In many gravel-bed rivers, grain size changes exponentially downstream and is mainly related to selective deposition of clasts on the streambed (according to size, shape and density), and abrasion (crushing, grinding, splitting, chipping, cracking and sandblasting) during transport. Most studies on the cause of downstream fining in rivers emphasize hydraulic sorting (or selective transport) as the dominant fining mechanism. The Band-e-Golestan drainage basin with a surface area of about 320 km2 is located southwest of Mashhad in northeast Iran. This catchment’s area consists of three subbasins (Mayan, Dehbar and (Jaghargh) that flow toward northeast connecting to the Kashafrud River. Grain-size variation and causes for downstream fining were studied at 57 sites. Compositionally, gravels in the study area consist of phyllite, slate and a small amount of quartzite. Consequently, these sediments are poorly sorted, positively skewed and mainly platykurtic. Upstream, most of the boulders are phyllites, while in the downstream part, slate makes up the framework of the gravel. Grain-size distributions are weakly bimodality in the upstream and become strongly polymodal downstream. Based on our study, the main factors in downstream fining of grain size in the study area are hydraulic sorting and abrasion.

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