Abstract: Watershed terrain and the relationship between drainage tectonic and erosion are one of the core content of geomorphology. The Helan Mountains area locates in the west edge of the Ordos, and it is a hinge area which links the western and eastern different geotectonic units of the north of China. Research on river system and watershed area in the Helan Mountains can help us to construct indepth understanding of river and landscape evolution in tectonic movement active regions. Many authors have studied the characteristics of the tectonic and climate of the Helan Mountains, But for rivers, the especially quantitative analysis of river morphology is relatively rare. Hypsometric integral is a relief variable that is widely used to measure the degree ofuvial landscape erosion and describes the distribution of elevations across the drainage basin area. It is a powerful tool to tell apart the tectonically active and inactive areas. We choose hypsometric integral and stream profile concavity to study the tectonic activity and evolution of the landforms in the Helan Mountains. The drainage network and drainage basins in the Helan Mountains and the hypsometric integral and concavity of the river profile have been extracted from GDEM data based on software Arcgis 9.3. There is little correlation between hypsometric integral, as well as concavity and river fall, river length, drainage area, in other words, the latter three parameters have little effect on hypsometric integral and concavity. Comparative analysis on hypsometric integral and concavity of the mountains’eastern and western segments, the HI values of the eastern Helan Mountains gradually decrease from the north to the south, and this infers that the tectonic activity is relatively higher in the northern part of eastern foothills than in their southern part; the average HI value in the eastern part is 0.46, and 0.40 in the western one, meaning that the tectonic activity of the eastern part is higher than that of the western one. The concavity values of R2, R3, and R5 are greater than that of R6, R7 and R8; and the tectonic activity of the eastern part is lower than that of western part. The concavity values gradually increase from the north to the south in the eastern and western parts of the Helan Mountains, meaning their northern part being with lower tectonic activity than their southern part. Obviously, this inconsistency may indicate that many problems still exist in the specific quantitative analysis of different basins when HI and Concavity are applied. Repeated studies through other indicators in the same area are needed. Statistical analysis of HI values of different basins shows that HI values could be divided into three categories: 0.60, which correspond to the geomorphic evolution of the topography of old, mature and youth age, respectively. The HI values indicate that, there is no basins in youth age, most drainage basins are in mature stage, R1、R6 are in old stage. In the point of Concavity value, all 9 rivers are greater than 1, meaning that the main work of these rivers is lateral broadening. Our main conclusion is that for the northern region of Rujigou, the topography is in old age yet, the topography in the southern Rujigou is in mature stage.

Key words: hypsometric integral, concavity, tectonic activity, Helan Mountain