The characteristic of many rivers is that the grain size of the bed decreases towards the downstream side, this phenomenon is known as fineness towards the downstream side. The causes of fineness towards the downstream side have been the subject of study for several decades, because changes in grain size are caused by sediment transport, bed size and hydraulic erosion (Frings et al., 2010). The textural characteristics of sediments are widely influenced by several factors, which include the region of origin, climate, distance and energy of sediment transport in sediment environments. The rate of grain size change is one of the important consequences for downstream changes and sediment transport (Kumar Maity and Maiti, 2016) as well as river ecology (Petts et al., 2000; Tanabe et al., 2023). There are two different methods for identifying sediments: facies analysis and using traditional approaches for particle size analysis (Flood et al., 2015). The heights of Gonabad are separated from each other by wide and flat plains with different trends. One of these heights was the southern mountain range of Gonabad, which has an east-west trend and is located more than 1500 meters above sea level. The lithology of these heights is composed of Mesozoic (mainly Shamshak Formation) and Cenozoic rocks. These southern highlands have many seasonal rivers. One of the longest rivers is the Khanik River (more than 13 km) (Figure. 1). This river originates 35 km south of Gonabad from a height of more than 2200 meters and leads to the central plain of Gonabad (Ghaemi and Sharivar, 2005). The aims of this research is to investigate the sedimentary parameters such as skewness, sorting and kurtosis in the sediments of the Khanik River. Methods and Materials: 19 samples were taken from the bottom sediments of Khanik River. These sediments were graded by sieve shaker and granulation from -6 to 4.5 φ and at 0.5 φ intervals. Classification of particles (Table. 1) and sediment parameters have been calculated using Gradistat V9.1 software and using the logarithmic method of Folk and Ward (1957). Results and Discussions: The size distribution of particles in sediments depends on the properties of the source rock, their weathering, abrasion and selective melting processes during transportation, which also affects the types of sedimentary facies (Snelder et al., 2011). The sediments of the Khanik River, based on the φ scale, are mostly composed of coarse-grained sediments in the size of gravel (average 57%) and the average particle size is about -1.26 φ. The name of the sedimentsis based on the classification of Folk (1980) from fine to coarse sandy gravel. According to the elevation changes between station 1 and 19 between 2257 and 1526 meters, the samples based on the average size have a negative trend with R² = 0.0901, which indicates the presence of downstream fining in this river (Maazallahi et al., 2023)(Figure. 2). Due to the type of particle size and high dispersion, the sediment particles have badly to very bad sorting. All the samples have multiple modes (Polymodal) as expected. This dispersion of particles caused the samples to be mostly platykurtic and mesokurtic and sometimes leptokurtic. Due to the abundance of coarse grain particles, the sediments have a fine to symmetrical skewness. Due to its geomorphological nature and relatively large height difference (731 meters height in 13 km length), Khanik River has had a high energy, which has caused the abundance of coarse grain particles, lack of good sorting, negative skewness, and mainly platykurtic kurtosis. Among the effective factors in this downstream fining, we can point out the decrease in current energy intensity, increase in bed width and decrease in the topography of the main channel to the downstream side of the basin (Wohl & Merritt, 2008). The amount of very slight downstream fining along the river can indicate the influence of factors such as sudden changes in bed slope due to lithological changes, the entry of sub-branches and the occurrence of floods along the river channel bed. The natural sorting process is also created as a result of the concentration of coarse grains in the upstream part and finer grains in the downstream (Parker, 1991; Tanabe et al., 2023). Bad to very bad sorting is due to the short transportation time and also the reduction of energy during grain transportation. It is that sedimentation has taken place with a sudden decrease in flow energy (Rice and Church, 2010) and the process of skewness is dependent on the energy of the environment (Duane, 1964). Coarse skewness is associated with high energy and washing of fine particles and fine skewness. It is related to the low energy of the environment (Fridemen, 1968). The fine skeweness is in natural river environments and indicates that there is not enough opportunity for the washing of fine particles by the water flow (Rice, 1999). Also, kurtosis is an important textural parameter for identifying different environments (Mason and Folk, 1958), so kurtosis from platykurtic to leptokurtic shows that the sediments have reached sorting. The change in kurtosis is affected by the characteristics of the current during the settlement (Kumar Maity and Maiti, 2016).

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