5:15 PM - 6:45 PM
[HGM03-P02] Landform development along the Yura River, the Haze River, and the Takeda River based on projected longitudinal sections of terrace surfaces
★Invited Papers
Keywords:projected longitudinal sections of terrace surfaces, former Lake Fukuchiyama, changing the flow path of the Yura River, terrace formation processes, Osadano fill-top terrace
In the basin of Fukuchiyama, Kyoto Prefecture, there are terraces with a relative height of 50 m, and the area is a vast flat surface measuring 4 km from east to west and 3.5 km from north to south, and this terrace is currently used as an industrial park. The Yura River, which flows through the Fukuchiyama Basin, flows into the Sea of Japan, but according to Okada and Takahashi (1969), it once flowed through the Hikami-corridor into the Kako River Valley and into the Seto Inland Sea. This former Yura River was dammed for some reason, and Fukuchiyama Lake was formed centered around the Fukuchiyama Basin. The river/lacustrine sedimentary surface deposited in this Fukuchiyama Lake is the Osadano fill-top surface. The age of formation is approximately much more than 270,000 to 190,000 years ago, based on analysis of the volcanic ash layers (Kato et al., 2006). Due to the formation and decline of Lake Fukuchiyama, the local erosional level changed, resulting in the formation of the Osadano surface, which is a fill-top terrace, and the erosional terrace group, which is a fill-strath terrace.
The purpose of this research is to create projected longitudinal profiles of terrace surfaces, reconsider the terrace division from the continuity of terrace surfaces, and consider the formation history of terraces. The survey targets are the Yura River, its tributary HazeRiver, and its tributary Takeda River. The Takeda River is an unconformable river that flows through a valley created by the former Yura River. Furthermore, since there are differences in the distribution of terrace surfaces among the three rivers, the causes of this should be investigated from the perspective of the formation of river valleys and differences in river scale.
As a result of the survey, we first clarified the distribution of Osadano surfaces, which are fill-top terraces, and divided seven erosional terraces formed by undercutting Osadano surfaces. For both the Yura River and the Haze River, terrace surfaces that are roughly parallel to the current riverbed were divided in the mountainous sections. These are fill-tops of river valleys corresponding to the rising water level of Lake Fukuchiyama, and eroded terraces due to the falling water level of Lake Fukuchiyama. In addition, nearly horizontal Osadano terraces are divided between the Fukuchiyama Basin areas at an altitude of 70-75 m, and a topographic surface with a gentle longitudinal gradient was also confirmed in the terraces that erode Osadano terraces. These are thought to be fan delta sedimentary surfaces created by the Yura River and Haze River. The terrace surface formed by erosion of the fan delta was wide at Haze River and narrow at Yura River. This is thought to be because the Yura River has a high ability to transport sediment, and the sandy delta sediments are easily eroded and are difficult to remain as terraces along the Yura River. In Haze River, the terrace surfaces are relatively well preserved, and the vast Osadano terrace area may have been preserved without being eroded.
On the other hand, the terraces of the Takeda River were terraces formed from sedimentary fans from branch valleys, and had a completely different character in terrace formation processes. The terminal elevations of these sedimentary surfaces varied slightly between 70 and 80 m, but they were judged to be the remains of an alluvial fan that jutted out into Lake Fukuchiyama. The reason why the main Takeda River was unable to develop a fan delta is thought to be that the wide and deep valley created by the former Yura River could not be filled with the amount of debris transported by the Takeda River.
As mentioned above, differences in the distribution characteristics of terrace surfaces were observed among the three rivers due to the formation and decline of the Lake Fukuchiyama. One of the important factors for this was the compatibility or incompatibility of the river valley formed by the former Yura River with the current scale of the river that flows through it. In addition, from the perspective of preserving the terrace surface, it has become clear that the medium-sized Haze River is more suitable than the large-scale Yura River.
The purpose of this research is to create projected longitudinal profiles of terrace surfaces, reconsider the terrace division from the continuity of terrace surfaces, and consider the formation history of terraces. The survey targets are the Yura River, its tributary HazeRiver, and its tributary Takeda River. The Takeda River is an unconformable river that flows through a valley created by the former Yura River. Furthermore, since there are differences in the distribution of terrace surfaces among the three rivers, the causes of this should be investigated from the perspective of the formation of river valleys and differences in river scale.
As a result of the survey, we first clarified the distribution of Osadano surfaces, which are fill-top terraces, and divided seven erosional terraces formed by undercutting Osadano surfaces. For both the Yura River and the Haze River, terrace surfaces that are roughly parallel to the current riverbed were divided in the mountainous sections. These are fill-tops of river valleys corresponding to the rising water level of Lake Fukuchiyama, and eroded terraces due to the falling water level of Lake Fukuchiyama. In addition, nearly horizontal Osadano terraces are divided between the Fukuchiyama Basin areas at an altitude of 70-75 m, and a topographic surface with a gentle longitudinal gradient was also confirmed in the terraces that erode Osadano terraces. These are thought to be fan delta sedimentary surfaces created by the Yura River and Haze River. The terrace surface formed by erosion of the fan delta was wide at Haze River and narrow at Yura River. This is thought to be because the Yura River has a high ability to transport sediment, and the sandy delta sediments are easily eroded and are difficult to remain as terraces along the Yura River. In Haze River, the terrace surfaces are relatively well preserved, and the vast Osadano terrace area may have been preserved without being eroded.
On the other hand, the terraces of the Takeda River were terraces formed from sedimentary fans from branch valleys, and had a completely different character in terrace formation processes. The terminal elevations of these sedimentary surfaces varied slightly between 70 and 80 m, but they were judged to be the remains of an alluvial fan that jutted out into Lake Fukuchiyama. The reason why the main Takeda River was unable to develop a fan delta is thought to be that the wide and deep valley created by the former Yura River could not be filled with the amount of debris transported by the Takeda River.
As mentioned above, differences in the distribution characteristics of terrace surfaces were observed among the three rivers due to the formation and decline of the Lake Fukuchiyama. One of the important factors for this was the compatibility or incompatibility of the river valley formed by the former Yura River with the current scale of the river that flows through it. In addition, from the perspective of preserving the terrace surface, it has become clear that the medium-sized Haze River is more suitable than the large-scale Yura River.