Not quite 5 miles long, Jerome Creek winds through farmland and Pleasant Prairie, Wis., about 35 miles south of Milwaukee. In some places, it's narrow enough to jump across. It fish population consists mostly of minnows.

Yet this unremarkable stream could be an ecological time bomb, for it's a crucial link in a chain of waterways hundreds of miles long that connects Lake Michigan to the Mississippi River. Experts say the creek might be a doorway through which invasive species _ including the much-maligned Asian carp _ will slip between the Mississippi drainage basins and the Great Lakes.

In recent years, the battle to prevent the plankton-gobbling carp from entering the Great Lakes and disrupting their $7 billion fishing industry has focused on the Chicago area, where a shipping canal forms what some call a "superhighway" for invaders. But the U.S. Army Corps of Engineers is taking a closer look at 18 other waterways from New York to Minnesota that could provide an aquatic back alley for the wily aggressors.

The corps is analyzing which of those secondary routes pose the biggest threats _ a task that became more urgent this month with the discovery of Asian carp DNA in the St. Croix River on the Minnesota line and an angler's catch of a bighead carp in the lower Wisconsin River. Neither river has a direct tie to the Great Lakes, but both are tributaries of the Mississippi. The finds were further evidence of the carp's steady northward march, which makes shielding the lakes ever harder.

"The Asian carp has proven over and over again that it defies predictions, it defies limitations that scientists want to put on it," said Thom Cmar, an attorney with the Natural Resources Defense Council. "It has repeatedly surprised scientists with its ability to spread and breed and proliferate in U.S. waters."

Imported from China, the carp escaped into the Mississippi from Deep South fish farms and sewage lagoons in the early 1970s. They've fanned out across dozens of rivers, creeks, ponds and reservoirs. Bighead carp have turned up in 26 states and silver carp, the other Asian species on the Great Lakes' doorstep, in 17 states.

The bighead can reach up to 4 feet long and 100 pounds, while silver carp are notorious for hurtling from the water like missiles when startled, at times slamming into boaters with enough force to shatter bones. Biologists say it's uncertain how much damage they would do if established in the lakes. But under a worst-case scenario, they would unravel the food web by consuming huge amounts of plankton _ tiny plants and animals on which most fish rely at some stage of life.

With so much at stake, federal and state agencies are working to identify potential gateways across the basin divide. In a report last year, the Army Corps identified 31 such locations. Most are natural wetlands and streams or man-made farm ditches and transportation canals. Eighteen were classified as a medium, high or acute risk.

Among them was little Jerome Creek, which runs near the basin boundary. During high-water periods, its headwaters could link up with a nearby stream that flows into Lake Michigan, several miles away. On the other end, it's a tributary of the Des Plaines River, which courses southward to the Illinois River, which intersects with the Mississippi more than 300 miles south of the creek.

The creek is considered a medium risk as an invasive species route. During the dry season, some portions have little water. But storm runoff can fill it quickly.

"There's a potential that during high flow or flooding, an invasive species could use the creek to get from Point A to Point B," said Bob Wakeman, invasive species coordinator for the Wisconsin Department of Natural Resources.

According to the report, the riskiest site outside Chicago is the 700-acre Eagle Marsh near Fort Wayne, Ind. There, outlying waters from the carp-infested Wabash River sometimes mingle with the headwaters of the Maumee River, a tributary of Lake Erie. That could produce a disaster, said Duane Chapman, a U.S. Geological Survey fish biologist. Erie is the shallowest and warmest of the Great Lakes and has the most abundant fishery.

Chapman told The Associated Press he and a colleague are about to publish a paper identifying several rivers feeding Lake Erie, including the Maumee, as suitable Asian carp spawning habitat.

"It looks like the Asian carp could invade Lake Erie effectively," he said. "It has the food they need, it has the right temperature, it has rivers that are the right length. I don't know what else could hold them back, other than not getting enough fish in there to reach critical mass."

Authorities placed a chain-link fence in the marsh last year to block adult carp from getting through while a permanent solution is devised. The Army Corps plans to make recommendations by year's end.

Other high-risk sites include an Ohio lake that links Lake Erie and the Ohio River, plus wetlands in Minnesota that bring together waters feeding the Mississippi and the St. Louis river, a Lake Superior tributary.

Also due this year is the more extensive Army Corps analysis of the likelihood that Asian carp or other critters will use any of the 18 potential entry points. The Army Corps last month released a list of 40 species considered high risks for crossing the Mississippi-Great Lakes divide, including the aggressive northern snakehead and other fish, mollusks and plants.