All three lakes together have a storage space of 154 million cubic meters and a water surface of 12.1 million square meters. The Brombachsee is the heart of the Franconian Lake District, an artificial lake landscape used for tourism and water management. It is the largest lake in Franconia and the second largest reservoir in Germany by area. It is named after its drain, the Brombach.
The reservoirs were created from the 1970s as part of the water regulation of the Main-Danube Canal and to supply water to arid northern Bavaria as well as for local recreation and tourism. In the event of high water at the Gern gauge near Ornbau, water is fed to the Altmühlsee via the Altmühlzuleiter. There it is temporarily stored and, if the level is exceeded, released to the Kleiner Brombachsee via the Altmühlüberleiter. From there the water first flows over the Großer Brombachsee and the Brombach into the Schwäbische Rezat, at Georgensgmünd into the Rednitz and then on to Nuremberg before it reaches the Main via the Regnitz. In this way, water that would normally flow via the Altmühl into the Danube is diverted, if necessary, across the main European watershed into the river system of the Rhine-Main area. The construction project was approved by the Bavarian state parliament on July 16, 1970 on the initiative of the deputy Ernst Lechner and carried out in several construction stages by the Nuremberg Dam Construction Office. Work was completed in the late 1990s and the entire structure was officially inaugurated on July 20, 2000.[
Source: Wikipedia