Rivers in general; a somewhat formal term used in contexts like river management, flow, and flood control.
大雨で河川が氾濫し、避難が必要になった。
Heavy rain caused the rivers to flood, and evacuation became necessary.
河川の水質改善が急務だ。
Improving river water quality is an urgent task.
河川敷でピクニックをする人が多い。
Many people have picnics on the riverbank.