Episodes of flash floods and boulder transport in the Upper Jordan River

Abstract Because direct measurement of boulder transport in natural rivers is difficult, there are very few field studies on the subject. Long-term field studies, 1969-1999, were conducted in the Upper Jordan channel to evaluate the geomorphic impact and persistence of a 1:100 year catastrophic flood, and to analyse the relationship between the river's hydrological regime and boulder transport behaviour. A channel reach of 210 m length and 25-35 m width, with 260 boulders larger than 500 mm b-axis, was resurveyed periodically over 30 years. Detailed field surveys, photographs, and painted rocks were used to assess the bed load transport. During the study period, four major flood events had the ability to transport boulders of sizes from D 50 1000 mm up to the largest, namely 1700 mm b-axis size. Stream power values increase where the depth-slope product is maximized. In the wide sections of the channel with lower slope values, the deposition of big boulders determines the formation of bar structures. The catastrophic flood has a decisive effect on the modification of the fluvial landform, being a geomorphologically effective event