monitor stage

An established river stage at a certain point along a river which is intended to represent the maximum safe water level which will not overflow the river banks or cause any significant damage within the reach of the river.

In a leveed stream …

In a leveed stream, monitor stage is the stage at which patrol of flood control project levees by the responsible levee maintaining agency becomes mandatory, or the stage at which flow occurs into bypass areas from project overflow weirs.

In a non-leveed stream …

In a non-leveed stream, monitor stage is the stage at which initial action must be taken by concerned interests, such as livestock warning, removal of equipment from lowest overflow areas, or simply general surveillance of the situation. This level may produce overbank flows sufficient to cause minor flooding of low-lying lands and local roads.

Source: DWR River Stage Definitions