A simple 40 minute route from the train station of la Saraz. The climb is of about 100 metres. In 40 minutes you arrive at the dam. Its a magical place filled with energy from the forest fairies. There are two dams and a small beach. The water doesn't seem too clean, but its from the amount of organic matter there is in the forest.
If you feel like taking a walk after lunch at Saint-Cergue.. there is a walk you can take, called 'le sentier Ballade à Béatrix'. It is welll indicated, with 16 points of information. It starts with a little steeply, but the forest is very pleasant, the views from the old castle are impressive, the invisible ruins, and it goes through a series of rocky formations with interesting caves.
A simple route, strongly recommended, with very little uphill.
We start at the camping place of Vallorbe, with a strong vertical slope untin we cross the col. From there, we descend through a minor road until Vailion. After crossing the village we enter once more at the forest, where we cross a defensive system (the toblerones) where we start once more to descend, following the river. In the last part, we walk next to the river and cross wooden bridges several times.