Spatial density and characteristics of woody debris in five mountain rivers of the Dolomites (Italian Alps)

Abstract

This research quantifies amounts and characteristics of woody debris in mountain channels of the Alps. All pieces of woody debris greater than 5 cm diameter and 0.3 m length have been surveyed along five channels of the Dolomites (Italian Alps) with channel slopes ranging from 0.04 to 0.26 (basin area 2.2–51 km 2). During the survey, channels were divided into uniform reaches and the mean gradient, bankfull width and depth of flow were measured. Overall, 4900 woody debris pieces, single and jam-forming, were measured and classified with respect to several qualitative attributes, such as presence/absence of rootwads, orientation to flow, and position in the channel. Results show that very large variations in the volume of woody debris per unit area of the streambed occur even within single streams, and that inputs mainly result from slope instabilities adjacent to the channels. Amounts of woody debris are low compared to most previous data published from different world regions, but match with other mountain rivers of the Alps. Debris transport seems to be of minor importance compared to local inputs from the slopes, because reach characteristics are poorly correlated with the volume of woody debris in a reach. A strong negative correlation is, however, observed between basin area and average density of woody debris in the five channels, but debris removal in the larger channels likely affects such a trend. Finally, the frequency of log steps is inversely correlated to drainage area, and positively related to the density of debris.