MB, welcome to the forum!!! By the "back" of the dam, do you mean the excavated dirt itself, or the dry side base of the dam? Tree roots will spread under the ground as far as the tree's drip line (outer circumference)..so if your tree's leaves are over the pond water, it's roots are that far also. Does the backside, base of the dam stay wet long after surrounding soil has dried? Dig down a foot or so and compare moisture levels with soils dug away from and above your pond.

Is there a chance construction upstream took away a lot of your runoff?