Wood storage within the active zone of a large European gravel-bed river

Wood storage within the active zone of the dynamic, gravel-bed, Flume Tagli amento, Italy, was investigated at eight sites along the river's main stem. The quantity, nature, and mode of wood storage revealed a number of trends related to active zone morphology, cover type, and distance from the river 's source. Relatively small quantities of wood were stored on open-gravel s urfaces (estimates ranged from 1 to 21 t ha(-1)),intermediate quantities we re associated with established islands (24-186 t ha(-1)), and large quantit ies were associated with pioneer islands (293-1664 t ha(-1)). Thus, variati ons in the geomorphological style of the river, which are associated with c hanges in these three cover types, are reflected in variations in the amoun t of wood that is stored in different reaches. In addition, although wood w as found in many locations within the active zone, it was preferentially st ored in three specific locations: (i) bar crests (the main open-gravel loca tion for wood accumulations and pk,neer islands): (ii) the margins and (iii ) surfaces of established islands. The proportion of the stored wood that w as living (sprouting) increased downstream and was higher on the open grave l than in association with established islands. There was a downstream grad ient in the dominant type of wood accumulation. Individual logs predominate d at the most upstream site. Thereafter, on the open gravel, whole shrubs a nd trees dominated the more confined sites in the headwaters and middle rea ches, whereas, jams were the most frequent form of accumulation in the down stream reaches. Jams were the most frequent type of accumulation associated with established islands throughout the river. In contrast to small stream s, where debris dams constitute the major type of wood accumulation, comple x patterns and trends of wood storage were revealed along the Tagliamento. Although further studies are needed, it is clear that erosion of woody vege tation, its subsequent transport and deposition, play a major role in struc turing the geomorphological and ecological character of this relatively nat ural, large European river-system. Insight into the mechanisms underlying t he observed spatial patterns will contribute to a better understanding of t he dynamic processes involved, and is essential for more effective manageme nt of river ecosystems. (C) 2000 Elsevier Science B.V. All rights reserved.