17:15 〜 18:45
[HQR04-P11] Differences in river terrace evolution on both flanks of the watershed: example from Laonong and Qishan Rivers of Taiwan
キーワード：river terrace, landslide, river evolutionary histories
The Laonong and Qishan Rivers are the two main catchments of the Gaoping River, which is the longest river in southern Taiwan, and the Yushan Range is the main watershed between these two catchments. In the eastern and western flank areas of the upstream watershed, many terraces underlain by fluvial/debris-flow gravels 50–250 m thick can be observed. Most of them are incised alluvial/debris fans emanating from tributaries mouths, which their development is likely to have caused the aggradation of otherwise incision-dominated trunk rivers.
Based on the currently available radiocarbon age data, the terrace ages and aggradation scales of the right-bank of the Laonong River on the eastern flank of the watershed are inconsistent. The age data indicate several periods of geomorphic development at 12.5–11, 6.5–5.3, 3.5–2.7, and 1.8–1.1 ka. In contrast, the terrace ages and aggradation scales of the left-bank of the Qishan River on the western flank of the watershed are relatively consistent. The geomorphic development ages are concentrated around 7.9–7 and 1.8–1.1 ka, and the heights of adjacent terraces are comparable. Furthermore, we believe that the observed terraces were generated following large landslides which supplied excess amounts of debris onto rivers. Although the two catchments have similar tectonic and climatic backgrounds, the development of these landslides and terraces cannot respond simultaneously to same extreme weather events. This leads to variations in sediment supply and frequency of occurrence in individual tributaries, which may have different effects on sediment transport and deposition in downstream areas. Therefore, we argue that it is such unevenly distributed landsliding that over time resulted in the observed diachroneity of river evolutionary histories. We also consider that the topographic characteristics of individual tributaries should be further investigated.
Based on the currently available radiocarbon age data, the terrace ages and aggradation scales of the right-bank of the Laonong River on the eastern flank of the watershed are inconsistent. The age data indicate several periods of geomorphic development at 12.5–11, 6.5–5.3, 3.5–2.7, and 1.8–1.1 ka. In contrast, the terrace ages and aggradation scales of the left-bank of the Qishan River on the western flank of the watershed are relatively consistent. The geomorphic development ages are concentrated around 7.9–7 and 1.8–1.1 ka, and the heights of adjacent terraces are comparable. Furthermore, we believe that the observed terraces were generated following large landslides which supplied excess amounts of debris onto rivers. Although the two catchments have similar tectonic and climatic backgrounds, the development of these landslides and terraces cannot respond simultaneously to same extreme weather events. This leads to variations in sediment supply and frequency of occurrence in individual tributaries, which may have different effects on sediment transport and deposition in downstream areas. Therefore, we argue that it is such unevenly distributed landsliding that over time resulted in the observed diachroneity of river evolutionary histories. We also consider that the topographic characteristics of individual tributaries should be further investigated.