[OT] altimeter calibration?

[spewey at comcast.net (Spewey)]

Hey military/airplane people,

I've got a question for you:  once when I was out sailing on a weekday
on Lake Pepin, two Minnesota National Guard C-130s flew repeatedly up
and down the lake at about 1000 ft.  Big birds low down wing to wing.

The lake is about 23 miles long and 3 miles wide, actually a wide spot
in the Mississippi River, probably the biggest open water for a hundred
miles.  It was an awesome sight but I presume it was not done for
jollies since they kept going back and forth.

Someone said that maybe they were calibrating their altimeters by flying
over a flat surface but I wondered how that would make sense if the
river level changes.  Is that what they were doing?