In a 5-quart saucepan, melt 3 tablespoons butter over medium heat. Add 3 cups sliced leeks and 1 cup sliced ramps; reduce heat to medium low. Saute until tender, about 10 minutes. Add 4 cups water, 1 cup milk, the potatoes, 1 tablespoon salt, and the pepper; bring to a boil over high heat. Reduce to medium low; simmer until potatoes are tender, about 7 minutes. Add watercress; increase heat to medium. Cook about 3 minutes, until watercress is tender and bright green. Cool slightly. Transfer in 1-cup batches to the jar of a blender; puree. Return all of it to pot over low heat; thin with remaining cup milk.
In a medium saute pan, melt remaining 2 tablespoons butter over medium-low heat. Add remaining 2 cups sliced leeks, remaining cup sliced ramps, and remaining 1/2 teaspoon salt. Saute, stirring frequently, until leeks are tender but still green, 5 to 10 minutes. Do not let them brown. Ladle soup into bowls; garnish with leek mixture. Serve hot.