The Study On River Terraces Of The Upper Heihe River In The Qilian Mountain

Rivers leave many traces and greatly shapped the earth surface during its evolution. The relationship between fluvial system and control factors remains one of the long-standing problems in geomorphology. Fluvial terraces, the most commonly step-like landforms, have recorded the history of the river evolution. In general, terrace formation has been widely attributed to either intrinsic changes in the dynamics of the fluvial system or extrinsic changes in variables such as tectonics, climate or base-level. The river behaviors in forming terraces are usually varied in different river systems with varied scales. In the middle Qilian Mountains near the Qilian City, staircases of fluvial terraces are well preserved along the Heihe River, Babao River and Dongsuo River, which have different drainage areas. This region supply us an ideal example for studying this problem.One erosion surface with altitude ranging from 2900-3100 m was developed near Qilian, and below this erosion surface, nine flights of fluvial terraces are preserved. The highest terrace is distributed at the junction between the Dongsuo River and the Babao River, southwest Qilian. The spatial distributions and longitudinal profiles of terraces are obtained by measuring the gravel altitude using differential GPS measurments, map analysis and terrace correllations. Along the Heihe and Babao rivers, terrace flights are continuous, and most of the terraces are strath terraces. Along the upper reaches of the Dongsuo River, several flights of filled terraces are developed. OSL(Optically Stimulated Luminescence) dating of the fluvial sand above the terrace gravels yeilds that river incisions along the Heihe can be grouped into two periods: T4, T5 and T6 are formed between 43.6 ka to 59.3 ka, and T1, T2 and T3 were formed between 4.4 ka to 13.6 ka. When comparing those ages with the climatic records, we find that the two periods river incisions can be correlated to two relatively warm periods, early MIS3 and MIS1. Formations of terrace surfaces can be correlated to relatively cold periods of MIS4 and mid-MIS3 to MIS2. This fact probably indicate that the transitions of river incising and depositing/grade were controlled by climatic changes.Terraces characteristics of the Heihe River indicate that because of regional rock types and sediment offers, stream power of the Heihe River is mainly used to move sediments, each fluvial incision event is completed by repeatedly cut and halt in turn.Incision of the Heihe River and the Dongsuo River is synchronous, but terraces of the Heihe River is more than the Dongsuo River, this indicate that the Heihe river is probably more sensitive to climate change.Terrace sediments combined with stream longitudinal profile demonstrate that the upper reaches of the Dongsuo River is in a state of aggrade, but the Heihe River, the Babao River and the down reaches of the Dongsuo River is all in an equilibrium state during T4 and T5 formation.Different response to the external factors change may be induced by the varied river scales. The Heihe River, The Babao River and the Dongsuo River incised synchronously in a warm period, the Heihe River is in an equilibrium state in cold period, and the upper reaches of Dongsuo River is in a state of aggrading.