To reach this resevoir, at the top of the Saas Valley, follow a small road from Saas Grund that offers beautiful views of the Saas-Fee glaciers. From the car park, allow 5min to reach the left end of the dam, which shows Stausee Mattmark Lake (1.76km2) surrounded by four glaciers (Hohlaub, Allalin, Schwarzberg and Seewjinen), from which many waterfalls descend. The lake has a capacity of 100 million m3 and a maximum depth of 117m. A path goes around it (2hr 30min).