How is the water level in the river measured?

The water level of this river is measured by scales set on both banks. There are many scales with e on the scale, and the difference between adjacent scales is one centimeter. The height of the ruler is calculated relative to the height of the zero point (such as the zero point of the Yellow Sea). The average velocity of a river section is the flow through this section divided by the underwater section area of this section. The velocity distribution on the river section must be measured point by point with a current meter. The velocity at a certain point refers to the distance that water moves there in unit time.