Betsy made its most strongest landfall near the mouth of the Mississippi River , causing major flooding of the waters of Lake Pontchartrain into New Orleans ; at the time it was the costliest hurricane in the history of the United States , and , as the first hurricane to cause over a billion dollars in damages , earned the nickname `` Billion-Dollar Betsy '' . Betsy made its most intense landfall near the mouth of the Mississippi River , causing significant flooding of the waters of Lake Pontchartrain into New Orleans . As it was mistakenly known as the first hurricane to cause over a billion dollars in damage ( 1965 dollars , not adjusted for inflation ) , it earned the nickname `` Billion-Dollar Betsy '' .