Our understanding of spatial and temporal dynamics of the distribution of the hyporheos can be considerably enhanced by an appreciation of surface-hyporheic hydrologic exchange. We illustrate this relationship using two case studies from very different lotic ecosystems: a large mesic river in France and an intermittent desert stream in Arizona. In both cases, there are strong correlations between direction and magnitude of hydrologic fluxes and the composition of hyporheic invertebrate assemblages. Hydrologists are likely to find that the distribution and abundance of hyporheos will provide useful corroborative data in studies of hyporheic flow dynamics, while ecologists should recognize the effects of hydrologic exchange between the surface stream and the hyporheic zone at a variety of scales. Thus hydrologic measurements of surface water and groundwater exchange should be included routinely in future studies of hyporheic invertebrate ecology. Such exchange controls the availability of nutrients, food, a...