3:30 PM - 5:00 PM
[HGM02-P06] Relationships between terrace development around tributary junctions and morphologies of tributary basin in the Doushi River, a tributary of the Sagami River, central Japan
Keywords:Fluvial terrace, tephra, Connectivity, Sediment routing system
To understand fill terrace development in the last glacial period, we have to evaluate the sediment supply from tributaries and slopes with diverse geomorphological and environmental settings in long-term timescales. Reconstructing the terrace development around tributary junctions is an efficacious way to reveal the history of the sediment supply from tributaries during valley-filling. This study reconstructs the terrace development around tributary junctions and its relationship with morphologies of tributary drainage basins in the Doushi River, a tributary Sagami River, central Japan.
Late Pleistocene terrace treads located in the right side of the Doushi River are classified into the below two; Sagamihara (III-S) and Tanahara (III-T) terraces. Lower III-S terrace deposits are the brown-colored finer deposits (sand and silt) with angular gravel clasts and laminae. These deposits outcrops at the Aone, Kajino and Mikage district and so on along the Doushi River. Several tephra beds such as Kikai-Tozurahara tephra (K-Tz) are intercalated into it. At the Kajino district, a weathered white ash bed (KJN-4 tephra) intercalated into the upper horizon than K-Tz outcrops along the tributary in Kajino and declines to the lower end of the tributary with the slope ca. 2-3%. This slope probably reflects the riverbed slope of the tributary in Kajino at the time when the KJN-4 was fallen. And, no diatom frustule was found from the samples of the finer deposits between KJN-3 and K-Tz. This cause is presumed to that the sedimentation rate or flow velocity was too large, or that the deposits have been dried and under an aerobic condition. These finer deposits are covered with the angular gravel bed which consists the III-S terrace treads declining with the slope >8% along the tributary.
Above it, it is considered that the finer deposits (sand and silt) with angular gravel clasts consisting the III-S terrace had been deposited under the environments the floodplain of gentle river or wetlands. Because of the base-level rise associated with the trunk river aggradation, a segment with the gentle riverbed or a lentic environment had been established around the lower reaches of the tributaries during the trunk river aggradation. Most tributaries with these finer terrace deposits around junctions tend to have the drainage basins with the smaller area and relief ratio than that of other tributaries in the Doushi River. Coarse gravel clasts yielded in the drainage basins of these tributaries probably were hard to reach the junction with the trunk river during the trunk river aggradation.
This study was financially supported by the River Fundamation (Kasen-Kikin) and JSPS Grants-in-aid for Scientific Research (No. 21K13151).
Late Pleistocene terrace treads located in the right side of the Doushi River are classified into the below two; Sagamihara (III-S) and Tanahara (III-T) terraces. Lower III-S terrace deposits are the brown-colored finer deposits (sand and silt) with angular gravel clasts and laminae. These deposits outcrops at the Aone, Kajino and Mikage district and so on along the Doushi River. Several tephra beds such as Kikai-Tozurahara tephra (K-Tz) are intercalated into it. At the Kajino district, a weathered white ash bed (KJN-4 tephra) intercalated into the upper horizon than K-Tz outcrops along the tributary in Kajino and declines to the lower end of the tributary with the slope ca. 2-3%. This slope probably reflects the riverbed slope of the tributary in Kajino at the time when the KJN-4 was fallen. And, no diatom frustule was found from the samples of the finer deposits between KJN-3 and K-Tz. This cause is presumed to that the sedimentation rate or flow velocity was too large, or that the deposits have been dried and under an aerobic condition. These finer deposits are covered with the angular gravel bed which consists the III-S terrace treads declining with the slope >8% along the tributary.
Above it, it is considered that the finer deposits (sand and silt) with angular gravel clasts consisting the III-S terrace had been deposited under the environments the floodplain of gentle river or wetlands. Because of the base-level rise associated with the trunk river aggradation, a segment with the gentle riverbed or a lentic environment had been established around the lower reaches of the tributaries during the trunk river aggradation. Most tributaries with these finer terrace deposits around junctions tend to have the drainage basins with the smaller area and relief ratio than that of other tributaries in the Doushi River. Coarse gravel clasts yielded in the drainage basins of these tributaries probably were hard to reach the junction with the trunk river during the trunk river aggradation.
This study was financially supported by the River Fundamation (Kasen-Kikin) and JSPS Grants-in-aid for Scientific Research (No. 21K13151).