Sentence:  In September 1822 two companies of infantry arrived at the <WSD>mouth</WSD> of the St. Peter's River , the head of navigation on the Mississippi , 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:  the point where a stream issues into a larger body of water