Pool Spacing in Forest Channels

Abstract

Field surveys of stream channels in forested mountain drainage basins in southeast Alaska and Washington reveal that pool spacing depends on large woody debris (LWD) loading and channel type, slope, and width. Mean pool spacing in pool‐riffle, plane‐bed, and forced pool‐riffle channels systematically decreases from greater than 13 channel widths per pool to less than 1 channel width with increasing LWD loading, whereas pool spacing in generally steeper, step‐pool channels is independent of LWD loading. Although plane‐bed and pool‐riffle channels occur at similar low LWD loading, they exhibit typical pool spacings of greater than 9 and 2–4 channel widths, respectively. Forced pool‐riffle channels have high LWD loading, typical pool spacing of <2 channel widths, and slopes that overlap the ranges of free‐formed pool‐riffle and plane‐bed channel types. While a forced pool‐riffle morphology may mask either of these low‐LWD‐loading morphologies, channel slope provides an indicator of probable morphologic response to wood loss in forced pool‐riffle reaches. At all study sites, less than 40% of the LWD pieces force the formation of a pool. We also find that channel width strongly influences pool spacing in forest streams with similar debris loading and that reaches flowing through previously clear‐cut forests have lower LWD loading and hence fewer pools than reaches in pristine forests.