Following the climax of the last ice age, the Lech-Wertach foreland glacier, which covered the entire Ostallgäu, gradually melted back from its maximum extent at Kaufbeuren.

In the course of its retreat, as in the entire Pre-Alpine area, new morainic ridges were formed from the masses of debris carried by the ice from the mountains piled up at the edge of the glacial tongue whenever the glacier halted temporarily or advanced briefly again.

North of the Lech Waterfall was one of these glacial lakes, Lake Füssen (Füssener See), a waterbody with an area of 60 km2 whose surface lay at a height of 790 m above NHN, which was formed as water levels rose behind the ridge (tectonically formed sediments of the molasse, which exhibit greater resistant to erosion than the rocks of the Cretaceous Flysch zone adjoining them to the south) at the southern end of the Murnau Depression at the northern end of the present-day Forggensee.

Lake Füssen also visibly silted up, but it also flowed out through the Illasberg Gorge, which the River Lech gradually carved out through the southern wing of the Murnau Depression near today's barrage no.

In 1898, Siemens & Halske bought the first plots of land in the area of the Lech water gap near Roßhaupten and received a licence to build a hydroelectric power plant, which, however, expired in 1907.

In 1910, the "Royal Supreme Construction Authority" (Königliche Oberste Baubehörde) published a memorandum on the utilization of water power on the Lech, according to which a dam with a height of 34 m, a crown length of 140 m and a total capacity of 65 million m3 was to be built.

A draft paper from 1936/37 envisaged a storage target of 784 m above NHN, i.e. three meters higher than later, and a concrete dam with a built-in power plant.

Of the affected buildings, only 14 houses from Deutenhausen survived: They were bought by Bavarian Hydropower in 1952 from Theodor Momm, the owner of the spinning mill of the same name in Kaufbeuren, demolished in the fall of 1954, and rebuilt by expellees in the area.

The former episcopal mill, which was moved from Waltenhofen to Forggen in 1644, was probably the most important building among the larger individual farms; today its foundation walls are crumbling on the lake bed.

At the northern end of the lake, the Dösinger local researcher Sigulf Guggenmos (1941–2018) discovered various important archaeological sites, including traces of Mesolithic hunting stations and a Late Celtic or Roman place of sacrifice.

[4] A few hundred metres from today's shore, on the former path from Brunnen to Forggen, you will come across the foundation walls and brick remains of a Roman villa rustica that were cleared in 1974.