Woody debris and channel morphology in first‐ and second‐order forested channels in Washington's coast ranges

While there are conceptual and theoretical reasons to believe small streams behave differently than larger streams, the lack of information on small streams has lead land managers to rely on data from larger streams to guide management decisions. In response to the need for descriptive information on habitat and channel morphology specific to small, non‐fish‐bearing streams in the Pacific Northwest, morphologies and wood frequencies in 42 first‐ and second‐order forested streams <4 m wide were surveyed. Frequencies and size distributions of woody debris were compared between small streams and larger fish‐bearing streams as well as between second‐growth and virgin timber streams. Statistical models were developed to explore dominant factors affecting channel morphology and habitat. Findings suggest geomorphological relationships, specifically the role of woody debris in habitat formation, documented for larger streams do not apply to headwater streams. Relatively small wood (diameters between 10 and 40 cm), inorganic material, and organic debris (diameters <10 cm) were major step‐forming agents while big woody debris pieces (>40 cm diameter) created <10% of steps. Streams in virgin and managed stands did not differ in relative importance of very large woody debris. Because of low fluvial power, pool habitat was rare. These streams featured mostly step‐riffle morphology, not step‐pool, indicating insufficient both flow for pool‐scour. Stream power and unit stream power were dominant channel shaping factors.