I noticed last year that one of our mature maples has branches that are dying.
It has been a healthy tree.
We had some river rocks that had been dumped at the base and sat there for about 2
Years. Could this be the problem and can I do anything to reverse the damage?

Thanks in advance for any advice!

Please see the following link:

Maple Tree Decline (http://www.uri.edu/ce/factsheets/sheets/mapletreedecline.html)

The rocks might not be the problem. But, I would start by removing them and then see what happens.

Have any of the leaves taken on a different color? This can be a sign of disease.

What kind of Maple tree is it?

Please see the following link:

Maple Tree Identification (http://www.massmaple.org/treeID.html)

A few dead branches can be normal. A pile of rocks could cause a problem but it would have to be a large pile that effectively cut off oxygen and water. As a rock pile is generally faily porous allowing plenty of air and water to penetrate it would be unlikely to damage a tree very much. Unless there is a solid pile of them. Remove them, make sure the tree does not go through stress during any hot, dry times. Maples are fairly shallow rooted and have plenty of leaves to transpire water to the atmosphere. Water and fertilize it. I'm assuming no one has trenched through the root zone and no excavation or soil fill has been placed around it.