"冲刷河岸" can be translated into English as "erode the riverbank" or "scour the riverbank".
"Erode" emphasizes the gradual wearing away or destruction of the riverbank by natural forces like water flow.
"Scour" also conveys the idea of washing away or cleaning vigorously, often used to describe the action of water on the riverbank.
Both expressions are commonly used in contexts related to geography, hydrology, or environmental science.