In research with the aim of understanding where clay and other particles in rivers ultimately settle, laboratory experiments reveal surprisingly complex processes whereby microscopic interactions have macroscopic influence upon the flow. When clay is well-mixed with salt water, the particles flocculate and form a settling front that descends hundreds of times faster than an individual particle. However, in experiments with particles that precipitate out of solution, convective circulations can develop that slow their net descent. The situation is more complicated when the particles move within an advancing buoyant current passing over a denser ambient fluid (as for a river flowing over saline ocean water). Particles are found to act collectively, forming plumes that rapidly descend from the base of the current. These dynamics may have particularly important consequences for the fate of microplastics released in rivers, suggesting that a significant fraction of the plastics settle in estuaries, deltas and the coastal ocean.