Scott, do you have a water level comparison hole? A covered 8" hole would give you a way to determine how your pond level reacts to evaporation, run-off rises, etc. Seems to me if your pond can recover lost water quickly from the table, Willows sucking up water would allow fresh water to seep in and freshen the pond.

FWIW, when I had my dam repaired, the builder did all I asked of him, but he refused to build a small island for a willow tree. After I complained that the island he built was 2 feet below the surface, he smiled and said he knew I would have been unhappy with both the water loss and the willow control later-----he was right.