5:15 PM - 7:15 PM
[HQR05-P04] Evolution of the alluvial lowland around the Izunuma Lake in the lower Hasama River, northeastern Japan
Keywords:blocked-valley lake/swamp, landform evolution, backwater effects
River floods are one of the key geomorphic processes in alluvial lowlands and can pose hazards to human society. Landform classification and the study of landform evolution in alluvial lowlands are fundamental for evaluating and managing fluvial hazards. In Japan, research on landform classification and evolution in the alluvial lowlands of small- and medium-sized tributaries has been conducted to a lesser extent compared to that of large rivers. In recent years, backwater flooding frequently occurred in small- and medium-sized tributaries, yet few studies have evaluated this risk from a geomorphological perspective. Blocked-valley lakes and swamps form in alluvial lowlands where smaller tributary valleys are dammed by alluvial deposits from larger rivers, making them particularly susceptible to backwater effects. However, few studies have been conducted to clarify the evolution and structures of blocked-valley lakes and swamps in Japan.
Although the right bank of the Hasama River, where Izunuma Lake is located, is prone to backwater flooding from the mainstem river, the landform evolution of Izunuma Lake and the surrounding alluvial lowlands remains unclear. This study discusses the evolution of the alluvial lowland around the Izunuma Lake in the lower Hasama River based on landform classification, analysis of existing borehole logs, and boring surveys to obtain fundamental knowledge for evaluating flood risks and understanding the evolution blocked-valley swamps in alluvial lowlands.
Blocked-valley swamps, with elevations approximately 0.5–1.0 m lower than the mainstem floodplain, were found around the outlet of tributaries on the right bank of the Hasama River. A topological map published in 1913 shows that these areas were formerly lakes or wetlands. Borehole data reveal that the alluvium, approximately 20–30 m thick, is present around the outlet of the tributary valley of the Izunuma Lake. The lower part of the alluvium, at elevations of 15–20 m, consists mainly of mud containing shell fragments and is correlated with the Middle Clay Layer, which is interpreted as deposits accumulated at the bottom of an inner bay that expanded due to sea-level rise since ~10,000 years B.P. in the lower Kitakami River plain (Ito, 1999). This suggests that during the peak of the Jomon transgression, the inner bay expanded to the outlet of the tributary valley, and Izunuma Lake formed as a lagoon (a remnant of the sea).
To-a tephra was intercalated at a depth of 164–167 cm in a borehole core sampled from the former water area of Izunuma Lake, indicating that the lake had already formed by AD 915. Alternation layers of silt and fine to medium sand were found in the lower parts of the borehole cores sampled from the former water area of Izunuma Lake and an abandoned channel of the Hasama River. These deposits are interpreted as part of the interfingering between the deposits of the mainstem floodplain and blocked-valley lake of the tributary (Grenfell et al., 2010), formed by recurrent backwater effects from the mainstem. Radiocarbon dating of plant fragments from the abandoned channel of the Hasama River indicates that the mainstream of the Hasama River flowed through this area between 359 BC and 1220 AD. This suggests that Izunuma Lake has been maintained as a blocked-valley lake for at least the historical period. In another words, Izunuma Lake is a lagoon which also has the nature of blocked-valley lakes.
The contrast in sediment supply between the mainstem of the Hasama River and its right-bank-tributaries may have contributed to the formation of blocked-valley swamps in the lower Hasama River. Future studies should continue to accumulate knowledge on landform evolution in lakes and alluvial lowlands under similar topographical conditions to these of Izunuma Lake and compare findings with the results of this study.
This study was financially supported by Mt. Kurikoma Area Geopark.
Although the right bank of the Hasama River, where Izunuma Lake is located, is prone to backwater flooding from the mainstem river, the landform evolution of Izunuma Lake and the surrounding alluvial lowlands remains unclear. This study discusses the evolution of the alluvial lowland around the Izunuma Lake in the lower Hasama River based on landform classification, analysis of existing borehole logs, and boring surveys to obtain fundamental knowledge for evaluating flood risks and understanding the evolution blocked-valley swamps in alluvial lowlands.
Blocked-valley swamps, with elevations approximately 0.5–1.0 m lower than the mainstem floodplain, were found around the outlet of tributaries on the right bank of the Hasama River. A topological map published in 1913 shows that these areas were formerly lakes or wetlands. Borehole data reveal that the alluvium, approximately 20–30 m thick, is present around the outlet of the tributary valley of the Izunuma Lake. The lower part of the alluvium, at elevations of 15–20 m, consists mainly of mud containing shell fragments and is correlated with the Middle Clay Layer, which is interpreted as deposits accumulated at the bottom of an inner bay that expanded due to sea-level rise since ~10,000 years B.P. in the lower Kitakami River plain (Ito, 1999). This suggests that during the peak of the Jomon transgression, the inner bay expanded to the outlet of the tributary valley, and Izunuma Lake formed as a lagoon (a remnant of the sea).
To-a tephra was intercalated at a depth of 164–167 cm in a borehole core sampled from the former water area of Izunuma Lake, indicating that the lake had already formed by AD 915. Alternation layers of silt and fine to medium sand were found in the lower parts of the borehole cores sampled from the former water area of Izunuma Lake and an abandoned channel of the Hasama River. These deposits are interpreted as part of the interfingering between the deposits of the mainstem floodplain and blocked-valley lake of the tributary (Grenfell et al., 2010), formed by recurrent backwater effects from the mainstem. Radiocarbon dating of plant fragments from the abandoned channel of the Hasama River indicates that the mainstream of the Hasama River flowed through this area between 359 BC and 1220 AD. This suggests that Izunuma Lake has been maintained as a blocked-valley lake for at least the historical period. In another words, Izunuma Lake is a lagoon which also has the nature of blocked-valley lakes.
The contrast in sediment supply between the mainstem of the Hasama River and its right-bank-tributaries may have contributed to the formation of blocked-valley swamps in the lower Hasama River. Future studies should continue to accumulate knowledge on landform evolution in lakes and alluvial lowlands under similar topographical conditions to these of Izunuma Lake and compare findings with the results of this study.
This study was financially supported by Mt. Kurikoma Area Geopark.