Combine 5 cups water, potatoes, onion, celery, butter, bouillon cubes, salt, pepper, and garlic powder in a slow cooker. Cook until vegetables are tender, on Low for 8 to 9 hours or on High, about 6 hours.
Mix cold water and cornstarch together in a small bowl. Stir 1/2 of the cornstarch mixture, heavy cream, and evaporated milk into the soup and cook for 1 more hour until thickened, adding more of the cornstarch mixture if desired for consistency. Top with chives before serving.
Slow Cooker Potato Soup with Heavy Cream · Dinner Match Lab | Dinner Match Lab