Sentence:  In September 1822 two companies of infantry arrived at the mouth of the St. Peter's River , the head of navigation on the <WSD>Mississippi</WSD> , and began construction of Fort St. Anthony which , upon completion , was renamed in honor of its commander , Colonel Josiah Snelling . 
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