Hello,
Over the past couple of weeks my 50+ year old maple tree has been showing yellow-green leaves on the east/north side, even though it's too early for fall colors (I live in Denver, CO). Upon closer inspection, I noticed that some of these yellowish leaves are also curled and browning. Here are some pictures.

Can anyone identify the cause, and suggest treatment?

Thanks in advance,
Svarog

I should mention that it is only happening on one side.

What kind of maple is it? The leaves look more like an oak than maples I'm familiar with. It has interveinal chlorosis on those leaves - it is possible for iron chlorosis to affect one side of a tree. Did you check inside the curled leaves for pests?