The sources of uncertainty or error that arise in attempting to scale up the results from laboratory sediment transport studies include model imperfection,omission of important processes,lack of knowledge of initial conditions,sensitivity to initial conditions,unresolved heterogeneity and occurrence of external force.The sources of uncertainty that are unimportant or can be controlled on a small scale and over a short time may become important in the application on a large scale and over a long time.Control and repeatability,hallmarks of laboratory experiments,usually lack the large scale characteristic of large systems.Heterogeneity is an important concomitant of size,and tends to make large systems unique.Uniqueness implies that prediction cannot be based upon first-principles quantitative modeling alone,but must be a function of system history as well.In large systems,the construction of successful predictive models is likely to be based upon the discovery of emergent variables and a corresponding dynamics,rather than upon scaling up the results of well-controlled laboratory studies.