Bring chicken broth and water to a simmer in a saucepan (do not boil). Keep the broth warm over low heat.
Cut leek crosswise into thin slices. Heat oil in a large saucepan over medium heat. Add leek and celery, and sauté 2 minutes. Add rice and sage, and sauté 1 minute. Stir in wine, and cook 1 minute or until liquid is nearly absorbed, stirring constantly. Stir in 1/2 cup broth mixture, squash, and salt, and cook until liquid is nearly absorbed, stirring constantly. Add remaining broth mixture, 1/2 cup at a time, stirring constantly, and cook until each portion of broth mixture is absorbed before adding the next (about 20 minutes). Stir in Parmesan cheese, lemon juice, and pepper.