POOL SPACING IN FOREST CHANNELS

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 ch annels systematically decreases from greater than 13 channel widths pe r pool to less than 1 channel width with increasing LWD loading, where as pool spacing in generally steeper, step-pool channels is independen t of LWD loading. Although plane-bed and pool-riffle channels occur at similar low LWD loading, they exhibit typical pool spacings of greate r than 9 and 2-4 channel widths, respectively. Forced pool-riffle chan nels have high LWD loading, typical pool spacing of <2 channel widths, and slopes that overlap the ranges of free-formed pool-riffle and pla ne-bed channel types. While a forced pool-riffle morphology may mask e ither of these low-LWD-loading morphologies, channel slope provides an indicator of probable morphologic response to wood loss in forced poo l-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 strongl y influences pool spacing in forest streams with similar debris loadin g and that reaches flowing through previously clear-cut forests have l ower LWD loading and hence fewer pools than reaches in pristine forest s.