Water， water every where. 　　It lasted almost two months ， but in August it ended. It left 45 people dead and $10 billion worth of damage in nine states of USA. It was quite a big Mississippi flood ever recorded. 　　In St.Louis， Missouri， 9，000 people were forced to leave their homes while the city was in er. St. Louis is just downriver from the points where the Missouri and Illinois Rivers flow into the Mississippi. All three rivers were flooding. But the city escaped the worst when levees （堤） broke upriver. A levee is built of river sand and clay（粘土）。 　　Eleven miles of flood walls were built in the late 1960’s The walls are 18 inches thick and 5 to 22 feet high. They were designed to protect against a 52-foot flood. In St. Louis， water almost reached the top of flood walls. It measured 49.4 feet. The flood referred to in the passage _____.