Symbolism, Imagery, Allegory
In the very first chapter, on a gorgeous April day, Jody leaves his chores to build a flutter-mill (a waterwheel) in the stream, and basically thinks it's all that and a bag of chips: "Up, over, down, up over, down—the flutter-mill was enchanting" (1.16). His dad doesn't even mind that he totally blew off his chores to build a toy. "Leave him kick up his heels," Penny thinks: "Leave him build his flutter-mills. The day'll come, he'll not even care to" (2.24).
Think about it this way: as Toy Story 3 taught us, you don't care as much about your old toys and games as you get older. You know those Matchbox cars or the little plastic tea set you used to love, but stopped playing with years ago? That's what the flutter-mill represents at this point: easy, carefree, and happy childhood.
But get out your highlighters, Shmoopers, because this is Meaningful. When the story begins, Jody is just a kid, right at the beginning of what's going to be a tumultuous year full of hard life truths and growing pains. A year later, everything has changed. On his way home after running away, Jody stops by the same stream on another April day, one year later. He goes to look for the flutter-mill:
It seemed to him that if he found it, he would discover with it all the other things that had vanished. The flutter-mill was gone. The flood had washed it away. (33.83)
If the flutter-mill, the symbol of a carefree childhood, is gone, then means Jody's childhood has vanished, too, washed away by the storm and everything after. But something even better might be in its place: real maturity.