The end of a river (that is, the point at which it empties into
another body of water) is usually called the "mouth". The beginning
(the point at which the river starts) is sometimes called the
"source" or the "headwaters".
Boston is located on Boston Harbor, which flows out to the
Atlantic Ocean. Both the Charles River and the Mystic River
converge and empty into the beginning of Boston Harbor, right at
the northeast tip of Boston. The Charles River comes from the west,
and the Mystic River comes from the north.