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Aggradation
Aggradation
Aggradation
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Schematic of sediment accumulation (aggradation) in a river channel. The sediment is brown. The river is flowing on bedrock in the upper image, but because sediment was deposited over time the riverbed has risen. This has caused the house to be buried in the lower image.

Aggradation (or alluviation) in geology is the increase in land elevation, typically in a river system, due to the deposition of sediment. Aggradation occurs in areas in which the supply of sediment is greater than the amount of material that the system is able to transport. The mass balance between sediment being transported and sediment in the bed is described by the Exner equation.

Typical aggradational environments include lowland alluvial rivers, river deltas, and alluvial fans. Aggradational environments are often undergoing slow subsidence which balances the increase in land surface elevation due to aggradation. After millions of years, an aggradational environment will become a sedimentary basin, which contains the deposited sediment, including paleochannels and ancient floodplains.

Aggradation can be caused by changes in climate, land use, and geologic activity, such as volcanic eruption, earthquakes, and faulting. For example, volcanic eruptions may lead to rivers carrying more sediment than the flow can transport: this leads to the burial of the old channel and its floodplain. In another example, the quantity of sediment entering a river channel may increase when climate becomes drier. The increase in sediment is caused by a decrease in soil binding that results from plant growth being suppressed. The drier conditions cause river flow to decrease at the same time as sediment is being supplied in greater quantities, resulting in the river becoming choked with sediment.

In 2009, a report by researchers from the University of Colorado at Boulder in the journal Nature Geoscience said that reduced aggradation was contributing to an increased risk of flooding in many river deltas.[1] However, both degradation and aggradation are event driven.[2]

See also

[edit]
  • Avulsion (river) – Rapid abandonment of a river channel and formation of a new channel
  • Progradation – Growth of a river delta into the sea over time
  • Sedimentary basin – Regions of long-term subsidence creating space for infilling by sediments

References

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Aggradation

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from Grokipedia
Aggradation is a fundamental geomorphic process in which deposition leads to the progressive buildup and increase of a land surface, such as a channel bed, , or coastal shoreline. This deposition occurs when the supply of exceeds the stream's or coastal system's capacity to transport it, resulting in accumulation that raises the surface level over time. In fluvial environments, aggradation is the direct opposite of degradation, where lowers the bed through removal. The process is driven by several key factors, including a significant increase in upstream sediment supply from sources like landslides; reduced water discharge that diminishes transport capacity; or a decrease in channel slope due to meander development or downstream obstructions. For instance, in glaciated regions like Mount Rainier National Park, glacial meltwater carries substantial rock and sediment loads, causing riverbeds to rise by approximately 3 feet per decade (based on 1997-2010 data) in some areas, with rates accelerating due to climate change. In coastal settings, aggradation manifests as accretion, where waves and currents deposit sand and other materials to build up beaches or barriers, often in response to a positive sediment budget. Aggradation plays a critical role in shaping landscapes and river dynamics, influencing channel morphology, patterns, and formation by creating wider, braided channels or mid-channel bars that redirect flows and promote . It contributes to the evolution of floodplains and deltas, as seen in responses to climate-driven changes like accelerated glacier retreat or intense storm events, and can exacerbate risks to through elevated levels. Human activities, such as construction, further alter aggradation rates by trapping upstream, leading to downstream channel adjustments that affect ecological integrity and water management.

Fundamentals

Definition

Aggradation is the geomorphic process by which the elevation of a land surface increases due to the accumulation of sediment through deposition that exceeds erosion, resulting in upward or outward growth of the landform. This process primarily involves the settling of eroded materials transported by agents such as water, wind, or ice, leading to the construction of new layers atop existing topography. Key characteristics of aggradation include its occurrence in depositional environments where sediment supply outpaces removal, such as river valleys, deltas, and floodplains, where net accumulation dominates over erosional forces. Unlike , which lowers the through material removal, aggradation emphasizes the buildup of layers, often achieving a more uniform grade across the surface. It stands in contrast to degradation, the opposing process of surface lowering by . The term "aggradation" was coined in the late 19th century by geologist Rollin D. Salisbury in his 1893 report on surface geology, where it described the constructive phase of landscape modification through sediment filling. Salisbury introduced it alongside related concepts to distinguish building-up processes from degradational ones in fluvial systems. Basic examples of aggradation include the formation of alluvial fans at the mouths of mountain streams, where sediment spreads outward upon entering broader valleys, and the gradual raising of riverbeds through successive flood deposits over time. These features illustrate how aggradation shapes depositional landforms by layering sediments in low-energy settings. Aggradation, the process of sediment accumulation that raises land surfaces vertically, contrasts sharply with degradation, which involves the net removal of sediment leading to downcutting or lowering of surfaces, such as in incised river channels where dominates over deposition. In degradational environments, fluvial incision can deepen valleys and expose , directly opposing the constructive nature of aggradation by reducing topographic relief rather than building it. Progradation represents another related but distinct depositional process, characterized by the lateral extension of features like shorelines, deltas, or alluvial fans, often advancing seaward or basinward through buildup. While progradation and aggradation both involve deposition, progradation emphasizes horizontal growth—such as the outward migration of a delta front—distinct from the vertical thickening central to aggradation in riverbeds or floodplains. Landscapes frequently experience aggradational-degradational cycles, where phases of buildup alternate with in response to changing environmental conditions, notably in fluvial systems during glacial-interglacial periods. These cycles reflect dynamic equilibrium, with aggradation occurring during periods of high supply, such as post-glacial floods, followed by degradation as base levels stabilize and incision resumes. Key distinctions among these processes highlight their roles in geomorphic evolution: aggradation constructs and elevates through vertical accumulation, degradation erodes and lowers it via removal, and progradation expands it laterally; notably, aggradation and degradation can coexist in balanced profiles, such as graded rivers where upstream deposition offsets downstream to maintain a steady-state channel slope. This interplay underscores deposition as a fundamental mechanism shared across aggradational contexts, linking it conceptually to broader depositional dynamics.

Mechanisms

Sediment Transport and Deposition

Sediment transport in fluvial systems occurs through distinct modes that determine how particles contribute to aggradation. Bedload transport involves coarser sediments, such as and , that move along the channel bed via rolling, sliding, or saltation, typically comprising 5-20% of the total load in gravel-bed rivers. consists of finer particles, like and fine , that are carried within the turbulent above the bed, often making up the majority of the sediment flux in sand-bed rivers where suspension and bedload coexist. load refers to very fine clays and that remain in suspension even at low velocities due to their low rates, independent of the channel bed composition and sourced primarily from upstream . The initiation and cessation of sediment motion are governed by flow velocity thresholds, as illustrated by the Hjulström curve, which plots critical velocities for erosion and deposition against grain size. For erosion, the curve shows that fine particles like clay require higher velocities (around 50-100 cm/s) to dislodge due to cohesion, while medium sands erode at lower velocities (10-20 cm/s); deposition occurs when flow velocity drops below the settling velocity, with fines settling first at low speeds (e.g., <1 cm/s for silt). A key metric for the onset of motion is the Shields parameter, a dimensionless shear stress defined as τ∗=τ(ρs−ρ)gD,\tau^* = \frac{\tau}{(\rho_s - \rho) g D}, where τ\tau is the bed shear stress, ρs\rho_s and ρ\rho are the densities of sediment and fluid, gg is gravitational acceleration, and DD is the grain diameter; motion begins when τ∗\tau^* exceeds a critical value, typically 0.03-0.06 for non-cohesive sands. Deposition leading to aggradation is triggered primarily by a reduction in flow , which allows particles to settle out of suspension when the exceeds the turbulent rate. Increased supply relative to capacity can also promote fallout, as excess particles overwhelm the flow's carrying ability. Base-level rise, such as from sea-level changes or upstream damming, lowers the energy gradient, further reducing and facilitating widespread accumulation in channels and floodplains. In fluvial environments, channel patterns influence deposition sites. Braided rivers, characterized by multiple shifting channels around gravel bars, promote aggradation through frequent bar formation and sediment sorting in high-supply, steep-gradient settings. Meandering rivers, in contrast, deposit sediments in low-energy zones like point bars and oxbow lakes during overbank flows, where velocity decreases sharply on inner bends. Aggradation operates across varied timescales, from rapid events during floods that deposit thick layers (up to meters) in hours to days, to long-term valley filling over millennia, as seen in post-glacial alluvial sequences where cumulative deposition builds landforms over 10,000+ years.

Environmental Controls

Topographic factors play a crucial role in facilitating aggradation by creating conditions that reduce flow energy and promote sediment trapping. Low-gradient slopes, such as those found in deltaic environments with very low gradients (on the order of 10^{-4}), diminish stream competence, allowing suspended and bedload sediments to settle out rather than being transported further. Concave landforms, including depositional basins and wide, unconfined valleys (e.g., 5-7 km wide in river deltas), act as sediment traps by providing lateral space for overbank deposition during floods, enhancing long-term accumulation. Accommodation space in these basins, influenced by valley morphology and confinement, determines the volume available for sediment storage, with high channel-floodplain connectivity in low-gradient zones amplifying deposition rates. Vegetation and soil properties further control aggradation by stabilizing nascent deposits and contributing to organic buildup. Root systems of woody , such as junipers, physically anchor sediments, increase soil infiltration, and elevate critical for , thereby reducing sediment remobilization and promoting net accumulation. In wetlands, biogenic aggradation occurs through the vertical accumulation of from and plant decomposition, where biomass from species like Spartina alterniflora adds to elevation and helps marshes counteract or sea-level rise. , enhanced by vegetation cover, decreases erodibility, allowing deposited materials to consolidate over time and form stable landforms. Base-level dynamics regulate aggradation by altering the available space for deposition along river profiles. A rise in base level, such as from sea-level or lake-level increase, expands accommodation space, prompting rivers to aggrade as they adjust toward a new equilibrium profile—a balanced where and deposition rates match supply. This adjustment is most pronounced in coastal plains, where base-level fall can limit aggradation and induce incision, while rises create backwater effects that slow flow and enhance . The concept of equilibrium profiles underscores how rivers respond to these changes by modifying or channel geometry to maintain balance. Feedback loops amplify aggradation under favorable environmental conditions by linking initial deposition to subsequent process enhancements. For instance, early buildup in low-gradient channels reduces flow velocities, decreasing and favoring further deposition of fines, which widens and shallows the channel to perpetuate the cycle. In vegetated settings, this aggradation supports denser plant growth, which further slows and stabilizes soils, creating a positive reinforcement that sustains buildup over time. Such loops are evident in post-disturbance systems, where width modulates the pace, with wider sections delaying aggradation waves and intensifying local feedbacks.

Causes

Natural Drivers

Aggradation in natural systems is often driven by climatic factors that enhance supply and reduce capacity in fluvial environments. High events, such as intense rainfall, can trigger flows and increase mobilization from hillslopes, leading to deposition in channels when the load exceeds the stream's . Similarly, glacial during periods supplies large volumes of to proglacial and paraglacial zones, where rapid unloading of glacial results in widespread valley filling and development. For instance, in recently deglaciated landscapes like the , paraglacial has dominated post-glacial aggradation through the reworking of into valley fills and cones. Tectonic processes also play a key role in promoting aggradation by altering gradients and creating accommodation space for accumulation. Uplift in active orogenic belts steepens upstream slopes, increasing and yield, while simultaneously reducing downstream channel gradients, which favors deposition over transport. In contrast, associated with formation generates broad depocenters where from eroding mountain fronts accumulates rapidly; for example, in the Himalayan foreland, tectonic has driven aggradational stacking in wedge-top and foredeep zones, preserving thick alluvial sequences. Volcanic activity and associated mass-wasting events provide sudden pulses of that overwhelm river systems, causing rapid and localized aggradation. Lahars—volcanic mudflows formed by the remobilization of pyroclastic material during eruptions or heavy rains—can deposit coarse-grained sediments downstream, leading to channel infilling and elevation increases of meters to tens of meters in affected valleys. Landslides on volcanic flanks, often triggered by seismic activity or slope instability, similarly contribute to these pulses; at , for example, post-eruption debris flows have caused significant aggradation along the lower Toutle River, altering channel morphology for decades. Over timescales, aggradation patterns exhibit cyclic responses to glacial- climate oscillations. During warmer , increased precipitation and vegetation recovery enhance sediment delivery and fluvial deposition in valleys previously scoured by , resulting in net aggradation of alluvial fills. Conversely, glacial periods are characterized by enhanced due to advance and periglacial processes, which deepen valleys and limit deposition until subsequent phases. This alternation is evident in European river systems, where aggradation has built substantial terrace sequences atop glacial erosional surfaces.

Human Influences

Human activities significantly alter natural sediment dynamics, often accelerating aggradation through increased and sediment delivery to systems. Land-use changes, particularly and , remove vegetative cover and destabilize soils, leading to heightened rates that supply excess to downstream channels and deltas. For instance, in regions with , such practices can elevate sediment yields by several times compared to natural baselines, promoting depositional buildup in fluvial environments where transport capacity is exceeded. Engineering interventions like dam construction trap upstream , causing rapid aggradation within reservoirs. Globally, over 47,000 large have led to an average predicted 25% loss of storage capacity by 2050 due to sediment accumulation, with some areas like facing up to 50% reductions; this process reduces the reservoirs' effective volume and disrupts downstream sediment budgets. Similarly, systems confine flows, preventing overbank deposition and forcing sediment to accumulate within the channel, which elevates bed levels and increases risks over time. In stabilized channels, this confinement can result in progressive aggradation as natural is curtailed. Urbanization exacerbates aggradation in streams through impervious surfaces that accelerate runoff and mobilize sediments during intense storms. Construction activities and altered hydrology increase sediment loads from eroded banks and upland sources, leading to depositional events during flash floods that raise channel beds and widen floodplains. Studies in urban watersheds show that such changes can double peak flows and promote aggradation, potentially attenuating some flood peaks by storing water in expanded channels but at the cost of long-term instability. Mining operations directly contribute to aggradation by introducing substantial loads into rivers, often doubling suspended concentrations and causing downstream deposition. Artisanal and mining across tropical regions has elevated sediment fluxes in over 80% of affected rivers, smothering habitats and altering channel morphology through where flows decelerate. This human modification, compounded by indirect effects like intensified storms from , underscores the modifiable nature of aggradation drivers since the .

Impacts

Geomorphic Effects

Aggradation profoundly influences landscape evolution by facilitating the deposition of sediments that construct and modify various landforms. Alluvial plains form through the gradual accumulation of sediments in river valleys, creating broad, flat expanses that elevate the surrounding terrain over time. Terraces emerge as stepped features along riverbanks when aggradation raises the floodplain, followed by periods of incision that leave behind abandoned surfaces at higher elevations. Alluvial fans develop at the outlets of confined channels entering broader basins, where sediment spreads out in a conical pattern due to reduced flow velocity and increased deposition. Vertical aggradation specifically raises floodplain levels, reducing slope and promoting further sediment trapping, which stabilizes and expands these low-relief areas. In river channels, aggradation alters morphology by increasing sediment loads that lead to widening and shallowing, as deposited materials fill the bed and force lateral expansion to accommodate flow. This process commonly results in the formation of bars and islands, where accumulations of and create emergent features that divide the channel and influence flow patterns. Over longer timescales, aggradation adjusts the river's long profile toward a graded condition, where the channel slope balances and deposition, minimizing and promoting equilibrium. At the basin scale, aggradation contributes to sediment infilling of subsiding areas, which can redirect drainage patterns by filling depressions and altering flow paths across the landscape. In tectonically active regions, this infilling compartmentalizes basins into sediment-trapping zones, reshaping regional hydrology. A prominent example is the progradation of the Ganges Delta, where ongoing aggradation driven by high sediment supply from the Ganges and Brahmaputra rivers advances the shoreline seaward, building extensive subaerial and subaqueous landforms while maintaining pace with subsidence and sea-level rise. Aggradation rates vary by setting and scale, typically ranging from 1 to 10 mm per year in river systems, reflecting steady accumulation influenced by discharge and supply. In contrast, volcanic aggradation can occur rapidly during discrete events, with deposits reaching meters in thickness per eruption due to pyroclastic flows and lahars overwhelming . These differences highlight how aggradation operates across timescales, from episodic high-magnitude changes to gradual landscape building.

Ecological Consequences

Aggradation profoundly influences biological systems by reshaping habitats through accumulation, often creating new environments while altering or destroying existing ones. In dynamic systems, moderate aggradation forms expansive and wetlands that support diverse riparian , such as willows and cottonwoods, which stabilize banks and provide food and shelter for . For example, in glaciated mountain rivers like those in , flood-induced deposition scours and rebuilds channel beds, generating fresh and substrates essential for and habitats. However, rapid or excessive aggradation can bury preexisting soils to depths that disrupt root zones, leading to die-off of established plant communities and reducing habitat suitability for terrestrial dependent on mature . In meadows, such burial shifts toward disturbance-tolerant , altering community structure over time. The effects of aggradation vary with its rate and intensity, promoting richness in balanced depositional settings but causing homogenization and declines in extreme cases. Dynamic environments with periodic buildup foster heterogeneous landscapes, including bars, pools, and vegetated islands, which support a wide array of ; gravel-bed floodplains, for instance, act as ecological nexuses concentrating productivity and in mountain landscapes. benefit particularly, as aggrading channels create spawning grounds in newly deposited gravels, enhancing for salmonids and other rheophilic . Conversely, accelerated aggradation fills pools and embeds substrates with fines, diminishing complexity and leading to ; macroinvertebrate richness, including sensitive EPT taxa (Ephemeroptera, , Trichoptera), drops significantly above thresholds of 5-10 NTU, while populations experience reduced growth and survival due to smothered feeding areas. In unstable channels, such as those exhibiting braiding from high loads, aquatic organism recovery potential declines as niches for invertebrates and amphibians narrow. Sediment deposition during aggradation plays a dual role in nutrient cycling, delivering essential elements that elevate while posing risks of overload in aquatic zones. Floodplains trap nutrients like and bound to , enriching soils and fueling vegetation growth; in Amazonian systems, annual deposition of approximately 77 million metric tons of across floodplains contributes 0.98 million metric tons of particulate organic carbon, sustaining high net rates of 5.7-29.2 g C m⁻² day⁻¹. This process enhances fertility, with nutrient retention increasing with in riparian zones, thereby supporting robust food webs. Yet, in rivers with excessive aggradation, fine smother benthic algae and invertebrates, curtailing and disrupting nutrient regeneration; reduce light penetration, impairing growth and cascading to lower fish condition factors over time. Long-term ecological succession on aggraded surfaces transforms barren deposits into complex, mature ecosystems, fostering resilience and biodiversity over centuries. Pioneer species, such as black willow (Salix nigra) and cottonwood (Populus deltoides), rapidly colonize fresh silt and sand bars, binding sediments and initiating in the stand initiation phase. As deposits stabilize, succession progresses through stem exclusion and understory re-initiation, allowing shade-tolerant trees like American elm () and silver maple () to establish, creating multilayered forests with increased habitat diversity. In systems like the , this leads to bottomland hardwoods and cypress swamps over decades to centuries, enhancing nutrient cycling and supporting fisheries; however, human-induced alterations can accelerate or homogenize these trajectories, potentially reducing overall .

Study and Applications

Measurement Techniques

Field methods for quantifying aggradation primarily involve direct sampling and monitoring techniques to assess sediment accumulation rates and volumes in fluvial and coastal environments. Sediment coring uses devices such as the McNeil Ahnell sampler, a cylinder that excavates substrate to a depth of about 23 cm, allowing for the collection and analysis of deposited sediments in gravel-bed streams. This method enables analysis through wet or dry sieving, providing data on sediment thickness and composition to estimate aggradation volumes. Geophysical surveys, including (GPR), offer non-invasive imaging of subsurface , revealing depositional layers and bounding surfaces in sediments like sands and gravels with penetration depths exceeding 10 m in resistive materials. GPR profiles help delineate aggradation sequences in fluvial point bars and aeolian dunes by detecting changes in sediment architecture. Erosion pins, typically ¼-inch diameter steel rods installed horizontally in arrays along stream banks, measure vertical changes in surface elevation over time, capturing both and short-term deposition at the to quantify net aggradation rates during monitoring intervals of about two months. Remote sensing techniques enhance large-scale detection of aggradation by capturing topographic changes over time. (Light Detection and Ranging) generates high-resolution digital elevation models (DEMs) through airborne surveys, enabling differencing of pre- and post-event data to quantify elevation increases from deposition. For instance, multitemporal with point densities up to 6 points/m² and vertical accuracies of 5 cm has measured net aggradation volumes on the order of 578,000 m³ along river floodplains, with deposition thicknesses decaying exponentially downstream. complements by providing broader coverage for repeat surveys, calculating volume accumulation through algorithms that identify areas of topographic buildup. These methods are particularly effective for monitoring aggradation in dynamic river systems where field access is limited. Dating techniques are essential for establishing timelines of aggradation events and ages of sediments. Radiocarbon (¹⁴C) of organic materials, such as and macrofossils from core samples, tracks pulses of glaciofluvial aggradation, revealing cycles between 45 and 20 ka BP linked to glacial activity through rigorous pretreatment like ABA to ensure accurate ages. Optically stimulated (OSL) measures the dose accumulated in or grains since , providing ages for fluvial sediments as young as a few centuries, as demonstrated in headwater valleys where it dated aggradation phases from the 16th to 18th centuries associated with land-use changes. Cosmogenic nuclides, such as ¹⁰Be and in situ ¹⁴C in boulders, infer exposure timing on surfaces, accounting for inheritance from prior exposure to estimate aggradation around 5-12 ka in fans, with ¹⁴C offering reduced bias due to its shorter . These methods collectively constrain the tempo of depositional episodes. Modeling approaches simulate aggradation by predicting and deposition under varying hydraulic conditions. The (Hydrologic Engineering Center's River Analysis System) model employs quasi-unsteady and unsteady flow simulations coupled with sediment transport functions to route bed-load and suspended sediments, forecasting deposition patterns that lead to channel aggradation in one- or two-dimensional frameworks. This allows calibration against field data to predict long-term accumulation in rivers and reservoirs. Landscape evolution models incorporate equations to represent hillslope sediment transport, where flux is proportional to slope gradient, resulting in deposition in areas of positive curvature such as footslopes, thereby simulating gradual aggradation over geologic timescales. These numerical tools integrate field and data to hindcast and forecast aggradational responses to environmental forcings.

Case Studies

One prominent fluvial example of aggradation is the Holocene development of the in , where massive loess inputs from the have driven significant channel filling. During the , the river's sediment load reached up to 1.6 billion tons annually, leading to aggradation rates in the lower reaches exceeding 10 cm per year in periods of high deposition, such as the late . This process elevated the riverbed substantially, contributing to frequent flooding and the need for extensive systems, with total Holocene sediment volumes estimated at over 8 trillion tons across the basin. In coastal settings, the exemplifies progradational aggradation followed by reversal due to anthropogenic interventions. Since the early 1800s, sediment deposition from the built new land through delta lobe advancement, with progradation rates of 100–150 meters per year generating approximately 6–8 km² of annual land gain, resulting in roughly 500 km² of net area expansion during the before widespread construction and upstream damming curtailed sediment supply. By the late , this natural growth trend inverted, transitioning to net land loss as human factors like channelization reduced sediment delivery by over 50%, exacerbating and . A glacial context is illustrated by paraglacial aggradation in the following the end of the around 1850, where glacier retreat exposed unstable sediments that were rapidly reworked into depositional landforms. In regions like the central and in the Italian , post- deglaciation triggered sediment cascades from ridges, promoting the development of alluvial fans and valley fills through paraglacial processes such as debris flows and slope failures, with fan aggradation rates peaking in the decades immediately after glacier recession. These features continue to evolve, storing moraine-derived sediments and altering fluvial dynamics in formerly glaciated valleys. A modern anthropogenic case is the rapid reservoir sedimentation behind China's , operational since 2003, which traps vast quantities of River . By 2023, cumulative deposition in the had reached approximately 2.5 billion tons, with annual trapping rates averaging around 150–170 million tons in the initial decades (2003–2010s), though rates have since declined to about 100 million tons per year due to upstream construction and reduced influx. This , monitored via bathymetric surveys and , highlights the dam's role in altering downstream fluxes while concentrating aggradation in the impoundment zone. Recent studies as of 2025 have applied (InSAR) to measure aggradation in Himalayan rivers, such as the Marsyangdi, revealing rates of up to 0.5 m per year in ephemeral channels driven by monsoon floods and glacial melt, providing insights into climate change impacts on high-altitude sediment dynamics.

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