Log jams alter gradients in hydraulic head, increase the area available for hyporheic exchange by creating backwater areas, and lead to the formation of multiple channel branches and bars that drive additional exchange. Here, we numerically simulated stream‐groundwater interactions for two constructed flume systems—one without jams and one with a series of three jams—to quantify the effects of interacting jam structures and channel branches on hyporheic exchange at three stream flow rates. In simulations without jams, average hyporheic exchange rates ranged from 2.1 × 10−4 to 2.9 × 10−4 m/s for various stream discharge scenarios, but with jams, exchange rates increased to a range of 1.3 × 10−3–3.5 × 10−3 m/s. Largely due to these increased hyporheic exchange rates, jams increased stream‐groundwater connectivity or decreased the turnover length that stream water travels before interacting with the hyporheic zone, by an order of magnitude, and drove long flow paths that connected multiple jams and channel threads. Decreased turnover lengths corresponded with greater reaction significance per km, a measure of the potential for the hyporheic zone to influence stream water chemistry. For low‐flow conditions, log jams increased reaction significance per km five‐fold, from 0.07 to 0.35. Jams with larger volumes led to longer hyporheic residence times and path lengths that exhibited multiple scales of exchange. Additionally, the longest flow paths connecting multiple jams occurred in the reach with multiple channel branches. These findings suggest that large gains in hydrologic connectivity can be achieved by promoting in‐stream wood accumulation and the natural formation of both jams and branching channels.

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Large wood is inherently mobile in naturally functioning river corridors, yet river management commonly introduces wood that is anchored to limit hazards. Wood that is periodically mobilized is important for: replacing stationary large wood that performs diverse physical and ecological functions; contributing to the disturbance regime of the river corridor; diversifying wood decay states; dispersing organisms and propagules; providing refugia during floodplain inundation and in mobile‐bed channels; dissipating flow energy; and supplying wood to downstream environments including lakes, coastlines, the open ocean, and the deep sea. We briefly review what is known about large wood mobility in river corridors and suggest priorities for ongoing research and river management, including: structural designs that can pass mobile wood; enhancing piece diversity of introduced wood that is anchored in place; quantifying wood mobilization and transport characteristics in natural and managed river corridors; and enhancing documentation of the benefits of wood mobility.

We measured coarse particulate organic matter (CPOM) transport along a wood-rich, pool-riffle mountain stream in the Southern Rockies of Colorado, USA, to examine how spatial variations in storage features and temporal variations in discharge influence the transport of CPOM. Ecologists have found that the majority of annual CPOM export occurs during periods of high discharge. More recently, geomorphologists have begun to examine the transport of CPOM as bedload. There has been, however, little direct sampling of CPOM to evaluate how shorter (diurnal) and longer (seasonal peak flow) variations in discharge affect CPOM transport, and no examination of where CPOM is transported in the water column (primarily in suspension or as bedload). We collected CPOM moving as bedload, in suspension (at 0.6 of the flow depth) and at the surface to evaluate CPOM transport processes. Samples were collected at three sites: (1) in the backwater pool upstream from a channel-spanning logjam; (2) immediately downstream from the logjam; and (3) in a riffle about 10 bankfull-channel-widths downstream from any channel-spanning logjams. During sample sets, we collected samples over 15-min increments at approximately 4-hr intervals over a 24-hr period. Seven sample sets were distributed over a period of 2 months that spanned the rise, peak, and recession of the annual snowmelt flood.We found that the majority of CPOM is transported in suspension following a clockwise hysteresis loop in which CPOM peaks prior to discharge during the seasonal hydrograph.

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