Based upon the principle of least resistance,the stream passing around the Jiuduan sandbank is studied theoretically and the formula for predicting the development of the bank is derived.

• 本文根据流体运动的最小阻力原理；研究了长江口九段沙的分流；导出了它发育长度的理论公式；这一公式的建立加深了对该河段河床演变规律的认识.