Abstract

The Paleo-floods occurred in the past at oxbow lakes along the Đắk Bla River catchment are identified based on the analytical results of sedimentary grain size, the sedimentation distribution and the rapidity of sedimentation in oxbow lakes. Analytical results of 210Pb from drilling core with the length of 7 meters, in one oxbow lake in Đắk Bla river catchment pointed out 04 sedimentary layers, which characterize for Paleo- floods and have component of >50% sand, and the remaining is silts and clay. These sedimentary layers have an average grain size vary with depth, namely: Md = 0.266 mm, 0.210 mm, 0.180 mm and 0.262 mm corresponding to the depths of 15-41 cm, 142-145 cm, 190-407 cm and 419-462 cm, respectively. Average rate of sedimentation is about 4.02 cm/year on a total thickness of 7 m. There are 04 deposition phases, of which 03 phases have deposition rate greater than 9 to 27 times the the average sedimentation rate, corresponding to the sediment layers at OBS1-5 (15-41 cm), OBS1- 15 (190-407 cm) and OBS1-17 (419-462 cm). Combining grain size data and sedimentary accumulation rate, there are four major flood times happened in the Đắk Bla River catchment, which correspond to about 1972, 1984, 1996 and 2009. The frequency of the major floods in the Kon Tum region during the last 50 years is 12 years/time.