Fishes inhabiting streams and rivers in the interior of North America experience a continental climate. Water temperatures reach 0 8C in winter and are high in summer. There is a marked seasonal cycle in discharge. These circumstances make groundwater a crucial component of river habitats. Groundwater can in¯uence the distribution, reproductive success, biomass and productivity, behaviour and movements of ®shes, and is especially important in winter and summer. Winter ¯ows are minimal and are aected by ice. In winter, the importance of groundwater increases northwards. Groundwater provides overwintering habitat free of subsurface ice and ®sh may migrate long distances to take advantage of it. The melt season can account for up to half the annual discharge. In summer, groundwater is important for maintaining discharge and moderating stream temperatures. During critically hot weather, groundwater refugia protect species exposed to temperatures approaching their thermal limits. Since groundwater exerts such an important in¯uence on river habitats, its quality, quantity and sustainability should be considered before development proposals are approved which could alter it. Examples of the role of groundwater in the ecology of some species show how localised and critical habitats in¯uenced by groundwater can be, and, in consequence, how necessary it is to protect them. Protection is complicated because groundwater distribution pathways are often unknown and recharge areas may be remote from discharges. Scale becomes important in identifying potential risks to critical stream habitats from all types of landscape modi®cation and water abstraction. Groundwater temperatures re¯ect mean annual air temperatures and are likely to change as global climates respond to increases in the greenhouse gases in the atmosphere. This could profoundly change critical ®sh habitats, particularly those at the margins of species distributions or those that are already overcrowded. Such considerations emphasise the importance of developing proper strategies for the conservation of groundwater.