Stir celery, BBQ sauce, ketchup, onion, water, garlic powder, chili powder, salt, and pepper together in a slow cooker. Place roast into the mixture. Cover and cook on Low for 7 hours or High for 5 hours.
Transfer roast to a cutting board; shred meat with two forks. Return meat to the slow cooker, stir to combine with liquid, and cook for 1 more hour.