You are correct. The lake is actually a quarry pit and is one of many in the vicinity. The pits are located right next to the river. The whole area under the pits and river would be an aquifer or alluvium deposit that could be quite extensive. The aquifer/alluvium is full of water, thus the groundwater level would be the same level in all of the pits. You would have to drain the whole aquifer to lower the water level in the pits. While you would be trying to drain the aquifer, more water would be entering it from underground. The aquifer could be several miles wide and long and be hundreds, if not more, feet deep. Basically impossible to drain within our lifetimes. Great spot for wells.
The GPS coordinates for the location of where the car was found is : 42.808763, -93.232556. If the coordinates are entered into Google Earth you can see the whole situation.