Oil from a ruptured Shell Oil Co. pipeline floated down the Mississippi River Saturday and forced cleanup crews to work around the clock to try to limit effects of the spill, a Shell spokesman said.

Missouri Department of Natural Resources officials said the spill - the worst in the state's history - began Dec. 24 when a pipeline at Vienna, Mo., ruptured, allowing at least 336,000 gallons of dichlorobenzene, a petroleum product, to flow into the Gasconade River.The spill had not caused problems with water supplies by Saturday.