Water pollution is a global challenge due to the variety of pollutants and their seasonality. Human activities on land emit pollutants such as nutrients, pathogens, and plastics. This cocktail of pollutants enters our rivers and lakes. However, another season may bring another cocktail of pollutants to rivers and lakes due to changes in human activities and climate. Rivers and lakes are connected through a network, in which rivers export pollutants from land to sea. Lakes can affect the river export by storing pollutants. Such processes are, however, affected by seasonality (e.g. lower nutrient retentions due to lower temperatures). Understanding the seasonal pollutant flows in rivers and lakes is imperative for policymakers to ensure clean water. Therefore, this research aims to better understand how seasonality in lake retentions affects river export of multiple pollutants worldwide today and in the future. I will do so by advancing two existing models (a flow analysis model in Step 1 and an ecological model in Step 2) and link them into a new model system (Step 3) to explore future trends in seasonal pollution flows (Step 4). My research will provide novel insights into seasonal multi-pollutant flows in rivers and lakes, and will aid policymakers in identifying management strategies to ensure clean water for ecosystems and society across seasons.