Alps and the Jura was created in the Jurassic and Tertiary periods. The current Alpine Rhine was initially a tributary of the Danube. Over time, the basin was captured by the High Rhine as a result of headward erosion (fluvial erosion). The capture was not always only along the present Rhine valley; Lake Überlingen marks part of an older valley course. The river valleys were deepened during several cold periods by the Rhine Glacier from the valley of the Alpine Rhine (glacial erosion). Behind the present impressive traces of the Würm Ice Age, the effects of older cold periods can no longer be explored in any detail. Lake Constance now represents, above all, a zungenbecken or glacial lake of the Würm Glaciation. During a later phase of the ice age, only the Obersee was glaciated. As the glacier retreated further, the glacial meltwaters flowed out of the emerging Überlingen See through the older more northerly valley into the present High Rhine valley. Due to the advancing headward erosion, the present course of the High Rhine was finally (again) reconnected to Lake Constance. Like any glacial lake, Lake Constance will also silt up by sedimentation. This process can best be observed at the mouths of the larger rivers, especially that of the Alpine Rhine. The silting up process is accelerated by ever-increasing erosion by the Rhine and the associated reduction in the level of the lake. Tributaries The main tributary of Lake Constance is the Alpine Rhine. The Alpine Rhine and the Seerhein do not mix greatly with the waters of the lake and flow through the lakes along courses that change relatively little. There are also numerous smaller tributaries (236 in all). The most important tributaries of the Obersee are (counterclockwise) the Dornbirner Ach, Bregenzer Ach, Leiblach, Argen, Schussen, Rotach, Seefelder Aach, Stockacher Aach, Salmsacher Aach, the Aach near Arbon, Steinach, Goldach and the Old Rhine. The outflow of the Obersee is the Seerhein, which in turn is the main tributary of the Untersee. The most important tributary of the Untersee is the Radolfzeller Aach. Because the Alpine Rhine brings with it drift from the mountains and deposits this material as sediment, the Bay of Bregenz will silt up in a few centuries time. The silting up of the entire Lake Constance is estimated to take another ten to twenty thousand years. Outflows, evaporation, water extraction The outflow of the