| The
data most often collected at a gaging station are stage.
Stage is the height of the water surface above a reference elevation.
The most common source of river stage measurement is a gage
house (see diagram above). A gage house consists of a stilling
well dug along the riverbank with a surrounding shelter that protects
the equipment inside. Water enters the well through one or more inlet
pipes. The water in the well rises to the same level as the river. Recording
equipment in the gage house records the water level in the well (the stream
stage). Recorded data can then either be accessed by telephone or, if
the gaging station is equipped with an automated Data Collection Platform
(DCP), it may be transmitted via satellite to agencies such as the National
Weather Service and the Army Corps of Engineers. Data transmitted via
satellite are a very important source of information for many agencies.
Data are used to forecast river conditions, to issue flood warnings, and
to plan reservoir releases or water withdrawals. The availability of this
realtime data has become so important that the number of stations equipped
with DCP's has been steadily increasing. As of 1996, there were over 4,200
stations with DCP's.
Although
stage information is useful for some purposes, most users of streamflow
data find it necessary to have discharge
information about a stream. Discharge is the volume of flow passing
a specified point in a given interval of time and it is usually measured
in cubic feet per second (cfs). Discharge includes not only the volume
of water, but also any sediment or solids that may be mixed with or
dissolved in the water. Unfortunately, providing discharge data is more
difficult than providing stage data.
Discharge
is usually estimated
from
stage/discharge relations known as rating curves.
In order to develop a rating curve, USGS field personnel must visit
a gaging station to measure river discharge and compare it to the stage.
A Current Meter is the instrument that
is frequently used to perform discharge measurements (see FIG. A &
B). Year after year, discharge and stage data are gathered in the field,
sometimes as often as once a month. Over time, a stage/discharge rating
curve is gradually developed (see FIG. C). Unfortunately, rating curves
are never fully complete due to the constant changes occurring in rivers
and streams. Sedimentation, scour, changes in riverbed roughness, ice,
debris or aquatic vegetation may significantly alter stage/discharge
curves and must be adjusted for. In order to keep discharge estimates
accurate, it is necessary for personnel to continue taking discharge
and stage measurements in the field to keep the curves updated.
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FIG. A.
|
Current meter being used to measure
discharge. |
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FIG. B.
|
Current Meter. |
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FIG. C.
|
Stage/Discharge Rating
Curve |
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Voice (609) 883-9500 
clarke.rupert@drbc.state.nj.us
