Cities along the Mississippi River have been battling the worst floods in 15 years, which have killed at least 24 people and flooded more than 3.4 million acres. President Bush has declared more than 100 counties in Illinois, Missouri and Iowa disaster areas, and at many points only volunteer-laid sandbags are holding back the teeming floodwaters. And on Friday morning, in Lincoln County, Mo., the U.S. Army Corps of Engineers' efforts were undermined by another threat: muskrats.

The little critters made enough holes in the earthen berm to significantly weaken the Pin Oak agricultural levee, and overnight rains helped the floodwaters break through Friday morning around 5:30 a.m. The muskrats were looking for food or seeking to make dens in the levee, and emergency crews had had to patch up a hole earlier in the week created by a single burrowing muskrat, whose hard work was described as creating a "geyser of river water."

The levee was the last one remaining in Lincoln County and was protecting about 100 homes, which were immediately evacuated. Almost 700 homes in the area have been damaged by flooding.

Meanwhile, wildlife officials have expressed concern about the effects of the flooding on the local rodent population, including muskrats. The seminal scholarly study is still "Effect of Tidal Flooding on Juvenile Muskrats," which appeared in a 1990 edition of the journal Estuaries.