EMPLOYEE WAS FOLLOWING SUPERVISOR UP STAIRS FROM RIVER AT THE END OF THE DAY. THE RIVER HAD WASHED DIRT & SILT AROUND THE BOTTOM FEW STEPS. IT WAS WET & MUDDY. THE EMPLOYEE STEPPED UP WITH LEF T FOOT, AS HE BROUGHT HIS RIGHT FOOT UP THE LEFT FOOT SLIPPED BACK OFF STEP. HE CAUGHT HIMSELF WITH THE OUTSIDE OF HIS LEFT FOOT. HIS ANKLE TWISTED CAUSING INJURY.