An elusive one-ton concrete block appeared and then disappeared in Boulder County homeowner Mark Gerzon's front yard, blocking a culvert and causing thousands of dollars of damage to his yard and home during September's floods and heavy rains.

Gerzon called it a Boulder flood mystery that may never be solved.

He first noticed the concrete block, which he described as roughly 6 feet wide, 2 feet thick and 2 feet tall, about a week after the heavy rains that caused major flooding and damage across Boulder County.

He started wondering why, a week after the heavy rains had ended, the swollen Fourmile Canyon Creek near his home at 57th Street and Woodbourne Hollow Road wasn't receding. It had grown from about 2 feet wide to 75 feet wide because of the intense rain, Gerzon said, and covered a large portion of his yard.

When Gerzon went outside with a friend to see if debris had blocked the culvert that runs underneath 57th Street, he found a giant, one-ton concrete block sitting squarely in front of the culvert, stopping all water from rushing under the street. It had a metal hook on top, as if a crane or some other machine was needed to lift it, he said.

He called around to see who, if anyone, was responsible for the concrete slab that probably contributed to the creek overflowing into his yard and basement. Gerzon blames the concrete block for pushing floodwaters into his yard, destroying a two-room outbuilding on his 1.5-acre property and ruining all the appliances in the 1500-square-foot crawlspace beneath his home.

The water took so long to recede that Gerzon is still drying out his yard and the crawlspace, he said.

Gerzon called Boulder County, which maintains both the culvert and the roads in his neighborhood. Boulder County Road Supervisor Ted Plank said the county would only be hurting itself by blocking culverts during the flood. Because of the high number of roads washed out by the flooding, county road crews couldn't get to Gerzon's neighborhood until this week, Plank said -- almost a month after Gerzon saw the concrete block.

"The whole thing is a mystery to us," Plank said. "The county had no equipment in the area. We weren't doing any work and certainly we didn't block our own culvert, that would be silly. It had to have been by a private party."

Plank wondered aloud if a neighbor downstream deposited the concrete block in an attempt to keep some water away from their property during the heavy rains and flooding.

The day after he called Boulder County, Gerzon went outside to photograph the offending block. It was gone.

Gerzon said also he saw tire tracks in the mud nearby.

"Now I'm really getting curious," he said. "Coincidence that the next day it's gone? Who dropped this thing? Where'd it come from?"

The riddle persisted. Later, while he was out driving, Gerzon noticed huge stacks of concrete blocks behind a fence near 61st Street and Valmont Road. He called Martin Marietta Materials, which owns Boulder Ready Mix at that location, and the company explained it sold those types of concrete blocks every day. It could've been anyone, they told Gerzon.

The ditch company, which is responsible for maintaining the waterway itself but not the culvert, also denied any involvement or knowledge of the concrete block in Gerzon's yard. Boulder and Left Hand Ditch Company president John Brunner said it likely washed downstream and got stuck near Gerzon's culvert.

"I have no idea, other than it actually could've moved down there," Brunner said. "We don't have any reason to put anything in the ditch. Where it came from I have no idea. I would assume it was part of the debris that came out of the flooding."

Though he consulted several lawyers, Gerzon eventually decided not to take legal action, and because he was so busy trying to repair damage to his home from the flood waters, he didn't have time to investigate. Gerzon decided the mystery of the concrete block was one that would have to play out by itself.

"It'd like to get to the bottom of it," he said. "I'm not going to pursue legal action but I am hurt that someone would do that."

Contact Staff Writer Sarah Kuta at 303-473-1106, kutas@dailycamera.com or twitter.com/sarahkuta.