The Greenville Police Department said a non-toxic dye placed in the Reedy River caused the water to turn bright green Saturday.
Master Patrolman Johnathan Bragg, spokesperson for the department, said authorities aren't certain who placed the dye in the river.
Bragg said the color came from a dye used to track sewer leaks.
GPD issued a warning at 4 p.m. Saturday urging people to stay clear of the water.
A small crowd gathered atop the Reedy River Bridge as hazmat crews were seen processing the water coming from under the structure.
The river will remain green until the clean up efforts are completed, Bragg said.