This is the 2188th most frequent Japanese word.
河川
Rivers and streams.
河川 is used in a general sense to talk about rivers collectively.
日本にはたくさんの河川があります。
Japan has many rivers.
河川 refers to the specific rivers in a designated region.
この地区の河川は毎年氾濫します。
The rivers in this area flood every year.
河川 is used here in a technical sense, referring to river systems in relation to infrastructure and management.