The precipitation fell onto ground saturated by an unusually wet spring, running directly into streams already high from a melting snowpack in the mountains.
"A lot of those rivers have jumped up," National Weather Service (NWS) forecaster Mike Conger said. "They were just in the perfect location to get these consistent, moderate rain showers."
The brunt of the storm - which is actually two systems, one from Canada and another from California - dumped large amounts of rain from Wednesday night to Thursday afternoon, Conger said. Showers and thunderstorms were expected to taper off by this afternoon.
Some of the worst flooding occurred on the Logan River and its major tributaries, the Little Bear and Blacksmith Fork rivers; and along normally small creeks that flow through Box Elder County.
By 8 a.m. Thursday, the NWS had called a flood warning for Cache Valley.
About the same time, dozens of basements began to fill with water.
Ryan Yonk realized early that the two pumps in his Logan home's basement were not keeping up. By 1:30 p.m., the water had stopped rising and workers had siphoned 1,500 gallons out of the basement.
It would have been much worse, if not for volunteers and Logan City, who on Monday had placed hundreds of sandbags in the Country Manor subdivision, near the confluence of the Logan and Blacksmith Fork rivers. Flooding in 1997 and 1999 had prompted city officials to work in advance to prevent a similar disaster.
In other parts of the city, floodwaters covered Sumac Road and forced the closure of Park Avenue, approximately 600 West between 1000 South and 1800 South.
Five miles up Logan Canyon, DeWitt Springs was overflowing, prompting concerns about contamination of the drinking water source.
Logan officials were also concerned about some of the city's sewer ponds overflowing.
Further north, in Richmond, volunteers and firefighters helped divert raging floodwaters away from homes and into the streets. After the water was contained on State Street, workers and volunteers moved to 100 East where they continued to stack bales of hay with heavy equipment and sandbags with a human chain.
Portions of pavement were washed away near 400 South and 100 East.
Sandbaggers and county employees managed to control the floodwaters, though some flooding was reported inside a home at 1180 S. State St. and at Gilt Edge Flour Mills, 1090 W. 1200 North.
As the potential for disaster became apparent, Cache County Sheriff Lynn Nelson activated the county Emergency Operation Center midmorning for the first time in the new administration building that also houses the county jail. The sheriff's office enlisted 18 inmates to fill sandbags, said sheriff's Capt. Kim Cheshire.
Law-enforcement officials countywide had up-to-the-minute reports of paved roads being partially washed away, a bridge collapsing and reservoirs being tapped to prevent uncontrollable overflow.
Elsewhere in Cache County:
* Porcupine Road, beneath Porcupine Reservoir southeast of Hyrum, was overflowing as reservoir officials released water to prevent it from spilling uncontrollably. Officials were taking similar measures at Hyrum Dam.
* A bridge washed out at 3000 South and 3000 West in College Ward.
* Minor residential flooding was reported in Hyde Park.
* The Blacksmith Fork River flooded in Nibley, along Hollow Road, creeping perilously close toward homes. A bridge on State Road 165 in Nibley was closed.
* Sewage reportedly backed up in some homes in Wellsville.
In Box Elder County, two homes were evacuated and about a dozen others sustained basement flooding in Brigham City as a result of rising waters from Box Elder Creek, which runs through the town.
As the water continued to rise, Brigham City resident Linda Kay stood outside her home near 300 West and 500 North and watched as city crews and countless volunteers lined her back yard with sandbags.
"It's normally just a few inches high and hardly ever comes close to the edge," said Kay, whose basement was flooded slightly before volunteers corralled the water.
The water near 300 West slammed into the channel wall and spilled onto other properties, instead of heading under the road, where rainfall normally runs. Volunteers removed more than four backhoe loads of twigs, branches and tree parts near the area.
Authorities were also keeping an eye on Mayors Pond, a catch basin east of the city.
"It's very full and we're working to shore up the spillway to keep it channeled," said Brigham City manager Don Tingey.
Tingey said the city had been clearing Box Elder Creek of debris since early spring so the water had a place to go during flooding.
In neighboring Garland and Tremonton, city officials asked the Utah Division of Water Quality for permission to discharge raw sewage into the Malad River.
"They really have no recourse, unless they let it back up into people's basements. They don't want that and neither do we," said division director Walt Baker. "It's less of a risk to put it in the river."
Baker said his division also is watching about 10 industrial-size farms in Cache Valley, known as "concentrated animal feeding operations," which are permitted to discharge animal wastes only in case of a "25-year" water event, though he said it is too early to say whether Thursday's flooding qualifies as such.
The heavy rains are expected to ease drought conditions in northern Utah. The National Climate Prediction Center has forecast the drought will improve in the short term, according to Tess Davis, a research technician at the Utah Climate Center.
But Cache County Extension Agent Clark Israelsen said too much water may do more harm than good.
"As far as the crops are concerned, we have had sufficient - and then some," he said. "The ground is totally saturated. In fact, we have water running in every direction."
Perennial crops such as alfalfa and winter wheat will be less productive and spring barley and corn silage will have a shorter season after they go in the ground later than usual because of the weather conditions, Israelsen said.
"We went from snow to mud and standing water," he said. "We are happy about having moisture in the irrigation systems and in the soil, but we need a month of sunshine so we can get the crops in."
Jason Chaffetz, Gov. Jon Huntsman Jr.'s chief of staff, inspected the flooded areas Thursday by land and by air.
"The reservoirs are to the brink and streams are flowing as fast as they possible can," he said. Flooding ''is very widespread, especially in the agricultural communities."
Though city and county officials appeared to be working successfully to control the flooding, Chaffetz said the governor has offered them extra staff and equipment if necessary.
A flood watch will stay in effect today across northern Utah, but hydrologists said they do not expect any serious flooding.
The major storm was expected to move southeast of the state today, but smaller showers and cooler temperatures may persist through Saturday.
Tribune reporter Joe Baird contributed to this story.