里程差是指拟建水库中线里程与该库内河道里程之差。不论是山区或是平原水库，水库里程恒小于河道里程。所以由河道里程所计算的高程恒偏高，淹没的范围恒偏大。本文举Ａ、Ｂ、Ｃ、Ｄ四种类型水库比较，两种高程较差分别为１．３２、１．２１、０．８９、０．７９ｍ。因此水库纵断面的里程，建设沿水库中线量取，减少误差累积，提高回水计算精度． Mileage refers to the difference between the proposed reservoir midline mileage and the mileage of the river course in the reservoir. Whether mountain or plain reservoir, reservoir mileage is always less than the river mileage. Therefore, the elevation calculated by the mileage of the river is always high and the range of inundation is constant and large. This article cites A, B, C, D four types of reservoirs, the two elevation were poor 1.32,1.21,0.89,0.79m. Therefore, the longitudinal section of the reservoir mileage, along the construction of the reservoir to measure the amount of errors to reduce the cumulative increase backwater calculation accuracy.