Transient storage is mediated by watershed timber harvest in Southeast Alaska streams

Stream transient storage reflects a combination of hydrologic and geomorphologic characteristics including large wood volume, spatial heterogeneity, and pool-riffle sequences. Riparian timber harvest has been shown to decrease large wood recruitment, reduce benthic sediment size, and modify discharge, which should in turn influence transient storage metrics, but no previous study has investigated the influence of timber harvest explicitly. We quantified transient storage in 8 southeast Alaskan streams draining watersheds along a timber harvest gradient (6-70% watershed area harvested). Multiple conservative tracer releases were conducted at each stream at discharges ranging from 250-2000 L/s to determine if transient storage varies in relation to hydrologic conditions.  We conducted short-term rhodamine releases and quantified transient storage metrics using an advection-dispersion model. Results suggest that pristine streams have more dynamic and extensive transient storage, A_s:A ranged from 0.25-0.88 at varying discharges, representing a larger range than in harvested streams (A_s:A=0.20-0.25). Further, the importance of transient storage in stream dynamics was significantly larger in pristine (F_med²⁰⁰=0.049-0.079) compared to harvested systems (F_med²⁰⁰=0.021-0.058). Our study of larger streams examined under varying discharge regimes across a timber harvest gradient provides a novel opportunity to determine the underlying drivers controlling variation in transient storage metrics within and among streams.