The statistical results show (Figure 3A) that two channel types, i.e., gravelly and sandy channels, can be identified along the river section from the Jinta River to Hongyashan Reservoir. The section of the Jinta River from its source to Shaojiazhuang is a gravelly channel with a Md of 38–190 mm and an average of ∼106 mm (Figure 3B); the section from Shaojiazhuang to Hongyashan Reservoir is a sandy channel with a Md of 0.13–0.62 mm (Figure 3B). The modern riverbed sediments in the Xiying River are mainly gravels, with a Md of 50–103 mm and an average of 70.6 mm (Figure 3C).

As the distance from the river source increases, the particle size of the river sediment generally decreases exponentially, as shown in Formula Eq. 1: D i = D 0   e − ∂ L (1) where D _i is a characteristic linear dimension at some distance L, D ₀ is the characteristic dimension at L = 0, L is the distance from the river source, and ∂ is a size diminution coefficient that expresses the rate of grain-size decreases toward downstream.

In general, there is a strong exponential relationship (R ² = 0.9) between the sediment Md and the distance L in the reach from Jinta to Shiyang Rivers, and the ∂ is about 0.039 (Figure 3A). In the gravelly section of Jinta River, there is a significant exponential decreasing relationship between Md and the distance L, and the ∂ is about 0.033 (Figure 3B). In the sandy section of the Shiyang River, the refinement trend of Md is not obvious with the increase of distance L, which shows a fluctuating change of increasing first and then decreasing. The exponential decreasing relationship between Md and river length of the Xiying River is relatively weak, with a low coefficient R ² of 0.15, and a ∂ of 0.005 (Figure 3C).

The Md of riverbed sediment in the Shiyang River section showed a general trend of gradual decrease from upstream to downstream, which is consistent with the trend of refinement along the grain size in most studies (Table 1). A comparison of the global stream sediment refinement model (Figure 4A) shows that most of the gravelly channels are less than 10² km long, while the sandy channels can stretch for thousands of kilometers, indicating that the transport distance of gravel in the channel is limited, generally not more than 100 km. The fining coefficient of gravelly river is generally greater than 0.01, while that of sandy river is generally lower than 0.01, that is, the refinement trend of gravelly channels is more obvious than that of sandy channels (Figures 4B, C). In addition, the fining coefficient α of river sediments was inversely correlated with river length (Figure 4A), which probably attributed to the fact that the longer the river, the more complex the response of river sediment grain size to factors such as landforms, climate, and tributary confluence, thus showing a more complex change trend.

Abundant studies suggest that the downstream fining of river bed materials can be attributed to sorting and abrasion. However, lateral inputs of coarse sediment, basin geomorphic development stage, lithology, human activities and tributary confluence may break the downstream fining trend (Figure 3). For example, in the Jinta River, there is a sudden increase in the Md of river sediment near Qingdaban and below the Nanying Reservoir. The sudden change in Md of river sediment in the Xiying River mainly occurs at about 20 km from the source, near Jiutiaoling, and at the head of the Xiying River canal (Figures 3B, D), and the occurrence of these sudden change in grain size makes the distribution of river sediment along the river show an obvious segmental decreasing trend.

The results show that the Gravel-Sand Transition (GST) is evident in the Hexi Corridor area at about 28 km downstream from the outlet of the Jinta River (Figure 3B). Previous studies have shown that the GST of river sediments is usually related to slope, local base level, excessive sand supply or gravel wear, selective sorting, etc., (Yatsu, 1955; Gregory et al., 1995; Knighton, 1999; Dubille and Lavé, 2015; Ferguson and Ashworth, 2010; Ehda et al., 2021). Especially after the rivers flow out of the mountain, the channel is not limited laterally by topography, and the rapid decrease in sand transport capacity of the river is the main reason for the formation of the GST zone. In addition, the GST is accompanied by a shift in channel planform from multi-channel to single-channel (Dubille and Lavé, 2015), e.g., gravelly reaches in the upper part of the gravel-sand transition zone are usually braided streams with a large channel slope (Gregory et al., 1995; Venditti and Church, 2014), while downstream channels typically evolve into single-channel sandy streambeds (Frings, 2011; Dubille and Lavé, 2015; Dingle et al., 2020). From the mountain pass of Jinta River to Wuwei City, the landforms are dominated by alluvial fans, wide and shallow river channels. The river channel presents a braided river style, with the channel slope of 0.02. In the Hexi Corridor plain area, it is dominated by narrow and deep meandering rivers, with a slope of less than 0.005 (Figure 5). The GST occurs at the distance of about 10–40 km downstream from most river outlets (Dingle et al., 2017; Ehda et al., 2021), suggesting that the gravel transport distance is related to the geomorphic characteristics of the watershed, sediment supply, and the transport capacity of flowing water.

It was suggested that the GST occur at the distance of about 10–40 km downstream from the mountain pass (Dingle et al., 2017; Ehda et al., 2021), indicating that the gravel transport distance is limited downstream the outlet. After the river exits the mountain pass, the stream gradient decreases rapidly in the lower reaches, and the channel is no longer restricted, accelerating lateral swings, resulting in large deposits of coarse gravel and continued transport of fine-grained material (Paola et al., 1992). And in the GST region, the presence of some gravel further promotes the fragmentation of matter and the production of fine particles (Bradley, 1970; Kodama, 1994a). Therefore, under the condition of river hydraulic separation, fine particles are preferentially transported faster and farther downstream than coarse particles. In addition, some rivers passing through Wuwei City have been artificially channelized, which will also affect the original transport process of river sediments.

River sediments are mainly originated from the slope process, tributary inflow and river bank erosion along the river. Tributary confluence generally leads to a sudden increase in sediment grain size (Ichim and Radoane, 2010; Brewer and Lewin, 1993; Rice, 1998). If the transport capacity of the confluent section is weak, the coarse-grained materials will be deposited on the riverbed (Lane, 1995), and this deposition will in turn lead to a reduction in the channel slope in the confluence section, resulting in a reduction in runoff shear stress and deposition of fine-grained material (Dawson, 2010). Our results show that the confluence of Binggou River, Dashui River and Baita River, tributaries of the Jinta River, showed coarsening of the grain size of the riverbed material in the confluence section (Figure 5A). In addition, the confluence of Qingyang River and Shuiguan River, tributaries of the Xiying River, similarly caused an increase in the sediment particle size of the main channel (Figure 5B).

Especially the larger the volume of an input and the greater the grain size disparity between it and the main stem material, the greater is the expectation that the main-stem texture is changed notably. At confluences the situation is complicated by the concomitant influx of water which, if sufficiently large, modifies ambient stresses and, in turn, the bed material. A tributary which introduces a significant quantity of water but little sediment could, by increasing the capacity of the main stem, produce a significant change in texture. Therefore, the relative volume of a sediment input, its size characteristics relative to the recipient channel and, at tributaries, the relative contribution of water, may be controls on the occurrence of particle size discontinuities. The volume of material carried by tributaries depends on the volume of sediment supplied to the river and the ability of the stream transport, while the volume of water flowing out of the basin depends mainly on the climate, vegetation and watershed area (Knighton, 1980; Rice, 1998). In the Qilian Mountains, the Xiying and Jinta Rivers are located in the same climatic unit, the vegetation difference is small, and the watershed area can be used to indirectly reflect the tributary flow (Table 2). For example, probably influenced by flow and sediment size, some of the tributary confluences, such as the Xigou, Tuoluo, and Xiangshui Rivers, cannot break the along-channel refinement of the river sediments (Table 2).

After the confluence of the tributaries, the sorting action and abrasion process of the water flow will continue to act on the river sediments. Except for the area near the confluence, the overall pattern of longitudinal changes in river sediments is basically not affected, and the overall trend is decreasing.

Watershed lithology is an important factor influencing the sediment yield, which determines the initial grain size of the river sediment and the anti-erosion ability (Rice, 1999). The lithology of the Jinta River basin is distributed uniform, with Ordovician metamorphic rocks and Caledonian granites in the source area, Carboniferous-Permian and Triassic sedimentary rocks in the upstream area, and Quaternary loose sediments in the middle and downstream areas. The lithology of the Xiying River basin is complex, with Ordovician metamorphic rocks and granites dominating, Silurian granites and Triassic granites and sedimentary rocks distributed locally, with a few Carboniferous and Permian strata exposed near the watershed; some Cretaceous and Cambrian strata are distributed near the outlet (Figure 1B). From the viewpoint of the stratigraphic lithology along the river channel, the distribution of lithological resistance strength and weakness along the Jinta River channel is uniform, which indicates that the lithology of the source of sedimentary material in the river channel is relatively uniform. However, the distribution of lithological resistance in the Xiying River channel is more complex, which indicates that the sources of sedimentary material are diverse, thus the grain size changes are more complex along the channel.

The geomorphic parameters that reflect the development stage of the basin geomorphology (Bull and Mcfadden, 1977; Keller and Pinter, 2002; Peters and Balen, 2007), such as drainage density (Dd), hypsometric integral (HI), basin shape index (Bs), and valley width-to-height ratio (Vf) of the basin, are shown in Table 3. The Vf values in the Jinta River basin are larger than those in the Xiying River, while the HI and Bs values in the Jinta River basin are smaller than those in the Xiying River, indicating that the regional tectonic activity in the Xiying River is more active than that in the Jinta River basin. Moreover, the geomorphic development stage in the Jinta River basin is tend to a mature stage, and the river longitudinal profiles tend to be graded (Yatsu, 1955). Therefore, in the Jinta River basin, which is relatively tectonically stable and has a relatively mature development stage, the refinement of river sediment shows an obvious exponential decreasing, while in the Xiying River basin, which is relatively tectonically active and has a relatively young development stage, the trend of refinement of river sediment grain size is often broken.

The decrease of channel slope will weaken the transport capacity of the river and lead to the accumulation of coarser fractions in the river sediment. By comparison, it was found that the change of channel slope in the Jinta-Shiyang Rivers generally showed a decreasing downstream trend, which basically followed the same trend as the change of river sediment grain size. For example, the gravel-sand transition zone near Wuwei corresponds to a decrease in channel slope (Figure 5A), which may be due to the fact that this area is a transition area from mountainous to plain area, and the lower channel slope reduces the transport capacity of the river, which can only transport finer fractions of material downstream, so that there is an obvious gravel-sand transition zone downstream. Similarly, the Xiying River channel shows a general trend of fluctuating lowering of channel slope downstream, and the fluctuating change corresponds to the area of sudden increase of grain size (Figure 5B), indicating that, except for strong tectonic activities, the along-channel change of channel slope has a significant influence on the change of grain size of river sediments.

Increasing human activities, such as village settlements and the construction of dams and reservoirs, can regulate river runoff, breaking the original water-sand balance of the river, for example, leading to a reduction in the transport capacity of the river, and inability to transport coarse-grained material downstream (Surian, 2002). For example, in the upstream of the Xiying River near Jiutiaoling (Figure 5A), the construction of houses and bank stabilization have led to an obvious coarsening of river sediment grain size along the river. In addition, the construction of reservoirs, has changed the river environment and weakened the river transport capacity, and the downstream river sediment grain size of reservoirs has also coarsened (Figure 5B).

In addition, the construction of Tianma Lake near Wuwei reduced the riverbed sediment transported downstream, breaking the original refinement pattern of river sediment grain size, and an obvious abrupt change area of gravelly sand transformation appeared in the area below Wuwei. Below Tianma Lake, the impact of human activities decreased, and the grain size of river sediments gradually increased and continued to show a refinement trend below Siba.