作者首先叙述了钱塘江水系地理概况和洪水简史，认为：季风气候条件和七里泷峡谷对洪水的阻挡是洪水形成的两种主要原因。
The author considers that the floods are resulted from two main causes : the subtropic monsoon-climate condition and the narrow Qililong gorge as a obstructing to delay the floodwater velocity .
七里泷