( CNN ) -- The steep , picturesque canyons that wind through Colorado 's northern Front Range have been a draw for tourists , day-trippers and homebuyers for more than a century . But when record-setting rainfall began to hit last week , those picturesque canyons became funnels that poured hundreds of tons of water per second into towns like Boulder , Longmont and Lyons and left hundreds stranded in the mountains above . `` This flood was a direct result of the large amount of rainfall that occurred in a widespread area , essentially from Denver north to almost the Wyoming state line , and for a long duration -- for about a 48-hour period last week , '' said Robert Kimbrough , a hydrologist with the U.S. Geological Survey in Denver . Coloradans are still digging out from those floods , which have been up to 1,000-year events in some communities . A stream of unusually humid air that was sweeping over the area early last week helped fuel the storms . High pressure east of the mountains bumped up against a low-pressure system to the west , triggering several days of storms as moist air flowed up the peaks of the Rockies , where cold air wrung the water out of it . The rain began September 10 , with an inch recorded in Boulder . Nearly two inches fell the next day . Then on September 12 , Boulder recorded 9.08 inches of rain -- nearly double the city 's previous one-day record , and almost half its 20.5-inch yearly average . A forecaster in the National Weather Service office there called it `` biblical . '' More than 4 more inches of rain fell by the week 's end , bringing the September total to more than 17 inches so far . That 's boosted the yearly precipitation figure , measured from October through September , to more than 30 inches to date -- the most in 120 years of record-keeping . The biggest rainfall amounts occurred over the foothills of the Rockies , Kimbrough said . That water poured down the mountainsides into the major tributaries of the South Platte River -- Boulder Creek and the St. Vrain , Big Thompson and Cache la Poudre rivers . How to help . `` It really originated in the streams that drain off the east side of the Continental Divide into the front range of Colorado , but it all happened fairly quickly , '' Kimbrough said . `` Even though the leading edge was in the foothills , we did see it very quickly on the 12th in communities like Boulder and Lyons and Longmont . '' The result was six dead and about 200 who remained unaccounted for , though most of those are believed to be alive and well . Boulder County reduced its count from 109 people to four still unaccounted for on Wednesday , while in Larimer County , to the north , the number remained at 197 . Nearly 18,000 homes have been damaged statewide . Now the runoff is coursing down the South Platte into neighboring Nebraska , where forecasters are warning of flooding into the weekend . `` We 're in the Grand Island area anxiously waiting to see how the Platte River responds and to see how much of the water reaches us here , '' Mary Harner told CNN affiliate KHAS . Many in Colorado returned home to heartbreaking scenes . In Lyons , on the St. Vrain River 15 miles north of Boulder , high water pushed entire houses around like furniture . Streets were dotted with snapped utility poles . Stories of grief , generosity and gratitude . The extensive damage may present long-term problems for communities even as workers scramble to make repairs , Larimer County Sheriff Justin Smith said Tuesday . Questions of how to provide emergency services to areas where roads have been washed out or protect properties where the owners have been evacuated are still being worked out , he said . `` We have a new normal in Larimer County for months , if not a few years , '' Smith said . CNN 's Kyung Lah , Faith Karimi and Sean Morris contributed to this report .
Boulder News says only those still unaccounted for . Boulder lost nearly half its annual population in a single year . ` ` It all happened fairly quickly , '' a reporter says . Nebraska warily watches the rising South Platte .