Gauge section or control section of a river, stream or canal "Canal (engineering)") is a location, whether natural or prepared for this purpose, in which the elevation-flow curve has been determined. In this way, when required, by measuring the level, with a graduated ruler implanted in place, by interpolation in the curve, the liquid flow in the section can be determined.

To select a gauging section, some important factors must be taken into account, but the most important is to be certain that the shape of the section does not change over time, that is, it is a stretch of river or stream that does not suffer scour and is not in the process of sedimentation.

When these characteristics are not found in the section in which the gauging section is to be installed, a work must be implemented, such as a landfill. This work, depending on the size of the river or stream, can be an expensive work, and in some cases it may be more convenient to determine the flow by other methods.

The landfills used in these cases are also of several types:

Often a suitable section to implement a gauging section is in correspondence with a bridge, since this section has already been stabilized.

In wide rivers and streams, it is difficult and expensive to implement weirs with the sole purpose of defining the gauging section; in these cases the measurement is done with the use of a current meter. With the help of this instrument, the flow speed is measured in various parts of the section in order to establish curves of equal speed throughout the section. Based on the mapping of flow velocities, the liquid flow rate is calculated.

The calculation of the flow in this case is carried out with the following formula:.

Where:.

To measure the hair level free of water, the following are used.