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flows through unconfined aquifers at rates determined by the porosity of the substrata and the slope of the floodplain, and eventually upwells to the surface some distance downslope. Location of aquifer discharge is often related to bedrock outcrops or encroaching canyon walls (knickpoints in Figure 2). Effluent groundwaters may enter the channel directly or emerge as floodplain springbrooks that exhibit seasonally dynamic hydrology controlled by flow entering the floodplain from the river and from tributaries. These springbrooks usually occur in abandoned meander channels blocked at the upstream end by natural deposition of alluvium and woody debris. They have been referred to as wall-base channels in locations where they erupt from the substratum of old channels originally constrained by contact with the terrace or canyon walls (Peterson and Reid 1984). However, variations on this general theme may occur, depending on floodplain geomorphology and catchment hydrology (Amoros et al. 1982). Since spates frequently may overflow these springbrooks (in the Flathead River, Montana, these systems are flooded on about a 10 year return frequency, J. Stanford et al. unpublished), woody debris often accumulates, providing structurally complex lotic habitat. Moreover, relative to the main river channel, these springbrooks are characterized by relatively stable flows, moderated temperature regimes, high water clarity and elevated concentrations of plant growth nutrients, particularly nitrate and soluble reactive phosphorus. As a result, standing crops of attached algae and zoobenthos can exceed biomass in the channel by several orders of magnitude. Juveniles of native cutthroat trout (Oncorhynchus clarkii) are abundant (J. Stanford et al. unpublished). Therefore, it appears that these springbrooks are "hot spots" of bioproduction, although this relation has yet to be thoroughly documented. Wall-base streams are known to be critically important as spawning and rearing habitats for salmonids in Pacific Northwest streams (Peterson and Reid 1984); and, recent analyses suggest that aggraded floodplains and upwelling groundwaters historically were key production areas for anadromous salmonids (Oncorhynchus spp.) and resident bull charr (Salvelinus confluentus) in the Columbia River system (James Sedell, U. S. Forest Service, personal communication). In the Flathead River, Montana, native bull charr adults migrate upstream from Flathead Lake to spawn in specific habitats of 4th order tributaries (Figure 3, see also Fraley and Shepard 1989). Juveniles remain in riverine habitats for 2 or 3 years before migrating downstream to Flathead Lake where they mature. This phenology is termed adfluvial. Primary bull charr spawning sites are the groundwater upwelling zones of aggraded floodplain segments, which usually occur downstream from major attitudinal transitions (knickpoints) in the river continuum. Bull charr select only 4th order streams that are 6