A reconnaissance of toxic substances in the Jordan River, Salt Lake County, Utah, was made during July, 1980 to October, 1982 as part of a larger study of the river that included studies of sanitary quality, dissolved oxygen, and turbidity. Samples for toxic substances were collected at five sites on the Jordan River, at three major tributaries, and at six storm drains. The toxic substance that most frequently exceeded State standards was total mercury. About 78 percent of the 138 samples for total mercury exceeded the State standard of 0.05 microgram per liter. Other toxic substances that exceeded State standards were: Ammonia-18 percent of the samples analyzed, cadmium--9 percent, copper-9 percent, zinc--6 percent, and lead--2 percent. One sample for cyanide and one for iron also exceeded State standards. The diversity of toxic substances with concentrations large enough to cause them to be problems increased from the upstream sampling site at the Jordan Narrows to the next two downstream sites at 9000 South and 5800 South Streets. Concentrations of trace elements in stream-bottom materials also increased in a downstream direction. Substantial increases first were observed at 5800 South Street, and they were sustained throughout the downstream study area. Iron is transported in the greatest quantity of all the trace elements studied, with a mean load of 110 pounds per day. Notable loads of barium, boron, lead , and zinc also are transported by the river. DDD, DDE, DDT, dieldrin, heptachlor, methoxychlor, PCB, and 2,4-D were detected in bottom materials; and DDE, Silvex, and 2,4-D were detected in water samples. Of 112 organic compounds in the Environmental Protection Agency 's priority pollutant list, only chloroform was detected in the storm drains that empty into the Joran River. Several metals and phenol also were detected in the samples for priority pollutants. (USGS)