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Rates of river incision are widely used to track changes in rock uplift rate and climate over time and space. However, the relationship between the spatiotemporal variation of river incision and past tectonic or climatic processes is still unclear. Here, we focus on the spatial patterns of river incision in the interior and front of the Qilian Shan in the northeastern Tibetan Plateau. A sequence of late Quaternary terraces is well preserved along the Heihe River across the central Qilian Shan. We complement existing chronologic data with five new optically stimulated luminescence ages and one cosmogenic radionuclide 10Be exposure age of terrace abandonment. At the mountain front, the river incision rate was ∼1 mm/yr over the past 100 kyr, while the range was from 2 to 7 mm/yr during the Holocene. We attribute the fast incision rates during the Holocene to the progressive decrease of the river gradient induced by the warm and wet Holocene climate. In the interior of the mountains (the Qilian basin site), the river incision rates were close to 1 mm/yr since ∼53 ka, suggesting that the upstream incision rate remained stable from the late Pleistocene to the Holocene. This observation further suggests that the adjustment of the climate-induced longitudinal river profile is limited to the downstream reach, especially along the front of the mountain range. We conclude that the long-term incision rate (during the late Pleistocene) is explained by ongoing uplift driven by fault activity, which determines the background rate of fluvial incision (∼1 mm/yr). On the other hand, denudation rates on the millennial timescale estimated by previous studies in this region are < 1 mm/yr, suggesting that landscape-scale erosion lags behind trunk river incision.

Downstream fining of riverbed gravels is generally linked with the processes of hydraulic sorting and abrasion. Hydraulic sorting is when larger gravel clasts stop moving in response to decreasing flow energy, whereas, finer grains will continue to be carried downstream. Furthermore, transportation of gravel clasts causes abrasion, bringing about a gradual decrease in grain size. Hydraulic sorting and abrasion have different dominant effects on the downstream fining of clasts in rivers with different climatic and tectonic backgrounds. At present, most studies focus on humid areas, and relatively few studies have explored this issue for the northeastern margin of the Tibetan Plateau in arid and semi-arid areas. Detailed investigations of the grain size, lithology, and roundness of riverbed gravels have been performed here along the Taolai, Hongshuiba, and Fengle Rivers, which flow across the northeastern margin of the Tibetan Plateau and debouch into the arid inland of North China. The obtained data were subsequently employed in a hydraulic pattern of grain-size distribution of riverbed gravels in this area, which is characterized by the combined influences of tectonic activity and climatic aridity. Analysis reveals that there is no new rock type appearing in the lithological compositions of riverbed gravels along these rivers, only showing fluctuations in proportions of lithology even though they are adjacent to uplifting mountains. Fresh gravel material from these mountains does indeed mix into the fluvial bedload, inducing a notable decrease in roundness in the Taolai and Hongshuiba bedloads downstream from here. The downstream fining of gravel along the three rivers, with median grain sizes above 128 mm and falling into the range from 20 to 128 mm, can probably be attributed to hydraulic sorting and abrasion. Further analysis suggests that the former presents a high correlation with channel gradient, which may be sustained by fault activity at the northeastern margin of the Tibetan Plateau. The grain-size distribution in these riverbed gravels thus provides insights into the evaluation of fluvial responses to active tectonic uplift.