Shrinking Tonlé Sap and the recent intensification of sand mining in the Cambodian Mekong River.

This paper investigates the impacts of extensive riverbed mining in the Lower Mekong on the water level, extent and volume of the Tonlé Sap Lake, the largest freshwater lake in Southeast Asia. Our results indicate that the lake's volume has decreased from 1980 to 2018 (p-value = 0.016), with water levels at Phnom Penh Port and Phnom Penh Bassac showing decreasing trends since 1980 (p-values <0.0001). However, discharge at Phnom Penh Bassac (1960-2002) presented an insignificant trend (p-value = 0.147), indicating that riverbed incision due to extensive sand mining in Phnom Penh has affected the Mekong's water levels more than basin-scale climatic factors. Similarly, the modulation of a limited portion of water by upstream dams is unlikely to have caused dramatic inundation variation along the Lower Mekong River around Tonlé Sap. A hysteretic relationship between water levels at Prek Kdam and Tonlé Sap indicates that Tonlé Sap's water level is largely controlled by Tonlé Sap River and the Mekong, and declining water levels at Prek Kdam due to extensive sand mining in Phnom Penh is directly related to the shrinking of the lake. Although there are three main inflows into Tonlé Sap (from the Mekong, local tributaries and direct precipitation), the Mekong's contribution is the largest; tributary discharge and rainfall did not display any significant trend over the investigated period as well. Additionally, the analysis of high-resolution images revealed a recent intensification of riverbed mining, with Phnom Penh being a mining hotspot in the Lower Mekong. Considering its economic and ecological importance, the shrinking of Tonlé Sap would have grave repercussions for the region. Since sand demand is unlikely to weaken in the foreseeable future, these new insights can potentially help inform regulatory frameworks in ensuring sustainable sand extraction rates.