Stream ecologists have recently recognized that sediments below streams play an important role in lotic ecosystems. Water flows not only across the surface of stream channels, but also through sediment interstices; consequently, surface and subsurface biogeochemical processes are linked. Recent attempts to understand the influence of subsurface processes on stream ecosystems have tried to resolve the surface-subsurface hydrologic interactions, and to gain knowledge of the ecology of subsurface organisms.