In a bowl, combine the sour cream with the cucumber, radishes, shallot, capers, chopped dill and lemon peel; season with salt and pepper. Refrigerate until ready to serve.
Using kitchen twine, tie together the dill sprigs, parsley sprigs and bay leaves. Place the bundle in a medium skillet. Pour in the wine and add the salmon, skin side down. Pour in enough cold water to come just to the top of the fish (but do not cover completely). Bring to a boil over high heat, then immediately reduce the heat and simmer until just cooked through, about 8 minutes. (Spoon hot liquid over the top of the salmon if it looks raw.) Using a large spatula, transfer the salmon to a plate and let stand for 10 minutes.
In a small saucepan, bring the chicken broth and butter to a simmer. Stir in the couscous and chopped parsley. Turn off the heat, cover and let stand for 5 minutes.
Dress the watercress with the lemon juice and olive oil; add salt and pepper to taste. Serve the salmon with lots of sour cream sauce, the couscous and the watercress salad.