In the 19th century through civil engineering, the flow of the river was reversed to head toward the Mississippi River basin, away from Lake Michigan. This was done for reasons of sanitation through the Chicago Sanitary and Ship Canal. The Chicago River is also noted for the local custom of dyeing it green on St. Patrick's Day. The tradition of dyeing the river green happened by accident in 1961 when plumbers used fluorescein dye to trace sources of illegal pollution discharges. Now, the river is dyed using forty pounds of powdered vegetable dye for environmental safety reasons. The river is controlled for boats or ships going through the river.