Large Pin Oak Health Question

Hello,
I have a large Northern Pin Oak in my yard that was a large healthy tree when we built the house 28 years ago and has recently (the past 3-5 years at most) been losing some large branches and the foliage seems to be 60-70% of what it used to be. The tree is situated in a small gully that features a small natural spring. For as long as I've lived here the tree seems to have been quite healthy and only recently displayed this problem. There were also several larger black cherry trees that have in the past 5-10 years slowly died out and the remaining specimens seem to be in declining health. I'm not sure if this is a problem with too much moisture given the spring but I wouldn't think this because all of the trees seemed to have established well and grew to large mature sizes with the spring there obviously. I hope at this point something can still be done as I beleive the tree is probably close if not more than a 100 yrs old and would be shame if it dies. If any information or insight can be given, I would greatly appreciate it. Thank you in advance!!