Plant development in unshaded streams is reviewed within the context of a conceptual habitat model. Resource factors (incorporating light, nutrients and temperature) predominantly govern the processes of biomass gain and hydraulic factors (incorporating separate temporal and spatial scales) predominantly govern the processes of biomass loss. It is postulated that differences in hydraulic stability among streams over periods greater than a year govern whether a stream (or site) is dominated by periphyton, bryophytes or macrophytes. Hydraulic stability over periods of less than a year governs the average periphyton biomass. During steady-state, interspate periods the effect of spatial differences in hydraulics varies depending on the stage of plant development. Periphyton and macrophyte colonization is enhanced by low velocities, and growth rate and organic matter accrual can be enhanced by moderate velocities. However, high velocities retard colonization and organic matter accrual. For mature communities, the peak biomass of periphyton and macrophytes can be negatively correlated with velocity. This is in contrast with bryophytes, which are often restricted to areas of high velocity on stable substrata. Hydraulic habitat suitability curves have yet to be developed for plant communities in streams.