Cut stems and center ribs away from Swiss chard leaves. Slice stems and center ribs thin and reserve. Chop leaves coarse and reserve separately.
In a 10- to 12-quart heavy kettle heat oil over medium high heat sauté stems and ribs, stirring, until crisp-tender. Add garlic and sauté, stirring, until garlic is fragrant. Add leaves with water clinging to them and cook, turning with tongs, until wilted. Add water if needed and cover, cooking over moderate heat until leaves are tender, about 4 minutes. Season with salt and pepper.