Both study site rivers Wertach and Lech are originating in the northern Alps near the border between Austria and Germany, and are then running in roughly north direction through Bavaria towards the Danube River. Both rivers have historically been used for extensive fishing, timber rafting and water mills; a huge artificial lake (“Forggensee”) operating as a seasonal water reservoir has been built in the area where the river Lech leaves the Alps, in order to feed more smoothly the 31 hydropower plants constructed downstream. Both the rivers were historically channelized, the longitudinal slope of the channel increased, and the width decreased, which resulted in higher flow velocity. Also, the input of new sediment into the river was decreased by artificial bank stabilization, and by the construction of dams in the mainstem river as well as in tributary streams. As a result, both rivers incised their river bed for several decades, and thus lowered their river beds by several meters. As a result of river channelization, the water retention capability of the river channel was lost, so that the flood risk has risen again; floods with huge damages of several 100 million Euros have occurred. In addition, the ecological integrity of former river floodplains has been greatly affected by channel incision and by the lost of longitudinal continuity. Especially, formerly typical, abundant and economically valuable fish species as Huchen (Danube salmon, Hucho hucho) and Grayling (Thymallus thymallus) have mostly disappeared.
Basic information, location (graphics: M. Carolli, IGB), morphological evolution of the Lech River (source: F. Reebs, LFV) and view of the Wertach river (photo by M. Pusch, IGB).
In order to mitigate channel incision, to improve the ecological status and to facilitate the access to the river for recreational activities two multi-targeted restoration projects (called respectively “Licca liber” and “WertachVital”) and have been initiated by the water management agency (Wasserwirtschaftsamt). The “Licca liber” project focuses to stabilise and restore the lowermost river section, including the river mouth to the Danube. It is attempted to mitigate the severe impacts of past channel straightening, and restore some elements of the Lech’s natural channel morphology. Several measures aim to stop further incision, including large-scale artificial addition of sediment. It is expected that hydromorphological measures will also improve the status of the aquatic biotic communities. Through the “WertachVital” project a number of actions, such as in particular section widening, reactivation of sediment sources and construction of fish passages have been implemented with the aim to reduce the incision process and to restore habitat for aquatic fauna and flora.
Example of the reshaping of the Wertach River's cross-sections after the 2002 flooding.