Global sand and gravel (aggregate) extraction is changing landscapes, with 29,375x106m3 to 36,875x106m3 globally mined each year (Steinberger et al, 2010). Rapid economic development in South East Asia has markedly driven demand for sand in the Mekong River basin (Hackney et al, 2019). The Mekong River, a convenient source of high-quality aggregate, is the main source of sediment in the region (Kondolf, 1994; Bravard et al, 2013). Aggregate is used in the burgeoning construction industry, reclamation projects, and infilling of land along the Mekong’s low-lying floodplains (Pierdet, 2008; Doyle, 2012; Mailhe et al, 2019). Questionnaire-based research found all countries that the Mekong flows through extracted material during 2010-11, of these Cambodia extracted the largest volume of at least 21x106m3yr-1 of Mekong River sediment (Bravard et al, 2013). However, the study timeframe was limited, and specific destinations of this sand were not mapped or quantified with field measurements. This research develops differencing methods that tie the datasets together to facilitate direct comparisons between the 2000 (NASADEM) and 2013 (TanDEM-X) global elevation DEMs, a promising new approach that could be applied globally. For the Cambodia study area over 183±18 x106 m3 of floodplain infill (FPI) occurred, identifying FPI the likely biggest demand for sand along the Cambodian Mekong River. Timing of FPI can be determined with annual high-resolution imagery, with peak infill activity occurring between 2010 and 2011. FPI volumes measured in this period were over 4±2x106m3 greater than estimates of sediment extraction from the Mekong during 2011 (Bravard et al, 2013). FPI decreases flood risk, bringing economic and social advantages to property owners in the basin (Pierdet, 2008). But rising demand for sand in the basin has caused local channel changes and a sediment deficit being supplied to the delta, causing delta retreat and flooding, which has displaced people (Orr et al, 2012; Jordan et al, 2019). The timing and enormous scale of FPI quantified in this paper can inform future frameworks and policies, and the novel techniques provide a pathway to expanding this research elsewhere
