Abstract:

More than 50 river basins with gauging stations were chosen for an analysis on relationships between sediment yield and basin morphological characteristics in the middle reaches of the Yellow River. They have drainage areas ranging from 500 to 2500 km 2 each and can be categorized into 6 types of basins with different physiographical backgrounds:the loess hilly basins and highly weathered mudstone & siltstone basins north to Wuding River,loess hilly basins in north Shaanxi, west Shanxi and east Gansu provinces, loess hill + bedrock mountain basins, bedrock mountain + loess hilly basins, loess yuan basins and bedrock mountain basins. A set of data on river basin morphology were obtained by using morphometric method, including drainage density, percentage of inter gully area with slope steepness less than 15°, relief ratio, basin roughness, basin circularity etc. Different relationships between sediment yield and basin morphological characteristics were obtained for different types of river basins by plotting and multi variate analyses. The results showed that the best correlation exits between sediment yield and drainage density, but the relations between sediment yield and the other morphological characteristics are not as good as expected due to heavy disturbance of surface material, vegetation coverage, morphological development from various types of river basins. If taking the basin types as parameters, different fitness, intercept and slope of correlation lines were observed between sediment yield and drainage density for different types of river basins, showing a control of other basin characteristics, such as surface material, vegetation, landform development etc., on the relationships of sediment yield and basin morphological characteristics. The intercepts and slopes of lines tend to increase from loess hilly basins through loess hill + bedrock mountain basins and bedrock mountain + loess hilly basins to bedrock mountain basins subsequently. It indicates that basin morphology is an important factor affecting sediment yield of a river basin but can exert substantial influence on sediment yield of the river basin only when the basin characteristics are more homogeneous. Therefore a classification of river basin types is necessary before a correlation between sediment yield and basin morphological characteristics is established.

Key words: the Middle Yellow River, basin morphology, sediment yields