黄河源区典型河网平面形态特征及影响因素

Planform geometry and controlling factors of river networks in the Yellow River source zone

黄河源区不同地貌环境下的河网发育模式、平面形态等具有显著差异,存在羽状、矩形状、对称羽状和树状4种典型的河网类型。选取黄河源区83个典型子流域,计算了河网平面特征参数,探讨了河网参数与地形和气候因子的关系及河网类型的分布规律。结果表明,4类河网平面特征差异性通过流域宽长比、河网密度和流域内河流流向最大频数得到了较好的体现。流域坡度和降雨量对河网密度及流向的影响显著,且能较好解释河网密度及流向最大频数的变化;降雨对流域宽长比的影响显著。羽状主要分布在源区上游北部边缘地带,气候干旱,地表裸露,流域坡度均值为4.5°,流域高差均值为730 m。矩形状集中分布在若尔盖地区,气候相对湿润,且有大量的沼泽湿地,流域坡度和流域高差均值分别为2.3°和177 m。对称羽状处于高山峡谷地带,流域坡度和流域高差均值分别为16.9°和1167 m,降雨量变化范围大。树状分布在黄河源中游山区及中下游的冲积地貌,流域坡度和流域高差均值分别为15.4°和968 m,植被覆盖较好。结合4类河网的空间分布特征及河网参数与环境因子的多元回归分析结果分析,认为地形是决定河网平面形态分异的主要原因;当地形限制减小,气候条件和植被覆盖情况对河网的发展起了重要作用。

The landform of the Yellow River source zone is diverse, leading to various drainage patterns. To understand the drivers of planform geometry of river networks in the Yellow River source zone, 83 representative sub-basins, including dendritic, pinnate, rectangular and symmetric pinnate patterns are selected for studies. Attributes to characterize the planform geometry of river networks are calculated. The relationships between river network attributes and environmental factors are examined. The results show that the differences in the characteristics of the 4 drainage patterns are well reflected by the aspect ratio, drainage density and the maximum frequency of flow directions. Changes in drainage density and the maximum frequency of flow directions are well expressed by slope and precipitation variation. Aspect ratio is significantly influenced by precipitation. The pinnate networks are mainly distributed at the northern edge of the upper plateau where the climate is arid and the surface is bare. The mean basin slope of this pattern is 4.5°, and the mean relief is 730 m. The rectangular networks are concentrated in the Zoige basin where the mean basin slope and relief are 2.3° and 177 m,respectively. The climate of the Zoige basin is relatively humid and there are plenty of swamps and wetlands. The symmetric pinnate networks are more likely to occur in the high-relief valleys where the precipitation varies greatly. The average slope and relief of the symmetric pinnate networks are 16.9° and 1167 m, respectively. The dendritic networks are distributed mainly in mountainous areas of the middle reaches and fluvial plains in the northeast part of the source zone. The average slope and relief of the dendritic networks are 15.4° and 968 m, respectively. The vegetation coverage is better than that of the upper plateau. Our analysis suggests that topography is the main factor that leads to the differences of planform geometry among various drainage patterns. Climate and vegetation coverage play an important role in the development of river networks when the constraints of topography are reduced.