The spectacular Emosson dam was inaugurated in 1976, mainly to produce industrial electricity. It receives water from the Argentière and Le Tour glaciers, but also from the Bérard valley. The artificial lake is 5 km long and contains around 225 million cubic meters of water!
To get there, you have to take the Mont Blanc express from Chamonix station towards Martigny (stop at Châtelard) or go there by car via the Col des Montets, then Vallorcine and the Swiss border.
https://www.valleedutrient.ch/fr/dam-emosson-fp358