The Ahr river (basin drainage area of about 630 km2) is a major tributary of the Rienz river, which is in turn the most relevant tributary of the Eisack river. The maximum elevation of the basin is 3498 m a.s.l., the minimum 810 m a.s.l. (the confluence with the Rienz at the city of Bruneck). The lithology of the basin is dominated by metamorphic rocks (orthogneiss, micaschists, paragneiss), but intrusive rocks (tonalites) are also present in the eastern part. The lower valley features the typical U-shaped section carved by glaciers, which now cover an area of about 25 km2, strongly influencing the flow regime of the Ahr, which can be classified as nivo-glacial. The mean annual precipitation in the valley is about 950 mm, and the mean annual discharge is 21 m3 s-1.
Along the 10 km-long reach from Sand in Taufers to to Gais (just upstream of Bruneck) the Ahr River features an average channel slope of about 0.4%. At present, the river mostly presents a single-thread, sinuous to meandering channel fixed by bank protections and levees, where former floodplain areas are now no longer inundated even for 30-50 yr recurrence interval events. Such dramatic variations have most likely occurred in response to the combination of both intense
sediment mining in the reach from the 1960s to the 1980s, and to the reduced sediment supply from the upstream basin determined by a hydropower dam (trapping roughly a basin area of 100 km2) and by hundreds of control works (retention check-dams and grade-control structures) along
the tributaries. Starting in 2003, the Department of Hydraulic Engineering of the Autonomous Province of
Bolzano has undertaken a program of river restoration in the reach from Taufers to Bruneck, mainly aimed at re-establishing adequate conditions of soil moisture for the remnants of the riparian woodlands found on the valley bottom and at enhancing fish habitat. Restoration actions included channel widening by removal of ripraps coupled to channel bed filling, midchannel bar creation and bed stabilization by ramps.