The spot that stands out is a particular region that is locked in an enchanted, eternal autumn. It's always harvest season there, and the leaves are perpetually red, yellow, and orange. The rest of the map is depicted in summer, hence the color discontinuity.

I shaped the rivers to conform to some established borders and landmarks. I haven't yet worked out a scale, but the visible map is about 1,000 miles wide east-to-west. You may be right about the river length. How long do you think they should go in, and how far back should I detail the tributaries?