Abstract: Cumulative probability curve of particle granularities is one of the methods to study the sediments. However, the sediments in the studies were mainly sands/sandstone and silts/siltstone, and granule at most in the past. Recently the rivers with large gravel bed materials including pebbles, cobbles and boulders in mountainous regions have been studied more and more. But the distribution pattern of the granularities of coarse gravels such as pebbles, cobbles and boulders has yet to be well studied. In this study, taking the modern gravel sediments of the mountainous Ganxi Stream in Zhangjiajie scenic spot as an example, the cumulative probability curves of gravel granularity were studied for the sampling on the channel bed, mid-channel bar and channel bank. The granularities of these gravel sediments ranged from 23 mm to 663 mm, with most of them being pebbles, cobbles and boulders. The results showed that these fluvial gravel sediments followed a statistical pattern very well and exhibited a spatial variation trend that reflected the river flow hydrodynamics. For each sample the gravel granularities could be expressed by a cumulative probability curve clearly with two or three sections. The gravels in different sections reflected different fluvial power in different flooding stages. Granularity parameters showed that the gravel sediments have higher degree of sortability and sphericity. The median diameters of the gravels on channel bed have a decreasing trend downstream, reflecting the decrease of stream power in that direction. The reason for the various kurtosis of the gravels is that the blocks that fell to the river from the cliffs of "the sandstone forest" had not been modified sufficiently by the stream flow. The fine gravels intercepted in the apertures of cobbles and boulders in low flooding stages resulted in negative deflection of granularity distribution of the channel gravel sediments. By comparing the sphericity among different lithological gravels one can find that the gravels composed of carbonate rocks have the highest level of sphericity while the gravels composed of quartzose sandstone have lower level of sphericity. This phenomenon suggests that the quartzose sandstone gravels have higher ability to resist the flow erosion while the carbonate rock gravels have lower ability. This work is a useful attempt to study the distribution pattern of fluvial gravels including boulders, and has a revelatory significance to the quantitative studies of the coarse gravel sediments such as cobbles and boulders in mountainous fluvial rivers with steep gradients.

Key words: boulder, cobble, Ganxi Stream of Zhangjiajie, granularity distribution, gravel, mountain river, sediment