of textural variation and the evidence forfining toward downstream in the Dehbar River, Southwest of Mash had

Category Sedimentology
Group GSI.IR
Location 20th symposium on geosciences
Holding Date 16 April 2008

     DehbarRiver drainage basin is located about 35 km southwest of Mashhad. It is mainly gravelly river with a bedload and single channel and consequent stream type. Generally, In many gravel— bed rivers, the grain size changes exponential toward downstream and is mainly related to selective deposition of material on the streambed (according to size, shape and density), downstream distance, lateral and vertical sorting, abrasion (crushing, grinding, splitting, chipping, superficial cracking, sandblasting) during transport and weathering effects when particles are at rest. The grain size variation and causes for fining downward were examined at 20 sites. This sediments are poorly sorted, positively skewed and mainly platykurtic. In the study area, the rapid reduction of slope is one of the main geomorphological factor that causes fining toward downstream as well as the lithological differences between different grains. Gravels are mainly composed of phyllite, slate, and Quartzite.
The percentage of abrasion in slate is about 25.5 while the phyllite is about 28 (estimated by los Angeles Machine). The main factor in downstream fining is selective transport due to changing hydraulic condition and controlled by hydrology of drainge basin as well as catchment slope.