Possibly the oldest bridge in Plymouth County, this 1911 structure northwest of Le Mars is one of 470 bridges on the county's rural roads -- a record for the state of Iowa. [Click to enlarge]

If you could put all the bridges in rural Plymouth County end to end, they would span four and a half times the length of the entire Golden Gate Bridge.

The county boasts 470 bridges on the secondary roads -- more than in any other county in Iowa, according to County Engineer Tom Rohe. Putting their decks end-to-end, he said would stretch 42,176 feet.

Rohe speaks of the bridges with a familiarity that comes from 25 years at his job. There's what he estimates to be the oldest bridge in the county -- the concrete truss bridge just northwest of Le Mars on 165th Street that proudly boasts "1911" in the middle of its arch. Some of its high railing has crumbled.

"It got hit a year ago with a disc," Rohe said. "We're watching it to try to keep it open as long as we can."

Then last year, there was the bridge on Fir Avenue west of Craig that met its limit. A tractor with a grain cart attempted to cross it. The tractor made it. The grain cart didn't.

"I guess it was kind of like dropping an anchor," Rohe said. "We normally don't have incidents like that. It was a very light bridge."

The bridges, whether wooden, metal or concrete, new or weathered, spot the county with about one for every two square miles.

Rohe said part of the reason Plymouth County tops Iowa in bridge numbers has to do with the county's size.

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"We're a 24-township county, so area-wise we're larger than most," he said. The majority of Iowa counties have 16 townships.

The rest of the reason is the lay of the land. Plymouth County's terrain is laced with what Rohe calls "drainage structures." Creeks, streams, rivers run like veins across the county's 865 square miles.

But Plymouth County's boon of bridges may be par for the course: Iowa ranks fifth in the nation for number of bridges with a total of 25,075.

In any case, those bridges add up to a hefty budget for Plymouth County: for next year's construction program Rohe estimated about $1.3 million for replacement and most of that is bridges. That's not counting the $75,000 just for bridge maintenance work.

"We naturally have larger budget because of the size of our county," he said. "But we also do receive more Federal aid dollars for bridge replacement."

The bridges are demanding, Rohe said. The county engineering department, which is in charge of all 1,430 miles of secondary roads, has to dedicate much of not only their money but also their time to them. The keep bridges on a running 50-year replacement cycle.

"On some bridges be able to go longer," he said. "But wooden bridges have a 40- to 50-year age limit."

That means the county replaces at minimum 8-10 bridges a year, but Rohe said it's best they get more in. In 2007 about nine bridges are up for replacement.

"We just try to do as much as we can each year," Rohe said. "That's what most counties are doing."

The 1911 bridge northwest of Le Mars has lasted longer than most, and they're keeping an eye on it.

"All bridges are inspected on a two-year cycle minimum," Rohe said. "This is one we inspect on a yearly basis."

On a low-maintenance dirt road, the nearly century-old bridge has a lot riding on it, figuratively speaking.

"If it ever did get at the point where we needed to close it, we might close the road, too," Rohe said.

Some of the other oldest bridges are a 1915 beam bridge in Sioux Township on 235th Street in section 11, a 1917 pony truss bridge in Henry Township on 220th Street in section 2, a 1917 bridge in Remsen Township on 165th Street in section 5 and a 1917 bridge in Marion Township on Pioneer Avenue in section 3.

In other locations, bridges are being replaced with concrete culverts. Rohe said they replace four or five bridges each year with culverts.

So are bridges becoming a thing of the past?

Rohe said probably not.

"We decide between a bridge or culvert depending on the size of drainage area and opening we have. Only the smaller areas replaced with culverts," he said.

Back in the 1950s, he said, they built a lot more small wooden bridge structures rather than culverts.

"Maybe because lumber was cheap," Rohe said. "Wood by far was the material used in the 50s and 60s."

Wood now is used mainly for gravel road bridges, while the 109 bridges on paved secondary roads are mostly concrete.

Along with prices of materials, standards have changed over the years.

Bridges built then, Rohe said, weren't constructed for today's traffic. Now bridges have to hold up under weight of stronger tractors and trucks and heavier loads.

"One of the biggest challenges is replacing bridges for the current standards," Rohe said. "We just try to maintain as many structures as we can to maintain the road system."