Melt butter in heavy medium saucepan over medium heat. Add leeks and sauté until tender, about 10 minutes. Add 2 cups water, carrots, beans, pear and rosemary; bring soup to boil. Reduce heat to medium-low, cover and simmer until carrots are tender, about 15 minutes.
Purée soup in 2 batches in blender until smooth. Return to saucepan; Thin with more broth or water to desired thickness. Season to taste with salt and pepper.