Holocene palaeochannels at SiSatchanalai, north-central Thailand: ages, significance and palaeoenvironmental indications

Radiocarbon dating of sediments infilling palaeochannels in the northern part of the Central Plain of Thailand shows that they probably date from the early to mid-Holocene. The composition of bed material of the palaeochannels and the Yom River (the modern river) confirms that the palaeochannels represent earlier courses of the Yom. The most recent palaeochannel was abandoned about 1700 years ago when the modern course of the Yom was established, probably by avulsion during a period of increased runoff and flood activity which has also been identified in northeast Thailand. Bankfull discharges (Qbf) of the palaeochannels are estimated in two ways. The 'hydraulic' approach combines a palaeochannel's bankfull cross-sectional area and an estimate of the average velocity needed to transport the coarse fraction of the channel's bed material. The 'geomorphological' approach relies on published equations relating Qbf and channel dimensions. When applied to the grain-size and dimensional characteristics of the modern Yom, both methods give results consistent with modem gauging data. The palaeodischarge estimations indicate the greatest bankfull discharges in the early- to mid-Holocene palaeochannel; slightly lower, but still comparable, discharges in the present channel; and lower to substantially lower discharges during the mid- to late-Holocene. The modem discharge may be affected by human activity, indicating that the highest discharges of the Holocene occurred during the early- to mid-Holocene. This sequence of Holocene environments is in close agreement with independ ently derived regional palaeoclimatic reconstructions, suggesting that these fluctuations in discharge have been externally climatically controlled, and that they are not the result of internal adjustments associated with multiple channels during phases of channel avulsion. The possibility of one phase of coeval distributary flow through the two mid- to late-Holocene palaeochannels does not change this conclusion, but it lessens the inferred decline in precipitation between the early and late-Holocene.