Water Level and Ice Fishing
Name: Lon R.
The following question was recently posed by one of our
employees here at Pinkston-Tadd, Inc. of DeKalb, Illinois:
When ice fishing, a hole is bored or chopped into the ice to provide an
opening for fishing.
It was noted that once the hole has been bored deep enough to reach past
the ice to water,
water will rise up in the hole almost even with the top of the ice.
The question is this: what causes the water level to rise in the hole to
a level just below the top of the hole?
We have asked several students from NIU, have tried searching the
Internet, etc. Have returned with some interesting observations but am
not convinced we have found an explanation grounded in science.
OK Here goes my "explanation". The level of a frozen lake prior to
drilling establishes a balance of forces between the downward pressure
(force/area) of the ice and the upward pressure of the water beneath. This
balance is very complicated because as the ice freezes and the temperature
is lowered the porosity of the soil underneath and other considerations come
into play. But whatever the details there is an equilibrium of downward
pressure of the ice and upward pressure of the liquid water below. When you
bore a hole in the ice you have removed a certain volume of ice. This
reduces the downward pressure of the ice by a force (pressure) equal to the
weight of the volume of bored ice. To compensate for this reduced downward
force there is an upward force equal to the weight of water equal to the
volume of displaced ice. Since liquid water is about 10% more dense than ice
at the freezing point the water will rise to reestablish the balance of
upward and downward forces. So for a cylindrical hole the water should rise
about 90% of the thickness of the ice. An application of Archimedes'
Principle for ice fishers, maybe.
I believe the answer is very simple. Ice has a density of 917 kilograms/m^3
and water has a density of 1000 kg/m^3. Therefore, free floating ice is
91.7% under water. So if the ice where you chopped a hole is 10 inches
thick, the water should come to within 0.83 inches of the top of the ice.
That could well be considered "near the top of the hole".
Having lived in northern Wisconsin on the banks of the "Ol Wisconse", I have
often chopped holes, sometimes when the ice was about 3 feet thick! As I
recall, the water sometimes came out and covered the ice. I can only think
that such thick ice in a rather narrow channel of the river was anchored to
the shores so when the water level in the river rose, the ice was stuck and
did not rise with the water. I do know that sometimes the water fell and
the ice was much above the water level.
Best, Dick Plano...
Just like in your glass of ice water in the summer the ice is floating so
that only a small fraction of it is out of the water. If you imagine
yourself on one of the ice cubes in the drink and drill a hole through the
ice the water will rise to nearly the top of the hole (the level of the
water around it).
When you are ice fishing you the ice is floating on the lake water. It
displaces the amount of water that weighs the same as the ice and anything
it is supporting. Assuming you take the same amount of supplies (i.e.,
same weight) every time you go ice fishing, the amount of ice that remains
above water is an indication of the thickness of the ice. The thicker the
ice, the greater the height of the ice above the water.
Click here to return to the Physics Archives
Update: June 2012