What occurs to Gene as he gets a glimpse of the river?
I think that you are referring to chapter 1, before Gene flashes back to 1942. Gene is visiting the school 15 years after the incident that most of the book is about. He is wandering around the campus and thinking about how very little of the school has changed since he attended 15 years ago.
Gene starts moving away from the buildings toward the river because he is looking for a specific tree. What occurs to him while looking toward the river is his surprise at the number of trees present.
"There were several trees bleakly reaching into the fog. Any one of them might have been the one I was looking for. Unbelievable that there were other trees which looked like it here. It had loomed in my memory as a huge lone spike dominating the riverbank, forbidding as an artillery piece, high as the beanstalk. Yet here was a scattered grove of trees, none of them of any particular grandeur."
Gene is also surprised at how normal all of the trees look. In his mind, there was only one tree at Devon: the one that he and Finny would always jump out of.
Once Gene finds the tree, he is surprised at how much smaller it appears than he remembers. Gene begins to then reminisce about how things stay the same yet change. The tree is the same tree, yet has changed in his mind. After that, Gene begins to tell the entire story of why that tree is important to him.