Sentence:  Raymond Vernon reports that residents of East St. Louis have been driving across the <WSD>Mississippi</WSD> , through the heart of downtown St. Louis and out to the western suburbs for major shopping , simply because parking is easier at the big branches than it is in the heart of town . 
Meaning of the word marked with <WSD> tags:  a major North American river and the chief river of the United States; rises in northern Minnesota and flows southward into the Gulf of Mexico