"the river was navigable for 50 miles"
误 We row a boat in the river.
正 We row a boat on the river.
析 “在河里划船”的介词用on不用in。
哈尔滨在松花江畔。误 Harbin is by the Songhua River.
正 Harbin is on the Songhua River.
黄浦江上有座桥。误 There is a bridge on the Huangpu River.
正 There is a bridge over〔across〕 the Huangpu River.
析 “江畔有城”用介词on,“江上有桥”用介词over或across。