长江源通天河段辫状河道形态特征与变化规律

长江源广泛发育辫状河道,共同组成一个庞大的辫状河群,在水流泥沙与河床地形数据缺乏的条件下,长江源辫状河道形态的时空变化尚缺少研究。基于Landsat遥感影像解译,研究长江源通天河局部辫状河段的平面形态特征与分汊强度的时空变化规律。结果表明:1987—2016年辫状河段的水域面积与河床面积之比在年内呈现单峰变化过程,即汛期4—9月份逐渐变大,非汛期10月—次年3月趋于变小至基本不变,而且近30 a有所增加,间接反映了长江源径流量增加的趋势。辫状河段的36个横断面的汊道数指数和汊道长度指数分别为4~10和5~12,在1994,2005,2015年其均值几乎维持不变,表明此辫状河段的整体形态具有自相似性且河型不变。

Morphological Features and Spatial-temporal Change of Braided Reachof Tongtian River in the Yangtze River Source Region

Braided rivers widely develop in the Yangtze River source, forming a huge cluster of braided rivers. Yet, the spatial-temporal variation of the braided channel morphology remains unclear owing to the shortage of flow, sediment, and riverbed topography data. In the present paper, braided channel morphology and branching intensity in the local braided channel of Tongtian River were studied using the Landsat remote sensing images in 1987-2016 to extract water area. The ratio of water area to riverbed area above water changed periodically associated with the hydrological cycle in a year, i.e., the ratio in flood season (April-September) gradually increased, while in non-flood season (October to next March) decreased and eventually remained unchanged. In the past three decades, the ratio of water area to riverbed area displayed a trend of increasing, which implies the increasing trend of runoff in the source region of the Yangtze River. Of the 36 cross sections of braided river reach, the index of branch number and index of branching length ranged in 4-10, and 5-12, respectively; but the mean values of the two indexes in 1994, 2005, and 2015 almost remained unchanged, indicating that the braided river reach is of holistic self-similarity and invariant river pattern.