Lincoln, Neb. ? In a season of flooding by some of the nation’s biggest rivers, it’s streams most Americans have never heard of that could cause some of the worst problems in the Midwest.

Hundreds of tributaries that feed the congested Missouri River face a greater-than-normal flood risk this summer because of water levels that have kept them from draining. The Missouri is expected to remain near historic highs for months, which means the threat will remain through summer — a season when the Midwest often is beset with thunderstorms that can quickly dump heavy rain.

University of Iowa engineering professor Witold Krajewski, director of the Iowa Flood Center, compared the situation to a “traffic jam” of water. It started with the sustained release of massive amounts of water from dams on the upper Missouri, caused by spring rain and a heavy Rocky Mountain snowpack that filled reservoirs.

“It’s like a football game and baseball game getting out at the same time,” Krajewski said. “There are all these cars trying to move. It doesn’t take long before drivers can’t get out of the side streets.”

Backed-up tributaries in South Dakota and Missouri have already submerged streets and threatened homes near the point where they reach the surging river. In Hamburg, Iowa, work crews are keeping close watch on changing levels of the Nishnabotna River to their east, as they try to hold off the Missouri River from the west.

Some tributaries, such as the Nodaway and Big Sioux, are so backed up with high water in the larger river that local officials worry that a well-placed downpour could suddenly pose a new flooding threat.

Nebraska State Climatologist Al Dutcher said the problem won’t end soon.

“We’re probably going to see the tributaries be problematic all year,” Dutcher said. “If we get a monster storm that drops 3 to 4 inches of rain into a basin, all that water has to be evacuated. As it goes out toward the Missouri River, it’s going to back up.”

Bob Anderson, a spokesman for the Army Corps of Engineer in Vicksburg, Miss., said similar problems could have happened during spring flooding along the Mississippi River, but heavy rainstorms didn’t occur at key times and tributaries didn’t swell beyond their normal size.

The flooding along the Souris River that has devastated Minot, N.D. is due to heavy rains, not a tributary backup.

In southwest Iowa, crews in Hamburg have devoted most of their time to building a temporary levee to stop the Missouri, but officials said there is little they can do to protect against flooding by the Nishnabotna if an intense rainstorm hits the area.

In northwest Missouri, the Nodaway River is so clogged with water as it approaches the Missouri River that water is running backward nearly three miles upstream, said Andrew County Emergency Management Director Roger Latham. The township of Nodaway, a tiny community of about 25 people, has evacuated amid fears that the tributary will swell and swamp the area.

“If you want to drop a rubber ducky in the river, you can watch it float upstream,” Latham said. “The Missouri River probably will not affect the township. But if we get any significant rainfall, the Nodaway’s got nowhere to go but sideways.”

One resident, Gary Nold, said his riverside cabin has taken nearly 3 feet of water from the tributary — the most he’s seen since massive flooding in 1993. Dirty water from the Nodaway, filled with corn stalks and tree limbs, crept over its banks and forced the final holdouts in the area to seek dryer ground.

Nold said the cabin, about three-quarters of a mile upstream of where the Nodaway and Missouri rivers meet, has flooded several times before. But the 75-year-old retiree said all the past floods have come from intense rain upstream, not backed-up water.

“I was smart this time,” he said. “I took everything out ahead of time. It’s getting old, and I’m upset I’ll have to clean it. But I’ll be back.”

The tributary threat comes in the thick of the region’s rainiest season. Iowa, Missouri and eastern Nebraska traditionally receive their heaviest rain during the summer.

So far, areas that drain to the Missouri River have been relatively lucky.

Iowa State Climatologist Harry Hillaker noted that in central Iowa, Des Moines received about 10 inches of rain during the middle of June — but that water drains into the Mississippi River. In western Iowa, rainfall in the past month has been below normal, Hillaker said.

“Eastern Nebraska and western Iowa are the epicenter of the world for overnight rain,” Hillaker said. “We can get very heavy rains that go on hour after hour in the right conditions. That would be another issue for the Missouri. These big, flash-flooding kind of rains can really get to be a problem.”

Even without heavy rain, the Missouri has caused headaches in Dakota Dunes, S.D., where the Big Sioux River has backed up about a miles from the Missouri and covered several streets in the community.

After backing up, several creeks in Kansas have topped or broken through levees near the Missouri River, said Scott Watson, a National Weather Service hydrologist in Kansas City. Watson said the region’s wettest season won’t end until late July.