In Text below, a preposition is missing. Choose from the alternatives below the right preposition to fill in the blank.
St. Louis marks the divide between the upper and the lower Mississippi.
_____ the north, navigation is made possible by the system of locks and dams. The lower river is broad and level enough for riverboats to move more freely, although sandbanks and rocks demand full attention from river pilots.
(POLLARD, Michael. The Mississippi. Benchmark Books: Tarrytown, New York, 1998)
