The duration of the journey that an individual grain of sand takes as it bounces its way down the length of a river is incredibly difficult, if not impossible, to determine. Finding the answer to that question could be useful because the amount of time that sediment spends in a river can affect how pollutants or other materials move through the system. At the bulk scale, sediment balance measurements, comparing the inputs and outflows of sediment, can be used to help calculate how much sediment is moving through a waterway. Figuring out the storage time of particular grains is more difficult because in meandering rivers individual grains of sediment deposited along the banks may be subsequently eroded years or even millennia later—whenever the river meanders back to where the sediment was deposited. Being able to calculate the total trip times of particular bits of sediment means being able to properly model this long-term deposition-storage mechanism.