Place the chicken breasts, onion, celery, carrots, salt, thyme, sage, garlic powder, and wild rice into a crockpot. Stir in the chicken broth.
Cover and cook on high for 4 hours.
Remove the chicken breasts to a plate and shred with two forks.
Put back into the crockpot and cover.
In a small saucepan over medium-high heat, melt butter. Whisk in flour and heat for 1 minute, whisking constantly.
Pour in milk and continue to whisk until heated through.
Stir milk mixture into crockpot, and let heat for 30 more minutes.
Serve warm with crusty bread.