Boot sole flappin' in the breeze? It's happened to me a few times, after overzealous boot drying by a campfire softened the sole glue. Fortunately, there's an easy fix.
First, you'll need some tape and a high-quality waterproof glue like SeamGrip, Freesole, or Barge Cement. Then:
Peel the sole back as far as it willingly goes.
Wipe the sole and boot bottom with a dry cloth or sandpaper and let them air dry. Dry them by a stove or fire if you must, but be very careful not to overheat the old glue or even more of the sole will delaminate.
Following the glue manufacturer's instructions, slather on the goop, then carefully smooth the sole onto the boot.
Press the boot on a flat surface to smooth out lumpy spots, then tape the sole to the boot. Pile rocks on top of the boot to apply even pressure to the seal.
Let the glue dry overnight or longer.
Remove the rocks and tape. The repaired sole just might last as long as the original should have.