In 1900 for health reasons the course of what river was changes in the U.S. making it world's only river to flow backwards?
Chicago River
Right, August!
The river you are referring to is the Chicago River. In 1900, the course of the Chicago River was changed for health reasons, primarily to address the risk of water contamination. The river originally flowed into Lake Michigan, which was also the source of the city's drinking water. Due to industrial and residential pollution, there was an increasing concern that the city's drinking water supply was being contaminated by the river.
To resolve this issue, the city of Chicago constructed the Chicago Sanitary and Ship Canal, which was completed in 1900. This canal connected the Chicago River to the Des Plaines River, ultimately diverting the flow of the Chicago River away from Lake Michigan. As a result, the river was made to flow backward, away from the lake and towards the Mississippi River watershed.
This engineering feat was intended to prevent the pollution of Chicago's drinking water supply, as the polluted water now flowed away from Lake Michigan and towards the Mississippi River. The reversal of flow in the Chicago River made it the world's only major river to naturally flow backward.