Anyone who has spilled a drink into an open book knows how a flash flood works:

        • You tip the book to get rid of the liquid.

        • Everything that doesn't soak into the paper rushes down the groove in the center.

        • You're left with a mess.

        The same thing happens in gullies, washes and creeks.

        Sudden, heavy local rains overwhelm their ability to carry water away, and everything that spills over the banks is a flash flood.

        Tuesday night and early Wednesday morning, heavy rain turned Little Duck Creek and Sycamore Creek into deadly torrents. A father and daughter died in a Fairfax basement, and a teen-ager was swept away after rushing water in Symmes Township stalled the SUV in which she was riding.

        Up to 8 inches of rain fell on Hamilton County in about an hour, according to Patrick Karney, director of the Metropolitan Sewer District.

        “It falls so fast that it just won't sink into the ground,” he said. “It puddles. Then it runs off. That's going to hit surface streams and overwhelm them.”

        The speed with which rising streams sluice downhill adds violence that can float away a mobile home or push an SUV off the road.

        Flash floods on the Ohio and Licking rivers are rare because the intensity of local rainstorms gets reduced and it takes so much water to raise their levels, said Steven Buchberger, an associate professor and water-resource engineer at the University of Cincinnati. As a result, impending danger comes with hours or days of warning.

        But in a creek that usually is small enough to leap or shallow enough to wade, the sudden fall of a few inches of rain can fill the narrow confines to overflowing.

        “The stream can't contain it,” Dr. Buchberger said.

        Typically, he said, steep slopes leading to the creek or gully increase and accelerate the water flowing into it and leave little land to absorb the rain or store any overflow.

        Further adding to the danger is the likelihood that such rain-swollen streams follow steeper paths than larger rivers, and this multiplies the force with which the water hits anything in its way.

        In the same way, rising water flowing under a vehicle can make it buoyant, Dr. Buchberger said, increasing the risk that pressure will push it off the road.

        The rain gauge at his Montgomery home recorded more than 6 inches Tuesday night, he said. “I've never seen anything like it in the 12 years I've lived there.”

        The road to a nearby stream was closed by authorities because it was out of its banks, he added. “It was real bad.”

        Duck Creek has been a hazard for decades, but Sycamore Creek is a more recent problem, according to John Beck, a hydraulics technician with the Hamilton County engineer's staff.

        He said the creek's water-carrying capacity has been reduced by erosion, the buildup of silt, residents using it to discard yard waste, and fallen trees.

        All that has been worsened by development that left less land to absorb rain in the area drained by Sycamore Creek, he said.

        Finally, as with most small waterways, the violence of sudden, intense rains washes extra debris into them, and “that plugs up your bridges and culverts and compounds the problem,” Mr. Beck said.

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