Dozens of illegal immigrants each night enter the New River in Mexicali, Mexico, and float north, hoping to elude U.S. Border Patrol agents who usually avoid the river for fear of pollution. Mexicali dumps 20 to 25 million gallons of raw sewage into the river daily, according to the California Water Resources Control Board, and the river also picks up agricultural and industrial wastes as it flows north. By the time it reaches the border town of Calexico, Calif., it violates U.S. water quality standards by several hundred-fold and contains nearly 30 viruses ranging from hepatitis A to polio, as well as chemicals and heavy metals. Border agents who have dived in to rescue drowning immigrants have contracted skin rashes and infections.