The SIMPLER project aims to develop and validate new Earth Observation (EO) approaches to quantify input of marine plastic debris from rivers into coastal waters, using the medium-sized river system in Danang, Vietnam, as a pilot case study. Our key aim is development of EO-based algorithms to quantify microplastic flux rates from the river. Rivers flowing into oceans have been identified as one of the largest land-based plastic debris inputs to marine plastic pollution budgets, with Southeast Asian rivers contributing an estimated two thirds input to the annual global marine plastic debris budget. To date, EO detection algorithms have focused primarily on floating macroplastic detection in optically simple water bodies, such as the open ocean and relatively clear coastal waters. River mouths, with their often highly turbid and optically-complex waters, have long proven difficult for remote sensing detection of water constituent concentrations. EO detection of microplastics poses additional challenges, including: 1) microplastic concentrations often being not sufficiently high to change the water surface optical reflectance signal, and 2) strong absorption of infrared light, required for positive identification of hydrocarbons, within the water's surface. These limitations suggest that the only potential successful path to an EO algorithm for river mouth microplastic quantification will be dependent on using a proxy water surface signal for detection. Theoretical basis for intended flux quantification algorithms will be presented, and current state-of-the-art detection approaches discussed, including required assumptions and expected methodological limitations. As part of project activities, local stakeholders will be equipped with simplified techniques for plastic pollution measurement and reporting. Data generated by local monitoring agencies can be used for satellite validation into the future, thus contributing to study sustainability beyond the scope of the current project. Algorithms will be designed with an eye towards scalability to other Southeast Asian river systems.