[Sackett]

I think that symbolism fits very well Enzo, however, it might just be an expression of Jintan's attempt to cross the boundary of death, and a revelation of how impossible it is for him.

Thus the river represents the boundary of the dead from the living, hence Menma's attraction to it. Jintan rushes down there to try and stop Menma from crossing. Then Jintan tries to cross it himself in Menma's place. Time slows down as he tries to cross the barrier, but he can't. For Jintan it's just a river. Then he looks back and sees the evidence that Menma is already dead (has no reflection). Somehow the riverbank has changed sides, and he's still on the side of the living while Menma is on the side of the dead.

Jintan is separated from her, and he can't change that.

Instead of foreshadowing a future plotline, I think it's representing Jintan's character development since the start of the show, and now a conscious revelation to Jintan of what he's been learning all this time. That he can't go back to those days of having Menma with them all together as the Super Peace Busters.

Just like everyone else, Jintan is having trouble letting go of the past, but I think he's beginning to see that no matter how hard he holds on, the past isn't there to hold.

At least that's how I interpret the scene.

Also, why are you putting all this in spoilers? It's just speculation, not actual spoilers.