长河 is most often metaphorical:
历史的长河 ('the long river of history'),
时间的长河 ('the long river of time') - a stock literary image for the inexorable flow of time. Echoes Wang Wei's
大漠孤烟直，长河落日圆 ('In the vast desert, lone smoke rises straight; over the long river, the setting sun is round'), one of the most quoted couplets in classical poetry.
