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We present a statistically robust reconstruction of Thailand's Chao Phraya River peak season streamflow (CPRPF) that spans the 202 years from 1804 to 2005 CE. Our reconstruction is based on tree ring delta O-18 series derived from three Pinus merkusii sites from Laos and Thailand. The regional delta O-18 index accounts for 57% of the observed variance of CPRPF. Spatial correlation and 21-year running correlation analyses reveal that CPRPF is greatly influenced by regional precipitation variations associated with the El Nino-Southern Oscillation (ENSO). Periods of enhanced and reduced ENSO activity are associated with strong and weak ENSO-streamflow correlation, respectively. At the longer timescale, the Pacific Decadal Oscillation (PDO) appears to modulate the ENSO-streamflow correlations, with the most extreme flood events along the Chao Phraya River occurring during periods of increased frequency of La Nina events that coincide with extended cold phases of the PDO. The CPRPF reconstruction could aid management planning for Thailand's water resources.

Plain Language Summary We present a 202-year reconstructed record of the peak season streamflow from the Chao Phraya River in Thailand. Our reconstruction is derived from the average of delta O-18 from three tree ring sites in Thailand and Laos, upstream of the Chao Phraya River. We found strong connection between the streamflow and the Pacific Ocean climate modes. The result reveals short-term pulses in extreme flow conditions embedded in the longer-term climate variations. Such information could be used to assess streamflow variation and thereby aid water management planning.