The Mackenzie River Delta (MRD) has been recognized as an important host of river‐derived wood deposits, and Mackenzie River wood has been found across the Arctic Ocean. Nevertheless, we lack ...estimates of the amount of carbon stored as wood and its age in the delta, representing a gap in carbon cycle estimates. Here, we use very high‐resolution satellite imagery and deep learning to map wood deposits in the MRD, combining this with field data to measure the stock and age of wood‐based carbon. We find >400,000 individual large wood deposits, collectively storing 3.1 × 1012 g‐C, equating to 2 × 106 g‐C ha−1 across the delta. Sampled wood pieces date from 690 AD to 2015 AD but are mostly young with ∼40% of the wood samples formed after 1955 AD. These estimates represent a minimum bound on an important surficial, potentially reactive, carbon pool compared to other deeper carbon stocks in permafrost zones.
Plain Language Summary
The Arctic is warming rapidly, and this can increase landscape erosion. Consequently, carbon can be transferred into rivers and transported toward the Arctic Ocean. To date, work has tracked finer material dissolved in river water and the size of sand grains but has missed large pieces of wood that fall or slide into rivers. Wood is an important carbon‐rich material that may break down differently from the finer carbon pools, however, the amount of wood stored in Arctic river deltas has not been measured before. We also don't know the age of wood in those deposits. Here we study the Mackenzie River Delta, where river‐sourced wood is common and deposits extensive. We use very high‐resolution satellite images which show individual pieces of wood and use a machine learning technique to map wood across the delta. We also visited the deposits to measure their size and collected samples for radiocarbon dating. We find the wood is very young compared to other carbon pools carried by the river, and that the stocks of carbon are regionally important. Our work calls for further work to understand this overlooked carbon pool in river deltas and coastal regions of the Arctic.
Key Points
We use remote sensing and deep learning to measure wood‐based carbon in an Arctic delta, filling an important gap in carbon cycle estimates
The Mackenzie River Delta stores over 400,000 wood deposits in the delta totaling 3.1 × 1012 g‐C
Measured wood ages estimated from radiocarbon are younger than other river carbon pools, with ∼40% of the samples still growing after 1955 AD
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Legacy effects on river sediments are those which alter the location and volume of sediments and/or presence of contaminants within the sediments as a result of human activities. This review broadens ...the typical definition of legacy sediments and discusses human activities that create legacy effects on sediments in terms of three categories: activities that reduce sedimentation within river corridors — defined as channels, floodplains and riparian zones, and hyporheic zones; activities that enhance sedimentation in river corridors; and activities that contaminate river sediments with diverse pollutants. People reduce sedimentation within river corridors via three basic mechanisms: reducing sediment supply to the river corridor through changes in land cover or by trapping and storing sediment within the river corridor; increasing the ability of a river to transport sediments downstream by either increasing water supply to the channel or reducing physical channel complexities that promote flow separation, hydraulic resistance, and sedimentation; and disconnecting channels from floodplains, which are typically sediment storage zones and sediment sources for the channel. Conversely, enhanced sedimentation within river corridors results from increased sediment supply from uplands or upstream river segments and from decreased ability of a river to transport sediments downstream. Contamination of river sediments can result from nearly every conceivable human activity within a drainage basin (e.g., deforestation, agriculture, urbanization, industrial facilities, wastewater treatment). Contaminants can also enter a drainage basin from sources outside the basin boundaries because of contaminant transport within the tissues of migratory animals and via atmospheric deposition. Because many contaminants travel adsorbed to sediments, these pollutants can be concentrated within river corridors by activities that enhance sediment deposition. Legacy effects on sediments are now ubiquitous and abundant within river corridors around the world and can continue to alter river form and function long after cessation of the human activity that created the legacy. River management must be informed by accurate knowledge of the distribution and characteristic of legacy effects on sediments and geoscientists can contribute specialized knowledge to understanding and managing these sediments.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Planform geometry, spatial heterogeneity, and large wood abundance and distribution were characterized using combined remote imagery and field surveys along lengths of 20–28 km in four river ...corridors (channels and floodplains) in northwestern Montana. Study sites included four planform geometries: meandering, straight, braided, and multichannel. Planform spatial heterogeneity of channels and floodplains, such as proportion of the active channel in bars, sinuosity, braiding index, and the number of active channels, differs in relation to channel planform type. Braided and multichannel rivers have significantly greater spatial heterogeneity of channels and floodplains and store significantly greater volumes of wood in the channel. Wood is preferentially stored in jams, and jams are preferentially stored in shallow areas of the active channel (midchannel bars, inner bends, and secondary channels) and in abandoned channels on the floodplain. We interpret these results using a conceptual model in which boundary conditions create sufficient valley‐bottom width for the development of planform spatial heterogeneity, which then promotes storage of large wood. The results of this study can inform management that protects or restores spatial heterogeneity.
Key Points
Channel large woods differ by channel planform
Metrics of spatial heterogeneity differ by channel planform
Channel large wood loads correlate with metrics of spatial heterogeneity
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Floodplains perform diverse functions, including attenuation of fluxes of water, solutes, and particulate material. Critical details of floodplain storage including magnitude, duration, and spatial ...distribution are strongly influenced by floodplain biogeochemical processes and biotic communities. Floodplain storage of diverse materials can be conceptualized in the form of a budget that quantifies inputs, outputs, and storage within the floodplain control volume. The floodplain control volume is here defined as bounded on the inner edges by the banks of the active channel(s), on the outer edges by the limit of periodic flooding and the deposition of fluvially transported sediment, on the underside by the extent of hyporheic exchange flows and the floodplain aquifer, and on the upper side by the upper elevation of living vegetation. Fluxes within the floodplain control volume can also change the location, characteristics, and residence time of material in storage. Fluxes, residence time, and quantities of material stored in floodplains can be measured directly; inferred from diverse types of remotely sensed data; or quantitatively estimated using numerical models. Human activities can modify floodplain storage by: hydrologically and/or geomorphically disconnecting channels and floodplains; altering fluxes of water and sediment to the river corridor; and obliterating floodplains through alluvial mining or urbanization. Floodplain restoration can focus on enlarging the functional floodplain, reconnecting the channel and floodplain, restoring natural regimes of water, sediment, and/or large wood, or enhancing the spatial heterogeneity of the channel and floodplain. Each form of floodplain restoration can increase floodplain storage and resilience to disturbances.
Plain Language Summary
Floodplains perform diverse physical and ecological functions and these functions are highly interconnected. The chief physical function of floodplains is attenuation of fluxes of water, along with dissolved material, sediment, and organic matter transported by water. Although floodplain storage is a physical process, critical details of storage including magnitude, duration, and spatial distribution are strongly influenced by floodplain biogeochemical processes and biotic communities. Floodplain storage can be conceptualized in the form of a budget that quantifies inputs, outputs, and storage. Fluxes, residence time, and quantities of material stored in floodplains can be measured directly; inferred from diverse types of remotely sensed data; or quantitatively estimated using numerical models. Human activities can modify all of the factors influencing floodplain storage by hydrologically and/or geomorphically disconnecting channels and floodplains; by altering fluxes of water and sediment to the river corridor; and by obliterating floodplains through alluvial mining or urbanization. Floodplain restoration can focus on enlarging the functional floodplain, reconnecting the channel and floodplain, restoring natural regimes of water, sediment, and/or large wood, or enhancing the spatial heterogeneity of the channel and floodplain. Each of these forms of floodplain restoration can increase floodplain storage and resiliency to natural and human disturbances
Key Points
Storage of diverse materials constitutes a vital floodplain function
Characteristics of floodplain storage reflect floodplain size, connectivity, and spatial heterogeneity
Human activities can increase or decrease floodplain storage, but human alteration of floodplain storage is common
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Many of the conceptual models developed for river networks emphasize progressive downstream trends in morphology and processes. Such models can fall short in describing the longitudinal variability ...associated with low-order streams. A more thorough understanding of the influence of local variability of process and form in low-order stream channels is required to remotely and accurately predict channel geometry characteristics for management purposes, and in this context designating process domains is useful. We define process domains with respect to glacial versus fluvial valleys and lateral confinement of valley segments. We evaluated local variability of process domains in the Colorado Front Range by systematically following streams, categorizing them into stream morphologic type and process domain, and evaluating a number of channel geometry characteristics. We evaluated 111 stream reaches for significant differences in channel geometry among stream types and process domains, location and clustering of stream types on a slope–drainage area (S–A) plot and downstream hydraulic geometry relationships. Although individual channel geometry variables differed significantly between individual stream types in glacial and fluvial process domains, no single channel geometry variable consistently differentiated all stream types between process domains. Hypothetical S–A boundaries between bedrock- and alluvial-bed channels proposed in previous studies did not reliably divide bedrock and alluvial reaches for our study sites. Although downstream hydraulic geometry relationships are well-defined using all reaches in the study area, reaches in glacial valleys display much more variability in channel geometry characteristics than reaches in fluvial valleys, less pronounced downstream hydraulic geometry relationships, and greater scatter of reaches on an S–A plot. Local spatial variability associated with process domains at the reach scale (101–103m) overrides progressive downstream relationships in low-order mountain streams of the Colorado Front Range.
•Spatial differentiation of process domains exist within the river network.•Stream morphology differs significantly between glacial and fluvial process domains.•Streams below the glacial limit are more adjusted to water and sediment regimes.
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36.
Floodplains and wood Wohl, Ellen
Earth-science reviews,
08/2013, Volume:
123
Journal Article
Peer reviewed
Interactions between floodplains and wood date to the Carboniferous, when stable, multithread channel deposits appear with the evolution of tree-like plants. Foundational geologic texts, such as ...Lyell's, 1830Principles of Geology, describe floodplain–wood interactions, yet modern technical literature describes floodplain–wood interactions in detail for only a very limited range of environments. This likely reflects more than a century of deforestation, flow regulation, and channel engineering, including instream wood removal, which has resulted in severe wood depletion in most of the world's river networks.
Instream wood affects floodplain form and process by altering flow resistance, conveyance and channel–floodplain connectivity, and influencing lateral and vertical accretion of floodplains. Instream wood reflects floodplain form and process as the floodplain influences wood recruitment via bank erosion and overbank flow, and wood transport and storage via floodplain effects on stage-discharge relations and flow resistance. Examining turnover times for instream wood at the reach scale in the context of a wood budget, floodplain characteristics influence fluvial transport and dynamics (wood recruitment), valley geometry (wood transport and storage), and hydraulics and river biota (wood decay and breakage).
Accumulations of wood that vary from in situ jams and beaver dams in small channels to transport jams and log rafts in very large rivers can create stable, multithread channels and floodplain wetlands. Floodplain–wood interactions are best understood for a subset of small to medium-sized rivers in the temperate zone. We know little about these interactions on very large rivers, or on rivers in the tropical or boreal regions.
This review suggests that most, if not all, channels and floodplains within forested catchments in the temperate zone historically had much greater wood loads and consequently much more obvious and important influences from wood than do heavily modified contemporary catchments. For many rivers in the temperate zone, direct and indirect removal of instream wood very likely caused a fundamental shift in channel and floodplain process and form, as has been demonstrated in detail for specific rivers of diverse size in several regions. Failure to explicitly include floodplain–wood interactions creates a misleading conceptual model of floodplain dynamics in forested catchments.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
During the past 50 yr, the number and variety of papers written by U.S. fluvial geomorphologists that examine human alterations of rivers has accelerated substantially. From an initial focus ...primarily on how human-induced changes in land cover influence sediment yield and river dynamics, the literature has expanded to emphasize the effects of flow regulation, channel engineering, removal of large wood and beavers, and changing climate. These multiple human influences are now widely recognized to have resulted in global-scale cumulative effects including significantly altered fluxes of water, sediment, nitrogen, and carbon, and complete transformation of river networks across much of the planet. One outgrowth of this recognition is the increasing involvement of geomorphologists in diverse forms of river restoration, a form of river management that thus far has largely been dominated by engineers. Acknowledging the ubiquity of human alteration of rivers implies that (i) investigators cannot assume that even the most remote and seemingly pristine river segment has not been affected at least indirectly by people, (ii) the use of reference conditions requires careful consideration with respect to what reference sites indicate about past conditions, as well as their relevance for the future, (iii) detailed geomorphic understanding of the nature and timing of past human alterations of rivers is likely to be critical to effective restoration, and (iv) each scientist must decide how to engage within the context of research and advocacy with the issues of ecosystem degradation and loss of river form and function.
•Awareness of human alterations of rivers has grown during the past 50 years.•Human alterations of rivers are ubiquitous and diverse.•Investigators cannot assume that any river segment has not been affected by people.•Geomorphic understanding of past human alterations is critical to restoration.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Accumulations of wood in rivers can alter three-dimensional connectivity and facilitate channel bifurcations. Bifurcations divide the flow of water and sediment into secondary channels and are a key ...component of anastomosing rivers. While past studies illustrate the basic scenarios in which bifurcations can occur in anastomosing rivers, understanding of the mechanisms of bifurcations remains limited. We evaluate wood-induced bifurcations across thirteen anastomosing reaches in nine different streams and rivers in the U.S. Rocky Mountains to address conditions that favor different bifurcation types. We hypothesize that (1) wood-induced bifurcations exist as a continuum of different patterns in anastomosing rivers and (2) the position of a river segment along this continuum correlates with the ratio of erosive force to erosional resistance (F/R). We use field data to quantify F/R and compare varying F/R to bifurcation types across sites. Our results support these hypotheses and suggest that bifurcation types exist as a continuum based on F/R. At higher values of F/R, more channel avulsion is occurring and predominantly lateral bifurcations form. At lower values of F/R, banks are more resistant to erosive forces and wood-induced bifurcations are transitional or longitudinal with limited lateral extent. The relationship between F/R and bifurcation types is not linear, but it is progressive. Given the geomorphic and ecological functions associated with large wood and wood-induced channel bifurcations, it becomes important to understand the conditions under which wood accumulations can facilitate different types of bifurcations and the processes involved in these bifurcations. This understanding can inform river corridor restoration designed to enhance the formation of secondary channels, increase lateral and vertical connectivity, and promote an anastomosing planform.
River health can be defined as the degree to which riverine energy source, water quality, flow regime, habitat and biota match the natural conditions. In a healthy river, physical process and form ...remain actively connected and able-to mutually adjust, and biological communities have natural levels of diversity and are resilient to environmental stress. Both physical diversity and biodiversity influence river health. Physical diversity is governed by hydrology, hydraulics, and substrate, as reflected in the geometry of the river channel and adjacent floodplain, which create habitat for aquatic and riparian organisms. Biodiversity is governed by biological processes such as competition and predation, but biodiversity also reflects the diversity, abundance and stability of habitat, as well as connectivity. Connectivity within a river corridor includes longitudinal, lateral, and vertical dimensions. River health declines as any of these interacting components is compromised by human activities. The cumulative effect of dams and other human alterations of rivers has been primarily to directly reduce physical diversity and connectivity, which indirectly reduces biodiversity. Restoration and maintenance of physical diversity and biodiversity on rivers affected by dams requires quantifying relations between the driver variables of flow and sediment supply, and the response variables of habitat, connectivity, and biological communities. These relations can take the form of thresholds （e.g., entrainment of streambed sediment） or response curves （e.g., fish biomass versus extent and duration of floodplain inundation）. I use examples from Wyoming, Colorado, and Arizona in the western United States to illustrate how to quantify relations between driver and response variables on rivers affected by dams.
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