The
water level of a lake commonly is referred to as its stage. It
is important to know the stage of a lake because this is how
we can calculate how much water is stored in the lake. If we
know the shape of a lake basin by making a contour map of its
bottom, and if we know the stage,
we can calculate the volume of water stored in the lake for any
stage. It is useful to know the
stage of a lake for another reason. For example, if we reference
the stage to a common datum, such as sea level, and if we reference
other water in the lake’s watershed, such as streamflow
and ground water levels, to that same datum, we can determine
if ground water is seeping into the lake or if lake water is
seeping out. We can also determine how much seepage occurs, and
where it is located in the lakebed – but more on that later.
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| Lake stage recording device |
How do we measure stage? Stage is measured using a recording device
that has a clock, to keep track of time, and a float that rides
on the lake surface inside of a stilling well. The float is attached
to a steel wire or steel tape, and a counter weight is attached
to the other end. The tape rides on a wheel that turns as the stage
goes up and down. A chart is placed
 |
| Lake stage chart |
on the drum of the recorder
and a pen is lowered onto the chart.
The pen moves
across the chart according to the clock speed (a
week for Mirror Lake), and the pen moves up and down according
to the lake stage. The stilling well is necessary to dampen wave
action and to protect the float and counter weight. The elevation
of the lake above sea level is determined by surveying from an
official government benchmark to a reference level of the lake
that is assigned a specific altitude above sea level. This altitude
is assigned to a specific number on the
staff gage
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| Staff gage |
and the pen (representing the float level) is positioned on the
chart to match the number on the staff gage. These data can be
used to plot a hydrograph of the lake stage.
Finally, by developing a stage-to-volume relationship
we can determine the volume of water in the lake for any time and
stage shown on the hydrograph.
