When you've seen something one million times, you stop noticing it.

But the little concrete structures with windows atop Lock 15 at Davenport/Rock Island have held my curiosity for as long as I can remember. Not unlike the offices that were built into the piers in the underbelly of the Interstate 74 bridge (Off Limits I), the fact the structures contain windows has been the driving force behind my need to know.

As Lockmaster Derrick Glisan turned his key this past week in the first "pier house," closest to the Arsenal Island shoreline, I searched my imagination. But I landed on no real expectation.

What is all that stuff? How does it work? Who keeps it running?

The roller dam at Lock & Dam 15 is the longest in the world and the first to be built on the Mississippi River.

From its aged, yet pristine machinery to the strange debris it collects in its rollers, this Off Limits place delivered.

The dam houses

When the door swung open, I wasn't entirely sure what I was seeing. Directly inside, my eyes fell first on a long brass plate, mounted at about eye level, with a needle that clearly was marking the level of something.

To its left was five or six concrete steps. Each was about two times deeper than regular steps, which made the railings on either side especially helpful for pulling yourself upward.

The narrow, metal walkway at the top overlooked a set of gears, and the main gear is the size of a small Ferris wheel.

"Watch out, there," Glisan warned. "Birds fly in from underneath, and they get grease on their feet from the gears, and then they land on the railings."

But it was too late. If there's a railing nearby, you can count on me grabbing it, so the grease already was in hand.

One thing repeatedly occurred to me during the tour: That dam is in terrific shape for its age. The engineers thought of everything, including how to get a broken-down gear out of a concrete-sealed pier house.

So, they built the roofs (tiled in terracotta) in such a way that they can be removed by a crane. They even built an "eye" in the center of the roof, so the crane has something to grab.

In some ways, the roller dam seems complicated — with its massive gears and components for raising and lowering the gates and controlling the flow of the river. But it's really the massiveness of it that makes the engineering feat all the more impressive.

The brass plates that are mounted inside each of the 11 pier houses have needles, indicating the vertical opening of the gates in feet. There are only four buttons on the controls: Stop, Raise, Lower and Bypass. When the gates are raised as high as they will go, their ends fit neatly into the concrete pier houses, which were sculpted for a perfect fit.

"The end shields (on the gates) fit right up into their spot," Glisan said. "You can see the red paint on the walls from the gates being raised. Imagine setting those gates. The engineering is impressive, but think about the manpower.

"Think about getting those gears up here. It was all manpower. Our job, primarily, is the maintenance. Preventative maintenance on these structures is huge."

The collection of gears in the pier houses turn what Glisan described as an enormous bicycle chain. It moves the giant, red cylinders, or gates. There are 11 roller gates, so there are 11 pier houses.

An 83-year-old dam with an important purpose in life — helping to sustain Mississippi River navigation — requires constant maintenance.

Twice a year, Glisan and the 12-person lock staff pull on Tyvek suits and climb into the gear housings with brushes and buckets of grease to lubricate the gears.

They also have to access the gears and the giant bike chain that are not visible from the walkways inside the pier houses. This means they must climb straight down ladders that are built into the concrete, leading all the way down to the river.

Inside the houses, there are plenty of railings, but the workers must wear life jackets and safety harnesses when performing maintenance in the bottom of the pier houses.

"As long as you're tied off, you feel OK down there," Glisan said.

The birds have been enjoying the space just fine. For years, the owls, hawks and large numbers of pigeons that were getting into the pier houses were making a terrible mess. But Glisan and crew climbed into the bottom of the pier houses and stuffed chicken wire in the spaces along the floors to keep them out.

"There for a while, it was like a chicken coop in there," he said. "The wire seems to be working, so I'm sure the pigeons are unhappy."

The dam approach

As we walked under the Government Bridge, which connects Davenport to the Rock Island Arsenal, I saw something I never noticed.

People who have toured Lock & Dam 15, or who have been bold/hapless/unwise enough to boat under the bridge, probably know about them: Dozens of ropes, hanging from the underside of the bridge. At the bottom of each rope is a block of wood about half the size of a shoebox, suspended just above the surface of the water.

Dubbed "last chance lines," the warning devices offer a final caution that the dam is dead ahead while also extending a last-minute lifeline.

For me, they were the "last chance" to notice something other than the massive concrete dam we were about to climb.

When navigating concrete stairs above the Mississippi River, some of us aren't as keen as others on taking in the sights. We prefer to concentrate on the task at hand, surviving.

For better or worse, arriving at the top becomes more of a surprise when you stare straight ahead on the way up.

I never would have guessed there is, basically, a roadway on Dam 15.

From one end of the 1,203-foot-long dam to the other is a wide, metal-surfaced pathway that is used both for human access and for the crane that is on constant standby. It is wide enough for any car or SUV. But it also contains rails (just like on a railroad) for moving the crane back and forth. The pathway would be a spectacular place to watch fireworks.

"It's definitely a good view for being at work," Glisan said.

He pointed out a well-protected structure, separate from the pier houses, that contains one of the oldest hydro turbines in existence.

"Its serial number is 02," he said of the power-maker, which dates to the year the dam began operating: 1934.

Today, the turbine produces enough electricity to supply the lock and dam, Visitors Center and Clock Tower, leaving some to sell to the Arsenal.

Weird stuff in the water

From the windows of the pier houses, the rolling downstream water is mesmerizing -- like staring into a fire, if the fire made you slightly dizzy.

Spotting a vortex against one dam wall, I saw a circulating "eye" the size of a manhole cover and asked Glisan what it was.

"I just call it the death circle," he answered.

Floating nearby, the body of a pelican offered an answer, too: The force of the water, churning and whipping in violent, unpredictable circles, is every bit as dangerous as it looks.

"Walleye fishermen get as close to the dam as they can," Glisan said.

Another member of our tour group, Mike McKean, Park Ranger at the U.S. Army Corps of Engineers' Visitors Center at Lock & Dam 15, said some fishermen are repeat offenders. When they are spotted getting too close to the dam, and they are recognized from being too close in the past, there are ways to catch them.

"We'll get a picture of the boat — try to get the registration number — and report it to the DNR," McKean said. "We've had a boater climb up and get caught walking on the dam wall. The DPD (Davenport Police Department) has been called."

Before coming to Lock & Dam 15 a couple of years ago, Glisan worked at Lock & Dam 16 in Muscatine. He said he frequently would see a "black cloud" circling the dam there, which turned out to be a giant school of catfish. This, of course, reminded me of the tales of catfish the size of humans, making their living by feeding at the dam floor.

In the Visitors Center, I'd noticed pictures of fishermen with catfish weighing more than 100 pounds, but they were caught on the Mississippi River in Missouri.

"I did see one monster here," Glisan said. "It was dead, but (holding his arms out to his sides like an airplane) it was this big."

The roller dam can be hard on animals. Deer have died there, along with many birds. More than one human drowning victim has been pulled from the river there, too. On a brighter note, Glisan said, ducks and geese frequently build their nests upon the driftwood that collects at the roller gates. And I spent a lot of time taking inventory of the debris.

"You could start a sporting-goods store if you could get out all the stuff," the lockmaster said. "You get frustrated by what people throw in the water."

And I had no idea so many ballgames are played around the river. We must have seen 20 balls in that one tour, with basketballs and soccer balls leading the collection. There were plastic water bottles by the dozens, tires, channel markers and some things we couldn't quite identify.

"The problem is, there's really no safe way to get it out of there," Glisan said, adding the collection of debris at pier house 11, closest to the Davenport shore, always has the most junk.

"They throw more stuff in the river on the Iowa side," he said.

But it was just a dam joke.

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