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A new algorithm has been developed to quantify floating macro-debris transport on river surfaces that consists of three fundamental techniques: (1) generating a difference image of the colour difference between the debris and surrounding water in the CIELuv colour space, (2) detecting the debris pixels from the difference image, and (3) calculating the debris area flux via the template matching method. Debris pixels were accurately detected from the images taken of the laboratory channel and river water surfaces and were consistent with those detected by visual observation. The area fluxes were statistically significantly correlated with the mass fluxes measured through debris collection. The mass fluxes calculated by multiplying the area fluxes with the debris mass per unit area (M/A) were significantly related to the flood rising stage flow rates and agreed with the mass fluxes measured through debris collection. In our algorithm, plastic mass fluxes can be estimated via calibration using the mass percentage of plastics to the total debris in target rivers. Quantifying riverine macro-plastic transport is essential to formulating countermeasures, mitigating adverse plastic pollution impacts and understanding global-scale riverine macro-plastic transport. Quantifying the transport of macro-debris floating on the world's rivers, which are major sources of ocean debris, is essential in formulating countermeasures to mitigate the adverse impacts of land-based loads. In particular, the adverse impacts on aquatic ecosystems of plastics containing toxic chemicals (e.g., persistent organic pollutants (POPs)) 1-4 are recognized as a serious concern in the global aquatic environment 5-8. Many of the plastics in the oceans originate from land 9 , and thereafter, macro-plastics (>25 mm in diameter) 10 are evenly broken down into smaller plastic fragments known as meso-(5−25 mm in diameter) and micro-plastics (<5 mm in diameter) 10 due to photo-and thermo-oxidative degradation 6. Micro-plastics are rarely removed from the aquatic environment when released and are thus gradually transported far away due to ocean currents. Hence, to formulate countermeasures against oceanic plastic pollution, the macro-plastics in rivers must be efficiently captured before being fragmented into smaller pieces and/or being released to the oceans. In particular, because most macro-plastics float on the surface, it is important to understand how floating macro-debris is transported via rivers and then released into the oceans. Recently, a few studies have attempted to estimate the plastic waste emissions from land 9,11. Jambeck et al. 9 , for instance, estimated that 4.8 to 12.7 million tonnes of mismanaged plastic waste could be entering the oceans from 192 coastal countries in 2010 by considering the waste management level in each country and the coastal population. In their estimate, 0.02 to 0.06 million MT of plastic waste can enter the Pacific Ocean from Japan. Moreover, Lebreton et al. 11 estimated that between 1.15 and 2.41 million tonnes ...