Daniel N. Scott

Wood and sediment storage and dynamics in river corridors

Large wood along rivers influences entrainment, transport, and storage of mineral sediment and particulate organic matter. We review how wood alters sediment dynamics and explore patterns among volumes of in-stream wood, sediment storage, and residual pools for dispersed pieces of wood, logjams, and beaver dams. We hypothesized that: volume of sediment per unit area of channel stored in association with wood is inversely proportional to drainage area; the form of sediment storage changes downstream; sediment storage correlates with wood load; the residual volume of pools created in association with wood correlates inversely with drainage area; and volume of sediment stored behind beaver dams correlates with pond area. Lack of data from larger drainage areas limits tests of these hypotheses, but the analyses suggest that sediment volume correlates positively with drainage area and wood volume. The form of sediment storage in relation to wood appears to change downstream, with wedges of sediment upstream from jammed steps most prevalent in small, steep channels andmore dispersed sediment storage in lower gradient channels. Pool volume correlates positively with wood volume and negatively with channel gradient. Sediment volume correlates well with beaver pond area. More abundant in-stream wood and beaver populations present historically equated to greater sediment storage within river corridors and greater residual pool volume. One implication of these changes is that protecting and re-introducing wood and beavers can be used to restore rivers. This review of the existing literature on wood and sediment dynamics highlights the lack of studies on larger rivers. Copyright

Transience of channel head locations following disturbance

We used annual re-surveys of two populations of channel heads affected by a severe wildfire in 2012 to monitor changes in channel head location with time following disturbance. Relative to channel heads in surrounding unburned areas, the median contributing drainage area of burned channel heads decreased by two orders of magnitude immediately after the fire, but then returned to values comparable to unburned areas within four years. We distinguish three types of channel heads. Permanent channel heads, which constitute 4% of the total population, occur in well-developed swales in association with stable features such as bedrock outcrops: these channel heads appear to have been unaffected by the fire. Persistent channel heads, which are 40% of the total population, also occur within hillslope concavities, but the exact location of the channel head moves upslope and downslope through time in response to varying inputs of water and sediment. Transient channel heads form on straight and convex slopes immediately following disturbance, but disappear as regrowth of ground cover limits overland flow and sediment movement. The majority of the position changes for persistent and transient channel heads occurred abruptly when viewed as annual time steps. Copyright

Evaluating carbon storage on subalpine lake deltas: Evaluating carbon storage on subalpine lake deltas

Mountainous regions are important contributors to the terrestrial organic carbon (OC) sink that affect global climate through the regulation of carbon-based greenhouse gases. However, mountain OC dynamics are poorly quantified. We quantified OC storage in subalpine lake deltas in the Washington Central Cascades and Colorado Front Range with the objectives of determining the magnitude of transient carbon storage and understanding the differences in storage between the two ranges. We used field, laboratory, and GIS techniques to determine the magnitude of and controls on the subalpine lake delta OC pool in 26 subalpine lake deltas. Soil moisture, soil texture, mean basin slope, and delta valley confinement are significantly correlated with soil carbon on deltas. Average soil OC concentration on subalpine lake deltas ranges from 3 to 41%, and stocks range from 140 to 1256 Mg C/ha. Surprisingly, the carbon content of subalpine lake deltas is not significantly different between the two regions, despite stark contrasts in their climate, vegetation, and total ecosystem carbon stocks. We present a conceptual model that invokes geomorphic and biogeochemical processes to suggest that carbon is more likely to reach subalpine lake deltas from the upstream basin in the Colorado Front Range compared with the Washington Central Cascades, thus accounting for the similarity in OC storage between the two regions despite differences in total ecosystem carbon stocks and climate. This points to a complex interaction among carbon production, transport, and stability in each region, and supports the idea that geomorphic and biogeochemical processes determine the magnitude of transient OC storage more strongly than primary productivity or climate. Copyright

River beads as a conceptual framework for building carbon storage and resilience to extreme climate events into river management

River beads refer to retention zones within a river network that typically occur within wider, lower gradient segments of the river valley. In lowland, floodplain rivers that have been channelized and leveed, beads can also be segments of the river in which engineering has not reduced lateral channel mobility and channel-floodplain connectivity. Decades of channel engineering and flow regulation have reduced the spatial heterogeneity and associated ecosystem functions of beads occurring throughout river networks from headwaters to large, lowland rivers. We discuss the processes that create and maintain spatial heterogeneity within river beads, including examples of beads along mountain streams of the Southern Rockies in which large wood and beaver dams are primary drivers of heterogeneity. We illustrate how spatial heterogeneity of channels and floodplains within beads facilitates storage of organic carbon; retention of water, solutes, sediment, and particulate organic matter; nutrient uptake; biomass and biodiversity; and resilience to disturbance. We conclude by discussing the implications of river beads for understanding solute and particulate organic matter dynamics within river networks and the implications for river management. We also highlight gaps in current understanding of river form and function related to river beads. River beads provide an example of how geomorphic understanding of river corridor form and process can be used to restore retention and resilience within human-altered river networks.

Natural and Anthropogenic Controls on Wood Loads in River Corridors of the Rocky, Cascade, and Olympic Mountains, USA

Wood in rivers creates habitat, shapes the morphology of valley bottoms, and acts as a pool of organic carbon (OC). Effective riverine wood management depends on a robust understanding of the spatial distribution of wood throughout river networks. This motivates the analysis of wood load in relation to both reachand basin-scale processes. We present wood load data coupled with precipitation, forest stand characteristic, land use, and geomorphic data across four basins in the Rocky, Cascade, and Olympic Mountains of the western U.S. We compare basins with differing land use within the same climatic region and basins in differing climates and statistically model intrabasin wood load variability. Wood load is a function of metrics that generally describe river corridor spatial heterogeneity, metrics that describe wood storage patterns, and, at a broader scale, metrics that relate to wood supply. From this, we generate a conceptual model to describe controls on wood load across spatial scales. We use this model to propose that spatial heterogeneity and wood storage pattern together determine reach-scale wood trapping efficiency. Trapping efficiency in turn regulates how wood supply to valley bottoms determines wood load. We also find that wood in an undisturbed basin stores significant amounts of OC and that wood load restoration has the potential to restore significant amounts of OC to valley bottoms. This conceptual model of wood load controls may serve as a framework to guide wood load modeling and restoration at multiple scales. Plain Language Summary Downed wood in rivers creates habitat and nutrients for organisms in streams and on floodplains. Humans have negatively impacted valley bottoms through the removal of downed wood. Wemeasured the amount of downed wood in valley bottoms in four mountain river basins to understand what factors, both local and regional, determine how much wood is stored in river corridors. We found that at the regional scale, logging, precipitation, and forest characteristics control the supply of wood to valley bottoms. At a more local scale, the shape of the valley bottom and the way in which wood is stored (either as accumulations known as jams or as individual logs) determine how much wood can be trapped in the valley bottom. We present a conceptual model that ties these factors together and can guide our understanding and management of how much wood is in rivers.

Bedrock fracture influences on geomorphic process and form across process domains and scales: Bedrock Fracture Influences on Geomorphology

Fractures are discontinuities in rock that can be exploited by erosion. Fractures regulate cohesion, profoundly affecting the rate, style, and location of Earth surface processes. By modulating the spatial distribution of erodibility, fractures can focus erosion and set the shape of features from scales of fluvial bedforms to entire landscapes. Although early investigation focused on fractures as features that influence the orientation and location of landforms, recent work has started to discern the mechanisms by which fractures influence the erodibility of bedrock. As numerical modeling and field measurement techniques improve, it is rapidly becoming feasible to determine how fractures influence geomorphic processes, as opposed to when or where. However, progress is hampered by a lack of research coordination across scales and process domains. We review studies from hillslope, glacial, fluvial, and coastal domains from the scale of reaches and outcrops to entire landscapes. We then synthesize this work to highlight similarities across domains and scales and suggest knowledge gaps, opportunities, and methodological challenges that need to be solved. By integrating knowledge across domains and scales, we present a more holistic conceptualization of fracture influences on geomorphic processes. This conceptualization enables a more unified framework for future investigation into fracture influences on Earth surface dynamics.