Dwight, The open area may be due to groundwater discharge to the pond. Would that side be hydraulically upgradient?
My gravel pit style ponds always open from the southwest. Groundwater flow at my place is from SW to NE, so this is the discharge point of relatively warm groundwater. If your pond is clay lined you can toss this theory on the scrap heap.
Here in Michigan my ponds are all completely open. There is no frost in the ground. I'm actually planting trees this weekend, unprecendented at least for me in mid-January.
