Hi Sryder,
How large is the unattached piece? If it's small enough you can put it in a platic cup (16oz cut in half) with some rock rubble. Place it away from direct water flow and it will attach. If it's big you can try nylon fishing line and sew it to some larger rock rubble. I've used wedding vail to cover frags with rubber bands to hold into place. One more idea? Plastic tooth pick, (the sword type), super glue into rock rubble and then super glue to colt.
Just some things that have worked for me,
The frags you see behind the hawk were done with the plastic cup method
Tom C.