had been one of the drier episodes in Fairbanks history has become one of the wettest, prompting the corps to cut the flow in the river.
"We lower those gates down into the river at settings that are called in from our hydrology section in Anchorage," said Tim Feavel, flood control project manager for the corps.
He said the decision to lower the gates is not taken lightly and is based upon information gathered from a series of monitoring stations upstream from the dam, as well as forecasts from the National Weather Service.
When water levels are expected to rise too high, the corps activates a hydraulic system that pushes the four 25-foot-wide gates down and holds them in place against the current, allowing 5 or 6 feet of water to flow beneath them. The gates have the effect of blocking thousands of gallons of water, which pools into a temporary reservoir that builds behind the dam.
The dam equipment restricts the river flow beneath the gates to a maximum of 8,000 cubic feet a second. For the most part, that level keeps water off the streets of Fairbanks and within the banks of the Chena.
Without the flood control project, there would have been flooding in the city last weekend. Over the past 30 years, the main story about high water in Fairbanks is about the floods that didn't take place and the hundreds of millions in property damage that never happened.
In many ways, the flood control project has changed the psychology of living in Fairbanks. While many people once looked at the Chena with a wary eye -- knowing its propensity to top its banks -- the prevailing attitude today is that the river does not pose the same threat and is under control. As a result, people have become more willing to build houses along the river, relying on the corps to keep them dry.
In 2010, the corps removed the floodgates and other equipment for inspection and found them "in excellent operating condition."
Fairbanks is built in a floodplain subject to routine spring flooding and some summer flooding from its founding in 1902 until 1979. Over that period there was a flood once every four or five years, usually associated with spring breakup.
It's ironic that in the first days of Fairbanks the overriding concern was about getting more water into the Chena to make it easier to get steamboats in and out, at a time when that was the only dependable means of transportation.
The town is in this spot because E.T. Barnette got stuck here in 1901. He was forced to get off the boat with his supply of trading goods because of low water. A gold discovery the next year turned his misfortune into a fortune. In 1904, the owners and captains of riverboats doing business in the new town started to excavate what is now called Piledriver Slough, upstream from Fairbanks, to divert more water from the Tanana into the Chena.
This would have boosted steamboat travel but it would have made floods more frequent and severe. As pioneer Charles Creamer told a reporter in 1938, had the excavation plan not been stopped by the War Department, Fairbanks would have been wiped out.
"If the river men had been permitted to have their way, there would be no flood control problem. The Interior Alaska metropolis would be on Birch Hill," Creamer said.
Still, even without a bigger channel, significant amounts of water from the Tanana flowed into the Chena. The first flood control project in 1938-41, the Moose Creek Dike, reduced the Fairbanks flood threat by cutting off that channel.
As floods continued in the decades that followed, there was more talk of additional flood controls but it was only after a devastating 1967 flood -- when 4 or 5 feet of water covered most of Fairbanks -- that Congress funded the flood control project that protects the town today. During the 1967 flood, the Chena hit a rate of about 74,000 cubic feet per second.
In addition to the concrete outlet works, the system includes the Moose Creek Dam and the Tanana River Levee, which together provide a water barrier that extends for nearly 30 miles, stretching around the southern boundaries of North Pole and Fairbanks.
When the floodgates are lowered, water backs up behind the Moose Creek Dam, an earthen structure 8 miles long. The dam is 50 feet above the streambed and goes all the way to the Tanana River.
On only one occasion, when the floodgates were closed for 18 days, has enough water backed up behind the dam to fill the floodway and reach the Tanana River to the south.
Most of the time the periods of high water are short enough that the floodgates are opened and the temporary reservoir empties into the Chena within a few days.
When it hasn't been raining, the river flow shrinks to about 1,000 cubic feet per second or lower during the summer, enough to tempt people to walk across it. But during the sporadic periods of heavy summer rain, the calm Chena turns into a torrent.
When the temporary reservoir is created, it spills onto a large floodway, up to 2,400 feet wide, that extends 7 miles south to the Tanana River. A dam that would create a permanent reservoir was originally planned for the Chena but the local geology didn't allow it.
"We're standing on about 600 feet of sands and gravels, so it's not tied to bedrock," Feavel said of the dam.
"It's like trying to keep water in a sandbox," he said of the porous material, adding that 150 relief wells along the project relieve pressure under the dam from the groundwater.
One trade-off that has proven controversial in the past is that when the dam is used for periods of longer than four or five days to divert a large volume of water, groundwater flooding has been a problem in some North Pole basements as it seeps under the dam.
Reach Dermot Cole at firstname.lastname@example.org.
By DERMOT COLE