Wood controls on pool spacing, step characteristics and sediment storage in headwater streams of the northwestern Cascade Mountains

Abstract

Wood influences channel morphology in headwater streams by creating steps and pools in the longitudinal profile and trapping sediment. We used field data from 32 headwater stream reaches in the northwestern Cascade Mountains to test the hypotheses that 1) larger diameter classes of large wood (LW, defined as pieces with diameter>10cm and length>0.5m) are critical step-keying materials in headwater channels despite narrow channel widths (1–4m), 2) wood-keyed steps trap more sediment than clast- and root-keyed steps, and 3) the negative relationship between LW frequency and the distance between pools observed elsewhere in large streams extends to headwater streams. We found that the frequency of step key pieces peaked in the 20–40cm diameter class. Similarly, 40–100cm diameter pieces were disproportionately associated with key-piece function compared to their overall association with step formation (wood with diameter<10cm was an important step-keying material in channels of width<2m). Steps keyed by wood were significantly more likely to store sediment than clast- or root-keyed steps. In contrast to previous work that did not detect a relationship between wood loading and pool habitat in step-pool channels, pool spacing ranged between ∼17 and 1.5 channel widths with an apparent relationship described by a negative exponential function of LW frequency (RMSE=2.85 channel widths/pool), although the range of LW frequency and the functional relationship were different than those of lower gradient channels. Additionally, linear and nonparametric models demonstrate that sediment storage and step and pool characteristics are related to wood loading and stand history with important distinctions based on channel width. These results confirm the importance of maintaining riparian buffers sufficient to provide functional wood to headwater streams.