267-5 Source to sink sediment characteristics in the mixed lithology watershed of Nahal Sa’ar, Northeastern Israel

Session: Advances in Fluvial Processes and Sediment Transport (Posters)

Presenting Author:

Authors:

Abstract:

Nahal Sa’ar watershed (26 km2) is located at the boundary between the basaltic plateau of the Golan Heights and the sedimentary Mesozoic rocks of Mt. Hermon anticline. The drainage evolution is controlled by the regional tectonics of the Dead Sea Transform (DST), the Anti-Lebanon Mountain Range, and alternating cycles of volcanic activity. The main channel is an 11 km long watercourse with an ephemeral flashflood hydrology regime, a mixed alluvial lithology with bedrock outcrops, and significant waterfalls. The channel starts on the steep slopes of Mt. Hermon, incises a 650 m wide and 100 m deep canyon, and terminates in a small alluvial fan on the left bank of the perennial Nahal Hermon.

We hypothesized that the alluvium of Nahal Sa’ar will demonstrate bimodal sedimentary traits that reflect different degrees of resistance to erosion of each lithology rather than characteristics typical to unimodal sediments displaying downstream fining. Seven sampling sites were chosen along the main channel. At each site we surveyed: (1) An extensive grain size distribution (GSD) sample, (2) A visual quantification between limestone and basalt clasts, and (3) The roundness and sphericity of both fine and coarse sediments.

Results show significant downstream coarsening in the upper parts of the basin and typical downstream fining in the lower sections. In several upstream locations, the alluvium is disrupted by abundant cuboid mega-boulders (up to 3 m) while the alluvium is mainly composed of limestone clasts from the nearby karstic slopes. Further downstream, basalt sediments predominate. These results are explained by faster erosion of the carbonate sediments compared to the more resistant basalt rocks. Sandstone clasts from exposed layers are quickly eroded, diminishing from 3% to 0.5% of the alluvium at the immediate downstream site and disappearing entirely after. Surprisingly, the alluvial fan GSD somewhat resembles the upstream sampling sites. This can be explained by floods of Nahal Hermon that transported large boulders at the confluence and truncated the coarser grain fractions. We conclude that despite the differing alluvial lithologies, Nahal Sa’ar’s alluvium generally resembles the basaltic streams of the Golan Heights and not the karstic streams of Mt. Hermon. 

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9401

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