Young-of-the-year (age-0) brook troutSalvelinus fontinalisthat were spawned in a lake were observed migrating into and inhabiting small (<2-m-wide) inlet streams. We estimated that up to 81% of the entire age-0 population in the lake system inhabited the streams by midsummer. Some age-0 fish overwintered in the streams and remained there during their second summer. Such behavior suggested fitness advantages were gained by stream residence. The inhabited streams had accessible, cold, and stable habitats that existed because the streams occurred in forested catchments where groundwater dominated base flows. This identification of these small streams as habitat for young brook trout and the importance of catchment hydrology for sustaining these habitats indicated a need to review habitat protection guidelines for lake-dwelling populations.