(CNN) The rain-swollen Cedar River has swamped thousands of homes across central Iowa, where volunteers have been struggling to keep the cresting river's floodwaters from doing further damage. Some 2,500 people have been evacuated from their homes in central Iowa, and state officials said that one-fifth of the state's 99 counties could qualify for federal disaster aid. Flooding also was a problem in Michigan, where torrential rains flooded a highway, and cars crawled through high water in Detroit.