Like many species of amphibians, Boreal Toads (Bufo boreas boreas, Bufonidae) are declining throughout portions of their range. Recent efforts have focused on describing the ecology of this species, yet few studies have evaluated demographic characteristics that may influence the persistence of Boreal Toad populations. Because Boreal Toads often convey themselves down valleys via stream channels in some areas, we set upstream-facing hoop nets in early to late summer in several first- to third-order tributaries in two western Montana river basins to assess the sizes of individuals using streams and examine temporal and spatial variation in captures. We made 923 captures of juvenile and adult Boreal Toads. Adult females were up to 125 mm snout–vent length, whereas males never exceeded 105 mm. Females tended to be heavier than males and female weights were significantly more variable. Early-summer captures were dominated by juvenile toads <40 mm, late summer catches were largely of individuals >70 mm, and toads of intermediate size were rare throughout. In tributaries of one river basin, captures of toads were more widely distributed in late summer than in early summer, whereas in tributaries of the other basin catches were similarly distributed in both periods. We infer from these patterns that frequent and perhaps far-ranging movements by juveniles and adults are typical of Boreal Toads in this region. We contend that netting streams in summer represents a useful complement to breeding site surveys for understanding the demographics and distribution of Boreal Toads, and perhaps other non-breeding amphibians near streams.