Shrinking Tonle 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 Tonle 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 sandmining in PhnomPenh has affected theMekong'swater levels more than basin-scale climatic factors. Similarly, themodulation of a limited portion ofwater by upstreamdams is unlikely to have caused dramatic inundation variation along the Lower Mekong River around Tonle Sap. A hysteretic relationship between water levels at Prek Kdam and Tonle Sap indicates that Tonle Sap's water level is largely controlled by Tonle 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 Tonle 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 riverbedmining, with Phnom Penh being amining hotspot in the Lower Mekong. Considering its economic and ecological importance, the shrinking of Tonle 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. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

The study reveals that extensive riverbed mining has negative impacts on the water level and volume of Tonle Sap Lake, with the erosion in Phnom Penh affecting the water levels in the Mekong more than basin-scale climatic factors. The water level of Tonle Sap is mainly controlled by Tonle Sap River and the Mekong, with declining levels due to sand mining in Phnom Penh leading to the lake's shrinking. The study also highlights the increasing intensity of riverbed mining, particularly in Phnom Penh, and the potential consequences of Tonle Sap's shrinking for the region.