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 exception is the Fiume Tagliamento, Italy, which retains a riparian woodland margin and unconfined river channel system throughout almost the entire 170 km length of its river corridor. A research programme is underway focusing on a range of related aspects of the hydrology, fluvial geomorphology and ecology of the Tagliamento. This paper contributes to that programme by focusing on large wood retention. The paper adopts a simple force:resistance approach at the scale of the entire river corridor in order to identify reaches of the river with a high wood retention potential. Information on the character of the river corridor is derived from 1:10 000 scale topographic maps. A range of indices measured at 330 transects across the river corridor supports a classification of the geomorphological style of the river which reflects the presence and abundance of properties previously identified in the literature as large wood retention sites. This classification provides a qualitative representation of the resistance of the corridor to wood movement and thus its overall wood-retention potential. The map-derived indices are also used to extrapolate estimates of the ten year return period flood to each of the 330 transects so that the downstream pattern of unit stream 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 features incorporated in the classification of geomorphological style, including 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 August 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 reaches. Established islands were estimated to store an average of 80 t ha-1 of wood. Nevertheless, there was considerable variability between sites, and pioneer islands, which are not represented on maps or readily identified from air photographs because of their small size, were estimated to store an order of magnitude more wood than established islands. Furthermore, the wood storage from this sample of eight sites did not reflect variability in estimated unit stream power. A series of areas for further research are identified, which can be explored using field data, and which will throw more light on the processes of wood retention in this extremely dynamic fluvial environment