Floodgates have been opened in the southern United States in a bid to spare large cities from becoming inundated.
US army engineers opened the Morganza Spillway in Louisiana for the first time in four decades.
The move will channel floodwaters towards homes and farms, but will prevent New Orleans and Baton Rouge – the state’s largest cities – from being affected.
It is a delicate operation and it could take three weeks for the huge amount of water to flow through.
There has been massive flooding along the Mississippi River and its tributaries following heavy rains and a large spring thaw.
Three million acres of farmland have been inundated, bringing comparisons with historic floods from before World War II.
Oil refineries and at least one nuclear power plant have been threatened.
The Red Cross has prepared shelters for thousands of people now forced to leave their homes.