Denudation Rates since the Middle Holocene Based on Sediment Storage in the Kiso-Nagara-Ibi and Shonai, and Yahagi River Basins, Central Japan

Abstract

This study estimated millennial-scale denudation rates during the past 6,000 years in the Kiso-Nagara-Ibi and Shonai, and Yahagi River basins, central Japan, based on calculated sediment storage in two fluvial-coastal plains (the Nobi Plain and Yahagi River lowlands) by analyzing existing borehole data using GIS.

Total sediment storage rates during the past 6,000 years were 17,747×10⁶m³ in the Nobi Plain (1,179km²) and 1,980×10⁶m³ in the Yahagi River lowlands (221km²), and the rates at 1,000-year intervals were 2,281-4,375×10⁶m³ in the Nobi Plain and 222-434×10⁶m³ in the Yahagi River lowlands. Denudation rates based on

these values were an average of 0.37mm/year (0.29-0.55mm/year) in the Kiso-Nagara-Ibi and Shonai River

basins (7,969km²) and an average of 0.22mm/year (0.15-0.29mm/year) in the Yahagi River basin (1,490km²).

Total sediment storage when considering accumulation outside the calculation area (alluvial fans, valley

bottom plains, and sea area) was estimated to be 21,775×10⁶m³ (2,911-5,128×10⁶m³/kyear) in and around the

Nobi Plain (2,700km²) and 2,990-3,671×10⁶ m³ (390-716×10⁶m³/kyear) in and around the Yahagi River lowlands (626km²). Denudation rates calculated from these values were an average of 0.46mm/year (0.37-0.64mm/year) in the Kiso-Nagara-Ibi and Shonai River basins and an average of 0.33-0.41mm/year (0.26-0.48mm/year) in the Yahagi River basin. Although these values are approximately 0.1-0.2mm/year larger than the previous values before considering accumulation outside the calculation area, they are not significantly different.

Denudation rates estimated from mean slope gradients in the mountains of the drainage basins were 0.45mm/year in the Kiso-Nagara-Ibi and Shonai River basins and 0.16mm/year in the Yahagi River basin. In particular, most denudation rates based on sediment storage exceeded the values obtained from the slope gradient in the Yahagi River basin. This may be due to the influence of factors other than slope gradient (e.g., weathered granite distribution) and the overestimation of sediment storage in Mikawa Bay. However, there is no significant difference between denudation rates calculated from sediment storage and estimated from the mean slope gradient in the Kiso-Nagara-Ibi and Shonai, and Yahagi River basins. Thus, it may be possible to understand millennial-scale denudation rates in a drainage basin based on sediment storage in the fluvial-coastal plain.

The denudation rate during the past 1,000-year period in both basins was the largest during the past 6,000 years. Increases in human activities such as deforestation since about 1,000 years ago may have been responsible for the change in the denudation rate.