Tributary‐Trunk Sedimentary Links in an Inter‐Dam River Reach: Lessons From Low‐Altitude High‐Gradient Forested Streams, the Dyje River (Czechia‐Austria)

ABSTRACT This study employs two measures of connectivity, valley confinement index (VCI) and effective catchment area (ECA), combined with a field survey of active sediment sources (ASS) to unravel the tributary‐trunk sedimentary links in the 42.5 km long inter‐dam reach of the Dyje River, Czech‐Austrian border. The coarse sediment budget of the inter‐dam reach depends largely on sediment supply from 30 tributary catchments ranging from 0.1 to 137.7 km 2 . Among 15 tributary catchments selected for analysis, confined and partly confined streams prevail, the proportion of the confined category varying between 0% and 75%. The ECAs delimited for slope thresholds 5°, 15°, and 25° showed median values of 12.3%, 3.5%, and 0.6%, respectively, of the catchment area connected to the tributary streams. Fluvial erosion and tree uprooting were the most frequent types of processes supplying sediment to the channels, along with mass wasting processes; the mean frequency of ASS varied between 2.1 and 10.5 per 100 m of channel length. The ASS frequency increased with the degree of channel confinement by valley slopes. Similarly, ECAs with higher threshold slopes showed higher frequencies of ASS. Despite the overall high structural connectivity and numerous ASS, the sediment delivery to the main river is low. The low sediment input from ASS, the retarding effect of large wood, the disconnection of a large portion of tributary catchments by anthropogenic barriers (ponds), and the low frequency of runoff events capable of transporting coarse bedload are possible explanations of tributary‐trunk disconnectivity.