A reference river system for the Alps: The 'Fiume Tagliamento'

A major deterrent to a full understanding of the ecological ramifications o f river regulation at the catchment scale is a lack of fundamental knowledg e of structural and functional attributes of morphologically intact river s ystems. For example, both the River Continuum and the Serial Discontinuity Concepts, in their original formulations, had the implicit assumption of a stable, single-thread channel from headwaters to the sea. The Flume Tagliam ento traverses a course of 172 km from its headwaters in the Italian Alps t o the Adriatic Sea. No high dams impede the river's passage as it flows thr ough the characteristic sequence of constrained, braided, and meandering re aches. The Tagliamento, the only large morphologically intact Alpine river remaining in Europe, provides insight into the natural dynamics and complex ity that must have characterized Alpine rivers in the pristine state. The T agliamento has a flashy pluvio-nival regime (mean Q = 109 m(3) s(-1), with flood flows up to 4000 m(3) s(-1)). Thousands of newly-uprooted trees were strewn across the active bed and floodplain along the river's course follow ing a major flood in the autumn of 1996. The active floodplain is up to 2 k m wide and contains a riparian vegetation mosaic encompassing a range of su ccessional stages. Up to II individual channels per cross section occur in the braided middle reaches. Islands are a prominent feature of the riverine landscape and island dynamics are postulated to play a key role in determi ning pattern and process across scales. Future studies will examine the rol es of island dynamics and large woody debris in structuring biodiversity pa tterns of aquatic biota and successional trajectories of riparian vegetatio n. The high levels of spatiotemporal heterogeneity exhibited by the Flume T agliamento provide a valuable perspective for regulated river ecologists an d those engaged in conservation and restoration. Copyright (C) 1999 John Wi ley & Sons, Ltd.