/ Classification of streams and stream habitats is useful for research involving establishment of monitoring stations, determination of local impacts of land-use practices, generalization from site-specific data, and assessment of basin-wide, cumulative impacts of human activities on streams and their biota. This article presents a framework for a hierarchical classification system, entailing an organized view of spatial and temporal variation among and within stream systems. Stream habitat systems, defined and classified on several spatiotemporal scales, are associated with watershed geomorphic features and events. Variables selected for classification define relative long-term capacities of systems, not simply short-term states. Streams and their watershed environments are classified within the context of a regional biogeoclimatic landscape classification. The framework is a perspective that should allow more systematic interpretation and description of watershed-stream relationships.Managers of streams and their associated resources face problems of understanding and managing nonpoint source pollution, evaluating the complex, cumulative impacts of changing land use on stream habitats and biological communities, and assessing the effectiveness of fish habitat improvement projects and other mitigation procedures. Scientists have developed few generally applicable perspectives or procedures to address such needs. Present approaches to these problems typically involve paired watershed studies, long-term before-and-after monitoring programs, or upstream-downstream comparisons. Yet there exists no integrative, systematic approach for understanding the considerable natural variability within and among stream systems and stream communities (Hall and Knight 1981). How do we select representative or comparable sampling sites in such diverse environments? How can we interpret in a broader context, or how far can we reasonably extrapolate, information gathered at specific sites? How do we assess past and possible future states of a stream?This article articulates a general approach for classifying stream systems in the context of the watersheds that surround them. The stream classification

Large catchment basins may be viewed as ecosystems in which natural and cultural attributes interact. Contemporary river ecology emphasizes the four‐dimensional nature of the river continuum and the propensity for riverine biodiversity and bioproduction to be largely controlled by habitat maintenance processes, such as cut and fill alluviation mediated by catchment water yield. Stream regulation reduces annual flow amplitude, increases baseflow variation and changes temperature, mass transport and other important biophysical patterns and attributes. As a result, ecological connectivity between upstream and downstream reaches and between channels, ground waters and floodplains may be severed. Native biodiversity and bioproduction usually are reduced or changed and non‐native biota proliferate.  Regulated rivers regain normative attributes as distance from the dam increases and in relation to the mode of dam operation. Therefore, dam operations can be used to restructure altered temperature and flow regimes which, coupled with pollution abatement and management of non‐native biota, enables natural processes to restore damaged habitats along the river's course. The expectation is recovery of depressed populations of native species. The protocol requires: restoring peak flows needed to reconnect and periodically reconfigure channel and floodplain habitats; stabilizing baseflows to revitalize food‐webs in shallow water habitats; reconstituting seasonal temperature patterns (e.g. by construction of depth selective withdrawal systems on storage dams); maximizing dam passage to allow recovery of fish metapopulation structure; instituting a management belief system that relies upon natural habitat restoration and maintenance, as opposed to artificial propagation, installation of artificial instream structures (river engineering) and predator control; and, practising adaptive ecosystem management.  Our restoration protocol should be viewed as an hypothesis derived from the principles of river ecology. Although restoration to aboriginal state is not expected, nor necessarily desired, recovering some large portion of the lost capacity to sustain native biodiversity and bioproduction is possible by management for processes that maintain normative habitat conditions. The cost may be less than expected because the river can do most of the work.

– Warm stream temperatures may effectively limit the distribution and abundance of Pacific salmon Oncorhynchus spp. in streams. The role of cold thermal refugia created by upwelling groundwater in mediating this effect has been hypothesized but not quantitatively described. Between June 21 and September 15, 1994, rainbow trout O. mykiss abundance within 12 northeast Oregon (USA) stream reaches was inversely correlated with mean ambient maximum stream temperatures (r=−0.7, P<0.05). Some rainbow trout used thermal refugia (1–10 m2 surface area) that were on average 3–8°C colder than ambient stream temperatures. Within the warmest reaches, high ambient stream temperatures (>22°C) persisted from mid‐June through August, and on average 10–40% of rainbow trout were observed within thermal refugia during periods of midday maximum stream temperatures. Frequency of cold‐water patches within reaches was not significantly associated with rainbow trout density after accounting for the influence of ambient stream temperature (P=0.06; extra sum of squares F‐test). Given prolonged high ambient stream temperatures in some reaches, the thermal refugia available in the streams we examined may be too small and too infrequent to sustain high densities of rainbow trout. However, these refugia could allow some rainbow trout to persist, although at low densities, in warm stream reaches.