The gage height (or stage) of a river most commonly is measured through the
use of a stilling well, a bubble system, or a wire-weight gage.
(pictured at right) is used when a gage house can be built immediately
adjacent to a river. The well is connected to the stream with pipes so
that when the water level in the stream changes, the water level in the well
also changes. A float in the well is
connected to a recorder or data-collection platform.
The Bismarck gage house uses a stilling well to measure gage height.
The bubble system
(pictured at right) can be used when construction of a well is not
feasible. The bubble system requires a long open-ended pipe that
from the gage house to the river. One end of the pipe is fixed securely
below the water surface, and pressurized gas (usually
nitrogen or air) is forced through the pipe from inside the gage house and out a
submerged opening called an orifice. The pressure in the pipe
is determined by how deep the water is over the orifice. A change in the
water level of the river produces a corresponding change in the pressure in the
pipe. The change in pressure is converted to an electronic signal by a
transducer inside the gage house. Data from the transducer then is fed to a
recorder, or data-collection platform, which records the corresponding gage height.
An outside reference gage, typically a vertical graduated ruler called
a staff gage, is read periodically to verify that the recorded gage heights
from the stilling well or bubble system are the same as the water levels in
A wire-weight gage
(pictured at right)
consists of a drum wound with a single layer of stainless-steel cable attached to
a bronze weight, a graduated disc, and a counter, all within a cast-aluminum box.
The weight may be lowered to the water surface using a hand crank.
When the bottom of the
weight is at the water surface, the gage height is indicated by the combined
readings of the counter and the graduated disc. The wire-weight gage commonly
is mounted on a bridge handrail, parapet wall, or pier for use as
an outside gage.