Legacies are persistent changes in natural systems resulting from human activities. Legacies that affect river ecosystems can result from human alterations outside of the river corridor, such as ...timber harvest or urbanization, or from alterations within the river corridor, including flow regulation, river engineering, and removal of large wood and beaver dams. Human alterations of river ecosystems have been occurring for thousands of years in some parts of the world and are now ubiquitous, yet both river scientists and the public may be unaware of the persistent effects of historical activities. Failure to recognize the legacy of historical activities that no longer occur can skew perceptions of river process and form and the natural range of variability in river ecosystems. Examples come from rivers of the Mid‐Atlantic Piedmont and the Pacific Northwest regions of the United States. Mid‐Atlantic Piedmont streams in which legacy sediment accumulated behind now‐abandoned mill dams experienced a complete transformation from wide, shallow, marshy valleys to sinuous rivers lined with tall cutbanks, but the existence and the cause of this river metamorphosis was not widely recognized until the first decade of the 21st century. Rivers of the Pacific Northwest from which large wood was removed have changed during the past century from spatially heterogeneous, multichannel systems closely connected to their floodplains via frequent channel avulsion and lateral migration to single‐thread channels with more homogeneous floodplains and less lateral connectivity. Again, this river metamorphosis has only been recognized within the past two decades. In each of these regional examples, river process and form have changed so substantially that the river ecosystems can be described as having assumed an alternative state. In these and many other examples, the alternative state provides lower levels of ecosystem services such as habitat, biodiversity, and attenuation of downstream fluxes of water, sediment, organic carbon, and nutrients. River management designed to enhance and restore these ecosystem services can be more effective if the continuing effects of these historical legacies are recognized. The grand scientific challenges resulting from historical human alterations of river ecosystems include the following: (1) to recognize the existence of a legacy that continues to affect river ecosystem process and form; (2) to understand the source of the legacy with respect to chronology, type, spatial extent, and intensity of human activities; (3) to understand the implications of the legacy regarding how river process and form and river ecosystem services have changed; and (4) to design management or restoration strategies that can mitigate the loss of river ecosystem services. In summary, the existence of forgotten legacies challenges river scientists to recognize the continuing effects of human activities that have long since ceased and also poses challenges for the application of scientific understanding to resource management. Societal expectations for attractive, simple, stable rivers are commonly at odds with scientific understanding of rivers as dynamic, spatially heterogeneous, nonlinear ecosystems. Knowledge of how human actions, including historical actions that have long since ceased, continue to alter river ecosystems can help to bridge the gap between societal and scientific perceptions of rivers.
Key Points
Historical human activities have altered river ecosystems in ways no longer visible
Failure to recognize these alterations skews perception of river form and process
Effective river management and restoration requires close attention to historical legacies
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
The North Fork Poudre and Poudre Rivers have incised deep, narrow canyons into Precambrian‐age granite and gneiss along the Colorado Front Range. Longitudinally discontinuous active strath surfaces ...and strath terraces are present along each river. Although the crystalline bedrock of the study area is resistant to weathering and erosion, differences in joint spacing and orientation create significant differences in rock mass strength along each river. The vertical spacing of active straths along the North Fork Poudre correlates strongly with the spacing of prominent subhorizontal joint surfaces. Strath terraces along the Poudre River occur in areas of wider valley bottom upstream from a profile convexity. Especially wide segments with the greatest downstream extent reflect the locations of shear zones that create more densely spaced joints and lower rock resistance. I interpret the absence of strath terraces in wider canyon segments downstream from the convexity to indicate that the Poudre River is capable of laterally eroding the entire valley bottom as the river incises, leaving behind only fill terraces. Upstream from the convexity, in contrast, lateral erosion has created canyon segments with wider valley bottoms, but the entire valley bottom has not been lowered at the same rate, leaving discontinuous strath terraces. The Poudre River is regionally unique in having relatively well‐developed strath terraces, but strath terraces in other major drainages of the Front Range are also only present upstream from convexities in the longitudinal profile.
We measured wood piece characteristics and particulate organic matter (POM) in stored sediments in 30 channel‐spanning logjams along headwater streams in the Colorado Front Range, USA. Logjams are on ...streams flowing through old‐growth (>200 years), disturbed (<200 years, natural disturbance), or altered (<200 years, logged) subalpine conifer forest. We examined how channel‐spanning logjams influence riverine carbon storage (measured as the total volatile carbon fraction of stored sediment and instream wood). Details of carbon storage associated with logjams reflect age and disturbance history of the adjacent riparian forest. A majority of the carbon within jams is stored as wood. Wood volume is significantly larger in old‐growth and disturbed reaches than in altered reaches. Carbon storage also differs in relation to forest characteristics. Sediment from old‐growth streams has significantly higher carbon content than altered streams. Volume of carbon stored in jam sediment correlates with jam wood volume in old‐growth and disturbed forests, but not in altered forests. Forest stand age and wood volume within a jam explain 43% of the variation of carbon stored in jam sediment. First‐order estimates of the amount of carbon stored within a stream reach show an order of magnitude difference between disturbed and altered reaches. Our first‐order estimates of reach‐scale riverine carbon storage suggest that the carbon per hectare stored in streams is on the same order of magnitude as the carbon stored as dead biomass in terrestrial subalpine forests of the region. Of particular importance, old‐growth forest correlates with more carbon storage in rivers.
Key Points
Total carbon stored in a logjam relates to forest age and disturbance history
Old‐growth forests result in greater instream carbon storage
This is the first study to correlate forest age with riverine carbon storage
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
We use four stream segments along a wood‐rich, pool–riffle mountain stream in the Southern Rockies of Colorado, USA to examine how spatial variations in wood load and variations in discharge during ...and after the snowmelt peak flow influence the magnitude of surface and subsurface transient storage. Segments range in complexity from a single channel with no large wood to an anabranching channel with closely spaced, channel‐spanning logjams. Discharges at which transient storage was assessed range from base flow to snowmelt peak flow. To explore these relations, we used 10 geomorphic variables representing channel morphology and bed substrate, four wood‐related variables representing wood load and associated backwater storage, and two measures of skewness from instream and bulk electrical conductivity breakthrough curves during tracer tests. Instream curves reflect surface and subsurface transient storage, whereas bulk curves primarily represent subsurface transient storage. Higher values of skewness indicate greater retention, and we used the values here as a metric of increased transient storage. Although limited sample size restricts the power of our results, our findings suggest that stream segments with greater instream large wood loads have more and larger pools, greater storage of fine sediment and particulate organic matter, and higher values of skew from instream conductivity. The results also suggest that the presence of instream wood, rather than changes in channel morphology associated with wood, is the most important driver of transient storage. This implies that river management designed to foster transient storage should focus on retaining instream large wood. We did not find significant correlations between geomorphic or wood‐related variables and the skew estimated from bulk conductivity, which may reflect the relatively thin alluvium present in the field area and the prevalence of surface transient storage in this system.
Logjam with electrical resistivity sensors deployed across the channel upstream (at left).
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Nearly 50years of research focused on large wood (LW) in rivers provide a basis for understanding how wood enters rivers; how wood decays, breaks, and is transported downstream; and how at least ...temporarily stable wood influences channel geometry, fluxes of water, sediment, and organic matter, and the abundance and diversity of aquatic and riparian organisms. Field-based studies have led to qualitative conceptual models and to numerical stimulations of river processes involving wood. Numerous important gaps remain, however, in our understanding of wood dynamics. The majority of research on wood in rivers focuses on small- to medium-sized rivers, defined using the ratio of wood piece size to channel width as channels narrower than the locally typical wood-piece length (small) and slightly narrower than the longer wood pieces present (medium). Although diverse geographic regions and biomes are represented by one or a few studies in each region, the majority of research comes from perennial rivers draining temperate conifer forests. Regional syntheses most commonly focus on the Pacific Northwest region of North America where most of these studies originate. Consequently, significant gaps in our understanding include lack of knowledge of wood-related processes in large rivers, dryland rivers, and rivers of the high and low latitudes. Using a wood budget as an organizing framework, this paper identifies other gaps related to wood recruitment, transport, storage, and how beavers influence LW dynamics. With respect to wood recruitment, we lack information on the relative importance of mass tree mortality and transport of buried or surficial downed wood from the floodplain into the channel in diverse settings. Knowledge gaps related to wood transport include transport distances of LW and thresholds for LW mobility in small to medium rivers. With respect to wood storage, we have limited data on longitudinal trends in LW loads within unaltered large and great rivers and on fluctuations in LW load over time intervals greater than a few years. Other knowledge gaps relate to physical and ecological effects of wood, including the magnitude of flow resistance caused by LW; patterns of wood-related sediment storage for diverse river sizes and channel geometry; quantification of channel-floodplain-LW interactions; and potential threshold effects of LW in relation to physical processes and biotic communities. Finally, knowledge gaps are related to management of large wood and river corridors, including understanding the consequences of enormous historical reductions in LW load in rivers through the forested portions of the temperate zone; and how to effectively reintroduce and manage existing LW in river corridors, which includes enhancing public understanding of the importance of LW. Addressing these knowledge gaps requires more case studies from diverse rivers, as well as more syntheses and metadata analyses.
•Large wood creates many beneficial geomorphic and ecological effects in rivers.•Knowledge gaps for large wood in rivers include wood dynamics in large rivers.•We know little of the consequences of widespread wood removal from rivers.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
26.
The Natural Sediment Regime in Rivers WOHL, ELLEN; BLEDSOE, BRIAN P.; JACOBSON, ROBERT B. ...
Bioscience,
04/2015, Volume:
65, Issue:
4
Journal Article
Peer reviewed
Open access
Water and sediment inputs are fundamental drivers of river ecosystems, but river management tends to emphasize flow regime at the expense of sediment regime. In an effort to frame a more inclusive ...paradigm for river management, we discuss sediment inputs, transport, and storage within river systems; interactions among water, sediment, and valley context; and the need to broaden the natural flow regime concept. Explicitly incorporating sediment is challenging, because sediment is supplied, transported, and stored by nonlinear and episodic processes operating at different temporal and spatial scales than water and because sediment regimes have been highly altered by humans. Nevertheless, managing for a desired balance between sediment supply and transport capacity is not only tractable, given current geomorphic process knowledge, but also essential because of the importance of sediment regimes to aquatic and riparian ecosystems, the physical template of which depends on sediment-driven river structure and function.
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BFBNIB, DOBA, IZUM, KILJ, NMLJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
This research demonstrates how vegetation interacts with physical processes to govern landscape development. We quantify and describe interactions among driftwood, sedimentation, and vegetation for ...Great Slave Lake, which is used as proxy for shoreline dynamics and landforms before deforestation and wood removal along major waterways. We introduce driftcretion to describe large, persistent concentrations of driftwood that interact with vegetation and sedimentation to influence shoreline evolution. We report the volume and distribution of driftwood along shorelines, the morphological impacts of driftwood delivery throughout the Holocene, and rates of driftwood accretion. Driftcretions facilitate the formation of complex, diverse morphologies that increase biological productivity and organic carbon capture and buffer against erosion. Driftcretions should be common on shorelines receiving a large wood supply and with processes which store wood permanently. We encourage others to work in these depositional zones to understand the physical and biological impacts of large wood export from river basins.
Key Points
Continued accretion of driftwood to shorelines alter morphology on large scales
Driftwood‐rich shores influence ecosystems, carbon capture, and erosional buffering
Rates and volumes of accretion are nontrivial for basin wood and carbon budgets
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Wood is an integral part of a river ecosystem and the number of restoration projects using log placements is increasing. Physical model tests were used to explore how the wood position and ...submergence level (discharge) affect wake structure, and hence the resulting habitat. We observed a von-Kármán vortex street (VS) for emergent logs placed at the channel center, while no VS formed for submerged logs, because the flow entering the wake from above the log (sweeping flow) inhibited VS formation. As a result, emergent logs placed at the channel center resulted in ten times higher turbulent kinetic energy compared to submerged logs. In addition, both spatial variation in time-mean velocity and turbulence level increased with increasing log length and decreasing submergence level. Submerged logs and logs placed at the channel side created a greater velocity deficit and a longer recirculation zone, both of which can increase the residence time in the wake and deposition of organic matter and nutrients. The results demonstrate that variation in log size and degree of submergence can be used as a tool to vary habitat suitability for different fish preferences. To maximize habitat diversity in rivers, we suggest a diverse large wood placement.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
Traditionally, stream channel planform has been viewed as a function of larger watershed and valley-scale physical variables, including valley slope, the amount of discharge, and sediment size and ...load. Biotic processes serve a crucial role in transforming channel planform among straight, braided, meandering, and anabranching styles by increasing stream-bank stability and the probability of avulsions, creating stable multithread (anabranching) channels, and affecting sedimentation dynamics. We review the role of riparian vegetation and channel-spanning obstructions—beaver dams and logjams—in altering channel-floodplain dynamics in the southern Rocky Mountains, and we present channel planform scenarios for combinations of vegetation and beaver populations or old-growth forest that control logjam formation. These conceptual models provide understanding of historical planform variability throughout the Holocene and outline the implications for stream restoration or management in broad, low-gradient headwater valleys, which are important for storing sediment, carbon, and nutrients and for supporting a diverse riparian community.
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BFBNIB, DOBA, IZUM, KILJ, NMLJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Published research emphasizes rapid downstream export of terrestrial carbon from mountainous headwater rivers, but little work focuses on mechanisms that create carbon storage along these rivers, or ...on the volume of carbon storage. Here we estimate organic carbon stored in diverse valley types of headwater rivers in Rocky Mountain National Park, CO, USA. We show that low-gradient, broad valley bottoms with old-growth forest or active beaver colonies store the great majority of above- and below-ground carbon. These laterally unconfined valley segments constitute <25% of total river length, but store ∼75% of the carbon. Floodplain sediment and coarse wood dominate carbon storage. Our estimates of riverine carbon storage represent a previously undocumented but important carbon sink. Our results indicate that: not all mountainous rivers rapidly export carbon; not all valley segments are equally important in carbon storage; and historical changes in riverine complexity have likely reduced carbon storage.
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