利用河階發育年代、堆積特徵與其分佈來探討荖濃溪之河流演育過程

Abstract

荖濃溪主要位於高雄縣，流域以東為中央山脈地質帶，以西為西部麓山帶地質帶。自敏督利颱風過後此區有多處崩塌，因而露頭多且更易採集定年物質，因而重新調查此區河階。藉由航照判讀等與野外調查瞭解荖濃溪河階之類型與沈積相，以及採取河階沈積層之定年物質得河階生成年代。其目的為進行河階比對進而建立荖濃溪流域的地形演育歷史，並探討影響荖濃溪階地分佈之因素。 本研究共採集到20個碳十四定年物質。因河階自美秀四社之間難以對比，四社以南河階為岩石河階，美秀以北則有數階為堆積河階，因而將河階分為南北兩群，美秀以北為NT群，四社以南則為ST群。NT群的共五階並約三千年以來生成，ST群也分為五階，其中第四階與第五階生成老於一萬年前。 藉由定年推估荖濃溪的長期下切速率約0.8cm/yr，並且有上游較快，下游較慢之現象。荖濃溪河階演育大至分為五期。第一期為27000 ~28000年前。第二期為8750~11500年前，第三期為3000~5400年前，第四期為2100~2800年前，第五期為900~1800年前。美秀以北的土石流規模較以南的大且年輕，並可能造成荖濃溪的堵塞與局部加積。局部加積影響所形成的主流河階，也造成荖濃溪河階上下游對比上的困難。 流域內支流土石流扇集中於右岸，其因素可能為地質與地形特性。因玉山山脈的北陡南緩，導致美秀以北的支流較易重複發生土石流，因而其北土石流扇與河階年紀因而年輕。

Lao-Nong River is mostly located in Kaohsiung County, whose drainage basin is bounded by Central Mountains on the east and the fold-and-thrust belt of Western Taiwan on the west. There have been multiple landslides since Mindulle Typhoon, and so many outcrops are present that it is easier to sample substances for radiocarbon dating. Therefore this study aims to re-examine river terraces in this area. By studying aerial photos and undertaking field investigation, this research aims to understand the types of Lao-Nong River terraces and their sedimentary faces, and also to obtain the age of these river terraces by radiocarbon dating. The purpose is to correlate river terraces with an eye to establishing the evolutionary history of Lao-Nong River’s drainage basin and examining the factors that influence the distribution of its terraces. This study has altogether sampled twenty radiocarbon substances. While it is hard to correlate the river terraces between Mei-Shiow and Shi-Sher, it may be identified that rock terraces span south of Shi-Sher while some fill terraces are scattered north of Mei-Shiow. Thus the river terraces in this section can be divided into two groups: northern terrace group (NT) and southern terrace group (ST). NT is located north of Mei-Shiow and ST, south of Shi-Sher. NT consists of five terraces, all of which were generated about three thousand years ago. ST can be subdivided into five terraces as well, the fourth and fifth of which were produced more than ten thousand years ago. By radiocarbon dating method, the downward erosion rate of Lao-Nong River is estimated to be around 0.8 cm/yr on average. This rate is higher upstream and lower downstream. The evolution of river terraces of Lao-Nong River may be grossly divided into five stages: The first stage spanned from 27000 to 28000 years ago, the second 8750 to 11500 years ago, the third 3000 to 5400 years ago, the fourth 2100 to 2800 years ago, and the fifth, 900 to 1800 years ago. Mudslide to the north of Mei-Shiow was larger and younger than that to the south, and it might cause obstruction and local accretion. The mainstream terrace generated by the impact of local accretion constitutes a major difficulty in correlating the river terraces of upstream and downstream Lao-Nong River. Debris fans of the tributaries in the drainage basin are mainly located on the right bank. Probable causes include geological and topographic features. Because it is steep on the northern side but even on the southern side of Jade Mountains, tributaries north of Mei-Shiow tend to form mudslides repeatedly, so debris fans and river terraces in this area are younger.