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河床演变是由输沙量的不均衡而造成的。输沙量因水流的大小和强度不同而异。而水流、输沙又是空间和时间的函数。描述水流运动方程遵守两个定律：1）不随其流动而改变质量守恒定律（连续方程）；2）表示其运动量变化的运动量守恒定律。通过分析水流特性，提出河床演变计算方法。当计算水面曲线时：，当缓流区根据下游给出条件依次向上游推算，急流区则从上游向下游推算；当缓流与急流混合时：当上游为缓流，下游为急流时，从临界断面向两侧推算；当上游为急流，下流为缓流时，根据急、缓流水面曲线计算原则得出2个水深，即为水跃的共轭水深。
Bed evolution is caused by an imbalance in sediment transport. Sediment discharge varies with the size and intensity of the water flow. The current, sediment transport is a function of space and time. Describe the equations of water flow obeying two laws: 1) do not change the law of conservation of mass with its flow (continuous equation); and 2) the law of conservation of mass that indicates its change in mass. By analyzing the characteristics of water flow, the calculation method of river bed evolution is proposed. When calculating the water surface curve: When the slow flow area is calculated according to the downstream conditions, the upper flow area is estimated from the upstream to the downstream. When the slow flow and the fast flow are mixed, when the upstream flow is slow flow and the downstream flow is rapid flow, The critical section is estimated on both sides. When the upper reaches are rapids and the lower flows are slow flows, two water depths are obtained according to the principle of curve calculation of rapid and slow flowing water surface, which is the conjugate water depth of water jump.