Abstract Flow regulation is a major constraint on fish populations in many river systems. Current scientific understanding of the direct and indirect influence of hydrology on salmonids between spawning and emergence is reviewed and used to identify key knowledge gaps. The review emphasises difficulties in linking ecological response to physical process understanding and, conversely, the difficulties in applying detailed process understanding to large-scale management. It is suggested that the lack of process understanding in the development of current environmental standards reflects differences in perspective, language and scale consideration between researchers and river managers. This presents considerable difficulties for the development of new and more defensible environmental standards. Although upscaling of process understanding should remain a focus for researchers, it is also necessary to acknowledge the ‘uniqueness of place’ and consequently that local data collection may frequently be required.