Nano-colloids (1-100 nm) and small colloids (<450 nm) are ubiquitous and a critical component of environmental processes influencing the fate and transport of solutes through watersheds. This includes solutes beneficial for ecosystems (nutrients and organic carbon) and harmful contaminants (metals and organic pollutants). With their small size and high specific surface area, nano-colloids have the ability to redistribute solutes throughout a watershed and among ecosystems. Much of our understanding of nano-colloids and small colloids comes from bench-scale system studies. These systems are generally less complex than natural systems, and because of this gaps remain in our understanding. This multidisciplinary session encourages discussions on the occurrence and properties of nano-colloid and small colloids throughout watersheds. This includes, but not limited to topics such as: ecosystem functions, trace metal cycling, nutrient and carbon cycling, pore-scale processes, distribution and transport in watersheds, adsorption and desorption, biomineralization, geochemical characterization, and theoretical and empirical models.