The slope of a river is the ratio of the fall of the river to the length of the section on which the slope is determined. The unit of measurement is called ppm (‰). The slope can be determined both for individual parts of the river and for the entire river. The essence of the measurement is to instantly find out the height position of the water surface at two points between which the measurement is made.
Select the location where you want to take the measurement. It can be a straight line, or it can be extended with many turns. Drive the pegs into the water at the measurement points between which you want to know the slope of the water surface. This is usually done in shallow places, on shallows, next to the water's edge. To determine the longitudinal slope of the river, places are selected on one bank of the river. If pegs are hammered on different banks, the data can be distorted due to the skewed surface of the water surface caused by wind or hydraulic structures or the shape of the channel.
Find out what altitude these pegs have in a conditional or absolute coordinate system. To do this, make leveling. There are various devices and methods for leveling. From the simplest spirit level to the modern total station. There is a state geodetic network in our country. It includes cataloged points (benchmarks) with known elevations. Take measurements from such benchmarks with known elevation marks to the pegs hammered in the river, thereby finding out the height position of the pegs.
Measure the distance between the pegs. Measure not in a straight line from peg to peg, but along the curve of the shoreline.
Measure the water level above both pegs at the same time. Place slats or rulers on the pegs and measure the height of the water layer from the pegs to the surface of the water to the nearest centimeter. Find out what is the height of the water surface for both pegs in the absolute coordinate system (add the water layer measured at the same time to the mark of the peg).
Calculate the difference between the water surface marks in centimeters. Divide the resulting figure by the distance between them in kilometers, and you get the desired slope of the river in ppm.