TEMPORAL AND SPATIAL CHANGES IN BRAIDING INDEX OF ALLUVIAL RIVERS IN JAPAN AND THEIR REASONS

Channel forms of alluvial rivers in the Meiji Era of Japan，I.e.1870至1910‘s，is assumed to be under the nearly natural condition，because no large dams were constructed before and during the Meiji Era.The temporal and spatial changes in channel braiding and sinuosity of 34alluvial river during the past about100years were examined with morphometry using1/50，000 topographic maps published repeatedly four or nine times in the different years since1870‘s.The results are summarized as follows.1)in the Meiji Era，the maximum value of braided index of a river tends to appear in the channed span of 1×10 I D1-3 i D1 in channel gradient，I.e.at the fan margin where the mean grain size of river sediment decreases from gravel to sand.2)In the present，as compared with the data for the Meiji Era，the maximum value of braided index has increased markedly in the fan and appears at the mid-fan of 3×10 I D1-3 i D1 in channel gradient。Whereas the maximum value has decreased markedly in the large plains composed of fan，meander plains and delta，but it appears at the same area as that in the Meiji Era.3)After the construction of weirs，the braided index increase in the upstream channel of the weirs in fan，but a little effect of weirs on the braiding appears in the meander plains。4)The maximum value of sinuosity tends to appear in the channel span of 1×10 I D1-3 I D1~1×10 I D1-4 I D1 in channel gradient。This threshold gradient is larger than those in the Mississippi River。These temporal and spatial changes in channel braiding and sinuosity is considered to be resulted from the river improvement and dam construction since the 1960‘s in particular.

Channel forms of alluvial rivers in the Meiji Era of Japan, I.e. 1870〜1910's, is assumed to be under the nearly natural condition, because no large dams were constructed before and during the Meiji Era. The temporal and spatial changes in channel braiding and sinuosity of 34 alluvial river during the past about 100 years were examined with morphometry using 1/50,000 topographic maps published repeatedly four or nine times in the different years since 1870's. The results are summarized as follows. 1) in the Meiji Era, the maximum value of braided index of a river tends to appear in the channed span of 1×10ィイD1-3ィエD1 in channel gradient, I.e. at the fan margin where the mean grain size of river sediment decreases from gravel to sand. 2) In the present, as compared with the data for the Meiji Era, the maximum value of braided index has increased markedly in the fan and appears at the mid-fan of 3×10ィイD1-3ィエD1 in channel gradient. Whereas the maximum value has decreased markedly in the large plains composed of fan, meander plains and delta, but it appears at the same area as that in the Meiji Era. 3) After the construction of weirs, the braided index increase in the upstream channel of the weirs in fan, but a little effect of weirs on the braiding appears in the meander plains. 4) The maximum value of sinuosity tends to appear in the channel span of 1×10ィイD1-3ィエD1〜1×10ィイD1-4ィエD1 in channel gradient. This threshold gradient is larger than those in the Mississippi River. These temporal and spatial changes in channel braiding and sinuosity is considered to be resulted from the river improvement and dam construction since the 1960's in particular.