THE GEOMORPHIC HISTORY OF HUMAN ACTIVITY IN SOUTHWESTERN CHINA (Invited Presentation)

Interdisciplinary, multi-scale research in SW China suggests a complex relationship between people, land use, natural hazards, and erosion. At a regional scale, we used in situ ¹⁰Be-derived basin-average erosion rates, daily sediment yield data from Chinese gauging stations, meteoric ¹⁰Be-derived erosion indices, and ¹³⁷Cs and ²¹⁰Pb_ex activity in detrital sediments to document erosion rates over decadal to millennial time scales. Overall, in situ ¹⁰Be-derived erosion rates correlate with mean basin slope, but in heavily agricultural regions they correlate with agricultural land use determined from global datasets. The ratio of gauging station-derived sediment yield to in situ ¹⁰Be-derived sediment generation rates is ~2; this ratio and meteoric ¹⁰Be-derived erosion indices, a measure of soil loss, correlate with agricultural land use. Together, ¹⁰Be data suggest that agriculture has increased erosion. <5% of detrital samples have ¹³⁷Cs while 25% have ²¹⁰Pb_ex; few samples with detectable ¹³⁷Cs suggests high erosion during ¹³⁷Cs deposition (1953-1964), implying that agriculture or deforestation stripped soil. Greater abundance of ²¹⁰Pb_ex indicates erosion has since slowed, possibly due to recent reforestation policies. To understand human effects on erosion at local scales we worked in a national park in NW Sichuan and a 30 km² watershed in SW Sichuan. In NW Sichuan we used archaeological and geological mapping, as well as interviews with local people, to understand relationships between where people live and geomorphic processes. Here we find archaeological sites on loess deposits and active landslides, suggesting that hazards are related to occupied areas. In SW Sichuan, we mapped soil thickness and valley bottom deposits to complement anthropological data on historic land use and ecological data on forest composition and health. Here indigenous peoples’ ~250 years of swidden agriculture caused widespread slow erosion. 1960s valley bottom deforestation caused the local river to be more mobile in its floodplain and incise the deposited sediments. Thus, although regionally we see a signal of agriculture increasing erosion and soil loss, long-term swidden agriculture and recent rapid agricultural expansion both contributed, and as yet we cannot distinguish clearly between them.