''Some of these things were as big as cars, and they'd piled so thick in one tree belt it looked like a sand dune,'' said John Rothstein, who owns an insurance agency here. ''People had to take front-end loaders and tractors just to uncover their cars.''

Tumbleweed is the common name for a variety of plants that include the Russian thistles prevalent in South Dakota. They grow into fat puffballs in untended fields and ditches and along fences. In late summer and early fall, it is common to see them rolling across a highway or clinging to barbed-wire fences.

The tumbleweeds that bombarded Mobridge grew to an unusual size in the fertile river valley soil that is normally covered by the water of Lake Oahe, a 200-mile-long lake that extends almost from Pierre, S.D., to Bismarck, N.D. Two years of drought dropped the level of the lake this summer to 38 feet below the high-water mark. That uncovered an area of lake bottom two miles long and nearly one mile wide just outside of Mobridge.

A powerful west wind gusting to 60 miles an hour snapped the stalks of huge tumbleweeds and pushed them into town. A Fire Hazard

''When I got up that morning, I couldn't see anything from most of my windows the weeds were so high and thick against my house,'' said Darlene Zahn. ''I couldn't even budge my door to let my little dog out, they were that tight.''