You can reach Switzerland's second-highest dam, the Barrage de Mauvoisin, by road bike. Or at least almost. The last few meters lead through a dark, wet tunnel, which isn't so easy to ride after the approximately 25-kilometer climb.
From Sembrancher, at a good 700 meters above sea level, the climb is fairly flat for the first nine kilometers. Only from Champsec does the elevation profile gradually become steeper, but the gradient remains moderate until the very end.