Actually, the lake is usually 18 inches or less down all winter.  What you are seeing is a natural artifact of the valve being open to downdraw the lake.  When water is flowing (out the valve), its’ level has to go down in the direction of the flow.  Conversely, if the level of a body of water has a sloped surface, then it is flowing in the direction of the slope.  Think of a stream or river.  While there is a lot of science involved here, that’s just how nature works.  This phenomena has been recognized and understood since people have tried to direct water in a desired direction.  Think irrigation.

The following video shows what you would see when the valve is open, the lake level is about 15 inches down and what happens when the valve is closed, as it would be for the summer.  It’s more dramatic when the lake is 18 inches down.  In that case, the water level at the dam can be completely below the bottom of the sight gauge.  This phenomena does not have linear relationship.

While the valve is open, the only part of the lake that is lower than the actual lake level, extends from the face of the dam to to the old outlet, about 50 feet out into the lake past the Golf Course Road Bridge.  The rest of the lake remains at whatever its’ current actual level is.

In fact, if the lake tries to get any lower than about 24 to 26 inches down, the lake water can’t even reach the valve.  The flow is blocked by the old outlet, dating back to the early 1700s.  The old outlet is now a shallow spot in the lake bottom and effectively limits our ability to downdraw the lake any farther.