Escalating demands for sustainable water resources management, anthropogenic disturbances (e.g. channelisation and impoundment) and changing environmental conditions (for example floods and droughts) has led to an increased need to understand the influence of 'flow variability' on in-stream ecological communities. In this thesis, the importance of hydrological variability in structuring macroinvertebrate communities is explored at a range of spatial and temporal scales for rivers across England and Wales. At the reach scale (individual river reach), the influence of flow velocity variability on the seasonal distribution of benthic macroinvertebrate communities is examined. At the mesoscale (regional), hydrological regime variability and macroinvertebrate community data (species- and family-level) for fourteen rivers (all located within the Environment Agency, Anglian northern region) are examined over an eleven-year period (1990-2000). At the macroscale (national), the hydrological regime and family-level macroinvertebrate community data for 83 rivers across England and Wales are explored for an eleven-year period (1990-2000) to identify macroscale ecological responses using a range of 'ecologically-relevant' hydrological variables (up to 201 indices).