Sleuthing the lower Ohio River along Illinois's southern tip for sunken wreckages of boating past, Mark Wagner and Bob Swenson often find the gumshoe work a frustrating race against the unpredictable waterway.

Once or sometimes twice a year when the Ohio gives them the chance, the Southern Illinois University staffers scour for suspected graves of a former Civil War gunboat or other vessels of the time. The water often teases them, receding to briefly give up skeletons of 19th-century navigational life, only to swallow up the evidence again.

Upriver dams, commonly used to raise water levels to accommodate barges, can wreak havoc on their plans, quickly swamping an exposed relic. River-swelling rains do not help, either.

"It can be very frustrating," said Wagner, an archaeologist with the university's Center for Archaeological Investigations. "If you want to do any work, you have to schedule it around the river."

So it went recently, when the two took advantage of extremely low Ohio River levels and scouted by helicopter a 45-mile stretch of the waterway, spotting the wooden carcasses of several former steamboats and wooden barges.

But more importantly, Swenson said, he saw something sticking out of the mud near the mouth of the Cache River, not far from Mound City, Ill., an outpost about 44 miles south of Carbondale. Rumors had it that folks there, when they were kids, spotted remnants of a Civil War gunboat, including a cannon, when the wreckage was visible decades ago.

Could it be the final resting spot of the Cincinnati, among the vessels swaddled in iron and weaponry to become a fighting ship when the North warred with the South? Such a find could be big, with no known "ironclads" having been found along the Ohio, Wagner said.

"We now have a place we can look more closely," said Swenson, an architecture professor. "If we're extraordinarily lucky, it would be the Cincinnati."

To Swenson's knowledge, no other gunboat has been salvaged, aside from the USS Cairo, said to be the first warship to be sunk by an electrically detonated underwater mine or torpedo. That 175-foot ship, sunk in late 1862, is now on display at the Vicksburg National Military Park in Mississippi.

The Cairo and the Cincinnati were among seven "city class" ironclads -- bearing the names of U.S. river cities -- built for Union forces by James Eads's company during the Civil War to help wrest control of the Mississippi from the Confederates. Three of those twin-engine, 13-gun ships ensconced in thick iron -- the Cairo, the Cincinnati and the Mound City -- were built in Mound City, the other four in St. Louis.

Such ships essentially were steamboats retrofitted with armor, making them floating artillery batteries. But their underbellies were still wooden, making them vulnerable to underwater debris or, in the Cairo's case, explosives.

The Cincinnati, commissioned in 1862, later was sunk twice and raised before being decommissioned in 1865, about four months after the end of the Civil War, according to the Missouri Civil War Museum's Web site. It was sold at auction in New Orleans in 1866, not long before it sank possibly near Mound City, according to what Swenson said were "very limited records."

"We'd be really interested in the Cincinnati," specifically to see what is left of it and how the boat was built, Wagner said. "We don't know all that much about how these boats were put together."

It is possible the remains still may include Civil War-era artifacts, including stoves and personal items.

Equally elusive has been the Red Rover, a 625-ton steamer initially built for commercial use but converted into a hospital ship with an operating room. Its staff included the first female nurses to serve aboard a Navy ship. The Red Rover was stationed at Mound City until late 1865, when it was decommissioned and sold. It is believed to have sunk in an accident across from Mound City, near the Kentucky shore.

No one knows how many wreckage sites remain underwater or beneath farm fields -- hidden there by a river's shifting course -- or were ripped apart by currents. But most agree that along the Ohio, Mississippi and Missouri rivers, the watery graveyards are legion.

Those waterways have claimed hundreds of smoke-belching steamboats and other wood-framed vessels, many of them sunk by such calamities as fires, explosions, ice floes and run-ins with toppled trees.

Sometimes, Swenson said, high waters that keep the shipwrecks hidden may be a blessing, safeguarding the sites from weathering that could warp and rot the wood.

Pinpointing the wreckages has required different approaches, from the use of a helicopter recently by Wagner and Swenson to the projection of multiple sound beams elsewhere along a river's floor to generate three-dimensional images of the outlines of individual ships. Such technology helped pinpoint the suspected location of the USS Chickasaw -- another Eads ironclad -- up the Mississippi from New Orleans's French Quarter.