Large wood retention in river channels: The case of the Fiume Tagliamento,Italy

After more than 300 years of widespread and intensive river management, few examples of complex, unmanaged river systems remain within Europe. An exce ption is the Flume Tagliamento, Italy, which retains a riparian woodland ma rgin and unconfined river channel system throughout almost the entire 170 k m length of its river corridor. A research programme is underway focusing o n a range of related aspects of the hydrology, fluvial geomorphology and ec ology of the Tagliamento. This paper contributes to that programme by focus ing on large wood retention. The paper adopts a simple force:resistance app roach at the scale of the entire river corridor in order to identify reache s of the river with a high wood retention potential. Information on the cha racter of the river corridor is derived from 1:10 000 scale topographic map s. A range of indices measured at 330 transects across the river corridor s upports a classification of the geomorphological style of the river which r eflects the presence and abundance of properties previously identified in t he literature as large wood retention sites. This classification provides a qualitative representation of the 'resistance' of the corridor to wood mov ement and thus its overall wood-retention potential. The map-derived indice s are also used to extrapolate estimates of the ten year return period floo d to each of the 330 transects so that the downstream pattern of unit strea m power can be quantified as an index representing 'force' in the analysis. Although input of wood is an important factor in many river systems, it is assumed not to be a limiting factor along the Tagliamento, where riparian woodland is abundant. Field observations of large wood storage illustrate that wood retention at eight sites along the river reflects the presence and abundance of the feat ures incorporated in the classification of geomorphological style, includin g the complexity of the channel network, the availability of exposed gravel areas, and the presence of islands. In general at the time of survey in Au gust 1998, open gravel areas were estimated to store approximately 1 t ha-( 1) of wood in single-thread reaches and 6 t ha(-1) in multiple-thread reach es. Established islands were estimated to store an average of 80 t ha(-1) o f wood. Nevertheless, there was considerable variability between sites, and pioneer islands, which are not represented on maps or readily identified f rom air photographs because of their small size, were estimated to store an order of magnitude more wood than established islands. Furthermore, the wo od storage from this sample of eight sites did not reflect variability in e stimated unit stream power. A series of areas for further research are identified, which can be explore d using field data, and which will throw more light on the processes of woo d retention in this extremely dynamic fluvial environment. Copyright (C) 20 00 John Wiley & Sons, Ltd.