The Paleochannel Of The Yihe River In The Urban Section Of Linyi City During The Last Glacial Maximum

The environment since the Last Glacial Maximum(LGM) has always been a hot research topic both at home and abroad. Revealing the environmental characteristics and their change rules since the LGM is an important content in the study of quaternary geology. As an important part of the research of paleohydrogeology and water cycle, the research of paleochannel is the important area of studying the change of the geographical environment in the past. The river regime characteristics and hydrological characteristics can be revealed according to the geometric morphology and characteristics of the sediment of paleochannel, and then the ancient geographical environment can be restored. Domestic research on the paleochannel in the eastern China since the last glaciation focus on large rivers that directly flow into the sea, however, rarely involves the regional rivers that not directly flow into the sea. The Yihe River, as an important tributary of the Huaihe River, is a regional river that not directly flows into the sea and has not been researched systematically on paleochannel.In this paper, we selected five paleo-valley cross-sections near the Xiejiazhuang village, the Liuhan rubber dam, the Jiuqu River Bridge, the G327 River Bridge and the Kailuo Bridge as the research objects and then restored the cross-section morphology of the paleovalley of the Yihe River in the urban section of Linyi city during the Last Glacial Maximum by collecting and analyzing relevant geophysical and drilling data as well as dating samples. The cross-section morphological characteristics mainly include river width, river depth, width and depth ratio, river pattern, river longitudinal slope and so on. The morphological characteristics and sedimentary distribution of the paleovalley were compared and analyzed, also the reasons which result in the cross-section morphological characteristics of the paleovalley of the Yihe River and the evolution to the modern characteristics of the paleochannel of the Yihe River were analyzed. The research conclusions are as follows:(1) The width and depth ratio of the paleovalley of the Yihe River in the urban section of Linyi during the Last Glacial Maximum takes on a tendency of increase from the upstream to the downstream. The width and depth ratio of the paleovalley in the Xiejiazhuang village section is 86.22, 123.20 in the Liuhang rubber dam section, 128.51 in the Jiuqu River Bridge section, and 168.02 in the G327 River Bridge section.(2) During the LGM, the paleochannel of the Yihe River in the urban section of Linyi city is type of braided river from the upstream to the downstream. Two different river type identification methods are applied to compute various parameters of ancient river channel from the Xiejiazhuang village section to the G327 Yihe River Bridge section and to identify the sedimentary characteristics. Finally, we made a conclusion that the paleochannel of the Yihe River is type of f braided river. The reason should be: the climate of the Yihe River basin during the LGM is dry and cold with little precipitation and uneven distribution of the year, which result in annual river runoff is small but seasonal river runoff change is evident with large amplitude, furthermore the vegetation in the basin is thinly scattered, the peak flow and sediment yield is much larger than we've expected. The strong hydrodynamic of the flood and a great deal of sediment brought by the flood can cause erosion of the riverbed obviously, but also in the flood ebb process, because of its carrying capacity of flow decreases rapidly, which often makes the braided channels where the mainstream located silt rapidly, forces the mainstream swerve into a new Branch, and make the position of the main channel oscillate back and forth, which create the conditions for the formation of braided river. In addition, the river channel and the riverbank of the Yi ancient river in the urban section of Linyi during the Last Glacial Maximum time, where the content of the silt and clay is little and the deposited mostly consists of medium sand and coarse sand, so the riparian erosion resistance is low and the position of the main channel is wobbly, which make it easy to form features of the braided river. Moreover, the longitudinal slope of the Yi ancient river is great during this period, the power flow increases, the slope is steep and the stream moves rapidly, the erosion force towards the river bank enhances, but also improve its sediment carrying capacity of rivers, viscous and thin particles is hard to deposit, which create the necessary boundary conditions for the formation of braided river.(3) The longitudinal slope of the paleochannel of the Yihe River in the urban section of Linyi city is relatively large, the average longitudinal slope reached 0.93‰, largest in the Liuhang section, relatively small in other sections, and shows irregular decreasing trend below the Jiuqu section. The differences and variation tendency of longitudinal gradient during different sections of the paleochannel of the Yihe River in the urban section of Linyi should be attributed to tectonic movement and the components of basement long the river channel as well as the differences of riverbed components.(4) During the LGM, the paleochannel of the Yihe River in the urban section of Linyi city is type of braided river. The Yihe River is a braided river in the middle reaches of the river(Bashan reservoir to the Benghe River estuary), but gradually evolves into a curved river in the lower reaches of the river(the Benghe River estuary to the Luoma Lake), straight or slightly curved river from the Benghe estuary to lizhuang of Tancheng county. The research section from the Jiuqu River Bridge section to the G327 River Bridge section located between the Benghe estuary and lizhuang, the paleochannel of the Yihe River in this research section evolved from the braided river to the straight or meandering one. Many contributing factors may involve this process, such as the alteration of river and sediment runoff, differences of riparian components, the longitudinal gradient change, the water storage and project construction, the sand mining activities and so on.