Drue, 4-5 ft at the deepest even with a slight slope of 4 or 5 to 1 ratio means you have some water that is even shallower. I would do a muck depth check and see what it takes to get to hard bottom.This would at least tell you how deep it was originally and that it held water at that depth. I would contact a local excavator and see what it would cost to cut the dam, drain and remove the muck. If you have a place on the property to dump the muck it should cost somewhat less. With the maturity of this pond and the depth and probable nutrient load I think you will battle FA and other assorted plants as it currently is. Aeration in this pond may only at best take a very bad algae problem and make it just a "bad" algae problem if the nutrient level is as I would expect.50 plus years means this pond has a great head start on filling itself in and creating a very high fertility level for FA and bottom rooted plants,Even though it is spring fed see what else makes up the watershed.Keep us posted> Good Luck Ted
