Watershed sediment budgets sum sediment sources (bank erosion and tributaries) and subtract sediment sinks (floodplains) to compute the watershed sediment output. Because computations are lumped, however, these budgets cannot quantify contributions from individual sources to the watershed output. We overcome this limitation by dividing the channel network into nested reaches, each with its own sediment budget. The nested framework partitions sediment between transport and storage as it moves downstream, quantifying the contributions from individual sources to watershed outputs. We quantify sediment fluxes using gaging station data, sediment fingerprinting, hydrodynamic modeling, geomorphic mapping, and measured rates of erosion and deposition. Bank erosion supplies 15 790 Mg/yr to the watershed budget, while upland hillslopes contribute 6 300 Mg/yr, with 10 380 Mg/yr stored on floodplains. The output computed from the budget (11 710 Mg/yr) is within the uncertainty of 9 300–43 000 Mg/yr of the measured output, so the budget balances. Reach-scale budgets identify local sediment hotspots, while routing computations indicate that bank erosion supplies 76 ± 5% of the watershed sediment flux, with upland hillslopes contributing 24 ± 3%. Legacy sediments comprise 34 ± 6% of the output. By quantifying individual source contributions to watershed sediment fluxes, the nested approach can improve sediment management decision-making.

A fluvial catchment consists of unchannelized hillslopes drained by a channel network. Catchments can be fully characterized by their three-dimensional (3D) topography and the bankfull characteristics of their channels. Here we use a probabilistic algorithm to generate a set of scale-free, two-dimensional (2D) pixelized river networks of increasing complexity. We then integrate reach-scale hydraulic geometry equations, originally developed for single-channel gravel-bedded river reaches, to reverse engineer the corresponding 3D landscape topography of these 2D synthetic networks (Reverse Engineered Fluvial Landscape, REFL). To do so requires specification of outlet flood discharge and a characteristic bed grain size. By incorporating hillslope-channel coupling, represented by a characteristic hillslope length and slope, we can fully specify the 3D topography of the entire watershed. Our results suggest that under appropriate constraints, the equilibrium hydraulic geometry hypothesis can be extended beyond isolated river reaches to encompass entire fluvial landscapes. The class of landscapes we consider are relatively low-slope montane catchments with subdued tectonics. The streams that drain the catchment are assumed to be alluvial or quasi-alluvial well upstream of the outlet. A simplified model analogous to the subgrid model of the Large Eddy Simulation model of turbulent flow is used to describe processes upstream of the limit of alluviated channels.

Understanding the relationship between a dam's size and its ecological effects is important for prioritization of river restoration efforts based on dam removal. Although much is known about the effects of large storage dams, this information may not be applicable to small dams, which represent the vast majority of dams being considered for removal. To better understand how dam effects vary with size, we conducted a multidisciplinary study of the downstream effect of dams on a range of ecological characteristics including geomorphology, water chemistry, periphyton, riparian vegetation, benthic macroinvertebrates, and fish. We related dam size variables to the downstream-upstream fractional difference in measured ecological characteristics for 16 dams in the mid-Atlantic region ranging from 0.9 to 57 m high, with hydraulic residence times (HRTs) ranging from 30 min to 1.5 years. For a range of physical attributes, larger dams had larger effects. For example, the water surface width below dams was greater below large dams. By contrast, there was no effect of dam size on sediment grain size, though the fraction of fine-grained bed material was lower below dams independently of dam size. Larger dams tended to reduce water quality more, with decreased downstream dissolved oxygen and increased temperature. Larger dams decreased inorganic nutrients (N, P, Si), but increased particulate nutrients (N, P) in downstream reaches. Aquatic organisms tended to have greater dissimilarity in species composition below larger dams (for fish and periphyton), lower taxonomic diversity (for macroinvertebrates), and greater pollution tolerance (for periphyton and macroinvertebrates). Plants responded differently below large and small dams, with fewer invasive species below large dams, but more below small dams. Overall, these results demonstrate that larger dams have much greater impact on the ecosystem components we measured, and hence their removal has the greatest potential for restoring river ecosystems.

We extend two hypotheses based on studies of 1st- to 3rd-order Piedmont watersheds of southeastern Pennsylvania, USA, by collecting data in a larger 3rd- to 5th-order watershed nearby. One hypothesis posits that presettlement river corridors were dominated by wetlands, and the other suggests that river valleys were filled by millpond sedimentation following European settlement. Both hypotheses support new river restoration practices, so their generality is important to assess. Ten lithofacies indicate depositional environments, while pedostratigraphic criteria and 14C dating define presettlement and postsettlement stratigraphic units. Basal gravels similar to modern stream bed sediments represent presettlement channels with active bedload transport. Wedge-shaped gravel deposits resembling modern bars further document presettlement bedload transport by channelized flows. Extensive presettlement and postsettlement units of massive, organic-poor, fine-grained sediment formed when overbank flows inundated floodplains. Peat deposits, exposed at a single site (but absent elsewhere), represent a presettlement wetland. Decimeter-thick, discontinuous, massive carbonaceous fine-grained sediments occasionally overlie basal gravels; these may represent localized wetlands adjacent to presettlement channels or hydraulic backwater environments. Laminated sand and mud accumulated behind one 3-m-high mill dam, but these millpond deposits are absent at other sites. Instead of being dominated by wetlands, presettlement river corridors are better described as a complex mosaic of riparian environments including older colluvial landforms, floodplains (some of which may have been seasonally inundated wetlands), primary (and possibly secondary) channels, and depending on geomorphic setting, either localized or valley-spanning wetlands. After European settlement, millponds were important locally, but their deposits represent a minor component of the stratigraphic record.

Variation in floodplain topography can lead to gradual flooding and increase river-floodplain connectivity. We show that incorporating flowpaths as an explicit measure of river-floodplain connectivity can improve estimates of floodplain sediment deposition. We focus on the floodplain of the South River, downstream of Waynesboro, Virginia, where measurements of mercury accumulation have been used to estimate decadal-scale sedimentation rates. We developed a two-dimensional Hydrologic Engineering Center's River Analysis System (2D HEC-RAS) hydrodynamic model and used simulated model results with sediment deposition data to create regression models describing sedimentation across the floodplain. All of our statistical models incorporated a flowpath length from the location on the floodplain downstream to the riverbank as an explicit measure of river-floodplain connectivity that improved our estimates of floodplain sediment deposition (r2 = 0.514). We applied our best regression model to our hydrodynamic model results to create a map of floodplain sedimentation rate and discuss differences of three separate sections of floodplain. We found that floodplains with variable topography had wider, bimodal probability distribution functions (PDFs) of sedimentation rate (aggregated spatially) than floodplains without this topographic relief (with narrower log-normal PDFs). Our work highlights how floodplain topography and river-floodplain connectivity affect sedimentation rates and can help inform the development of floodplain sediment budgets.

Fluvial sediment transport and deposition shapes river, floodplain, and estuarine systems but also can create water quality problems, which in some cases motivates efforts to reduce hillslope erosion. However, watershed management efforts rarely consider the fate of eroded sediments and the time scale of transport to estuaries. This study presents a modeling approach to estimate regionally averaged floodplain deposition, erosion, and sediment flux for the mid‐Atlantic U.S. caused by changing forest cover, urban development, and milldam construction. Regional regression equations estimate discharge at 3‐month intervals, and (temporally invariant) channel width and slope. Sediment concentrations are determined from a rating curve (defined for modern conditions by gaging‐station data). Tuning the model to floodplain stratigraphic data suggests that‐sediment concentrations prior to European colonization (i.e., before ∼1750) were 5%–8% of those prevailing today, while legacy (1750–1950) sediment concentrations were 25%–35% of present values. The calibration process, however, is only partly successful for the period before 1750 due to the limited number of older dated samples available. Nonetheless, the model accurately reproduces the observed age distribution of floodplain deposits. Computed sediment‐budget components increase monotonically from before 1750 to the present, and the ratio of budget components remains similar from one time period to the next. The model also predicts that millennial timescales are needed for mid‐Atlantic floodplains to equilibrate following a change in sediment regime, a finding with important implications for river corridor and watershed restoration planning.

New and previously published strati -graphic data define Holocene to present sedi-ment storage time scales for Mid-Atlantic river corridors. Empirical distributions of deposit ages and thicknesses were randomly sampled to create synthetic age-depth re-cords. Deposits predating European settle-ment accumulated at a (median) rate of 0.06 cm yr-1, range from -18,000 to 225 yr old, and represent 39% (median) of the to-tal accumulation. Sediments deposited from 1750 to 1950 ("legacy sediments") accumu-lated at a (median) rate of 0.39 cm yr-1 and comprise 47% (median) of the total, while "modern sediments" (1950-present) repre-sent 11% of the total and accumulated at a (median) rate of 0.25 cm yr-1. Synthetic strati -graphic sequences, recast as age distributions for the presettlement period, in 1900 A.D., and at present, reflect rapid postsettlement alluviation, with enhanced preservation of younger sediments related to postsettlement watershed disturbance. An averaged pres-ent age distribution for vertically accreted sediment has modal, median, and mean ages of 190, 230, and 630 yr, reflecting the pre-dominance of stored legacy sediments and the influence of relatively few, much older early Holocene deposits. The present age distribution, if represented by an exponen-tial approximation (mean age -300 yr), and naively assumed to represent steady-state conditions, implies median sediment travel times on the order of centuries for travel dis-tances greater than -100 km. The percentage of sediment reaching the watershed outlet in 30 yr (a reasonable time horizon to achieve watershed restoration efficacy) is -60% for a distance of 50 km, but this decreases to <20% for distances greater than 200 km. Age dis-tributions, evaluated through time, not only encapsulate the history of sediment storage, but they also provide data for calibrating wa-tershed-scale sediment-routing models over geological time scales.

Sediment connectivity is a conceptualization for the transfer and storage of sediment among different geomorphic compartments across upland landscapes and channel networks. Sediment connectivity and dysconnectivity are linked to the water cycle and hydrologic systems with the associated multiscale interactions with climate, soil, topography, ecology, and landuse/landcover under natural variability and human intervention. We review current sediment connectivity and modeling approaches evaluating and quantifying water and sediment transfer in catchment systems. Many studies highlight the interaction between sediment and water in defining landscape connectivity, but many efforts to quantify and/or simulate sediment connectivity rely on the topographic/structural controls on sediment erosion and delivery. More recent modeling efforts integrate functional and structural connectivity to capture hydrologic properties influencing sediment delivery. Though the recent modeling development is encouraging, a comprehensive sediment connectivity framework, which integrates geomorphic and hydrologic processes across spatiotemporal scales, has not yet been accomplished. Such an effort requires understanding the hydrologic and geomorphic processes that control sediment source, storage, and transport at different spatiotemporal scales and across various geophysical conditions. We propose a path for developing this new understanding through an integrated hydrologic and sediment connectivity conceptual model that broadly categorizes dominant processes and patterns relevant to understanding sediment flux dynamics. The conceptual model describes hydrologic–sediment connectivity regimes through spatial-temporal feedback between hydrologic processes and geomorphic drivers. We propose that in combining hydrologic and sediment connectivity into a single conceptual model, patterns emerge such that catchments will exist in a single characteristic behavior at a particular instance, which would shift with space and time, and with landscape disturbances. Using the conceptual model as a “thinking” tool, we extract case studies from a multidisciplinary literature review—from hydrology, geomorphology, biogeochemistry, and watershed modeling to remote-sensing technology—that correspond to each of the dominant hydrologic–sediment connectivity regimes. Sediment and water interactions in real-world examples through various observational and modeling techniques illustrate the advancements in the spatial and temporal scales of landscape connectivity observations and simulations. The conceptual model and case studies provide a foundation for advancing the understanding and predictive capability of watershed sediment processes at multiple spatiotemporal scales. Plain language summary: Soil erosion and movement across the landscape are closely linked to rain events and flow pathways. Landscape connectivity is a way to consider how soil erosion from different parts of the landscape is connected to the streams. We explore where soil erosion occurs and how eroded soil moves across the landscape through the interaction with rainfall and drainage. The comprehensive understanding of sediment connectivity and its dependence on rainfall characteristics and watershed hydrology may help to inform the effective distribution of conservation funds and management actions to address water pollution from excess sediment.

&lt;p&gt;Storage of sediment on floodplains delays downstream sediment delivery, increasing the timescale of catchment responses to forcing by tectonics, climate changes, and watershed sediment management practices.&amp;#160; Including floodplain storage in catchment sediment routing models, however, is challenging because the long timescales involved exceed the duration of stream gaging station and other observational data sources.&amp;#160; As a result, floodplain storage is typically ignored in catchment sediment modeling.&amp;#160; To quantify timescales of sediment storage for mid-Atlantic U.S. floodplains since the early Holocene, floodplain sediment thickness distributions are defined for three time periods by analyzing stratigraphic data: presettlement (deposited before 1750), legacy (deposited 1750-1950), and modern (deposited after 1950).&amp;#160; These data are used to calibrate a model that predicts the thickness, age, and storage time distributions of floodplain deposits through time.&amp;#160; The model uses empirical equations to estimate changes in flood magnitude and duration caused by changes in forest cover and urban development.&amp;#160; Simple hydraulic models predict the occurrence of overbank flow based on channel geometry (which changes through time as floodplains accrete) and the potential for backwater induced by nearby milldams during the 19&lt;sup&gt;th&lt;/sup&gt; Century. Overbank deposition during overbank flows is predicted based on sediment concentration, sediment settling velocity, and overbank flow duration.&amp;#160; Sediment erosion is predicted based on the age distribution of stored sediment and a power law function that specifies the exposure of sediment to erosion by age category, an approach that is similar to the StorAge Selection Functions often used in catchment hydrologic modeling.&amp;#160; &amp;#160;The calibrated model, &amp;#8220;tuned&amp;#8221; to reproduce observed stratigraphic data, predicts monotonically increasing fluvial sediment concentrations from presettlement to modern time periods, and sediment budget components (input and output fluxes and rates of sedimentation and erosion) that also increase through time. &amp;#160;Predicted sediment residence times (mean age of stored sediment) vary from ~450 years in 1750 to ~300 years in 2017, and the model accurately reproduces the full age distribution (0 to &gt; 5000 yr) of stored sediment documented by contemporary stratigraphic data.&amp;#160; This calibrated model can accurately represent floodplain storage for improved watershed scale sediment routing computations in the mid-Atlantic region, improving our ability to manage Chesapeake Bay restoration and other important watershed sediment management issues.&lt;/p&gt;

As sediment is transported through river corridors, it typically spends more time in storage than transport, and as a result, sediment delivery timescales are controlled by the duration of storage. Present understanding of storage timescales is largely derived from models or from field studies covering relatively short (≤102 year) time spans. Here we quantify the storage time distribution for a 17 km length of Powder River in Montana, USA by determining the age distribution of eroded sediment. Our approach integrates surveyed cross‐sections, analysis of historical aerial imagery, aerial LiDAR, geomorphic mapping, and age control provided by optically stimulated luminescence (OSL) and dendrochronology. Sediment eroded by Powder River from 1998 to 2013 ranges from a few years to ∼5,000 years in age; ages are exponentially distributed (r2 = 0.78; Anderson‐Darling p value 0.003). Eroded sediment is derived from Powder River's meander belt (∼900 m wide), which is only 1.25 times its meander wavelength, a value reflecting valley confinement rather than free meandering. The mean storage time, 824 years (95% C.I. 610–1030 years), is similar to the time required to rework deposits of Powder River's meander belt based on an average meander migration rate of ∼1 m/yr, implying that storage time distributions of confined meandering rivers can be quantified from remotely sensed estimates of meander belt width and channel migration rates. Heavy‐tailed storage time distributions, frequently cited from physical and numerical modeling studies, may be restricted to unconfined meandering rivers.

Earth and Space Science Open Archive This preprint has been submitted to and is under consideration at Journal of Geophysical Research - Earth Surface. ESSOAr is a venue for early communication or feedback before peer review. Data may be preliminary.Learn more about preprints preprintOpen AccessYou are viewing the latest version by default [v1]Getting Beyond the Bankfull Shields Parameter: A Continuum of Threshold Channel Types Illustrated by the Case of the White Clay Creek, PAAuthorsSophieBodekiDJames E.PizzutoKristen M.McCarthyRaphaelAffinitoSee all authors Sophie BodekiDCorresponding Author• Submitting AuthorUniversity of DelawareiDhttps://orcid.org/0000-0003-0042-6688view email addressThe email was not providedcopy email addressJames E. PizzutoUniversity of Delawareview email addressThe email was not providedcopy email addressKristen M. McCarthyUniversity of Delawareview email addressThe email was not providedcopy email addressRaphael AffinitoUniversity of Delawareview email addressThe email was not providedcopy email address

Based on well‐developed hydraulic geometry relations for width and depth, classic studies initially interpreted the Mid‐Atlantic White Clay Creek (WCC) as a quasi‐equilibrium, alluvial channel. Subsequent studies document the legacy of colonial‐age watershed disturbances and urban development, confounding earlier classifications. To investigate this matter, we contribute new data from reach‐scale geomorphic mapping, and observations and modeling of bed material transport. WCC's longitudinal profile reflects a history of bedrock incision, while hydraulic geometry equations for width and depth indicate quasi‐equilibrium cross‐sectional adjustment. Alluvial landforms such as pools and riffles, bars, and actively forming floodplains occur at all 12 study sites, but exposures of bedrock and colluvium are also common. The ratio of bankfull to threshold Shields stress averages 1.41 (range: 0.41–2.63), suggesting that WCC is an alluvial, threshold, gravel‐bed river. However, a numerical model of WCC bed material transport and grain size, calibrated to bedload tracer data, demonstrates that 22% (range: 8%–73%) of bed material is composed of immobile, locally sourced cobbles and boulders, while the remaining bed material represents mobile, sand to cobble‐sized alluvium; this leads us to classify WCC as a semi‐alluvial river. Additional computations suggest that channel morphology is insensitive to bed material supply. Field observations imply that bankfull Shields stresses do not represent channel adjustments to achieve stable banks; rather, width adjustment likely reflects cohesive bank processes. Despite the numerous and contradictory labels applied to WCC (i.e., quasi‐equilibrium, Anthropocene, bedrock, semi‐alluvial, and gravel‐bed), each term contributes insight that any single conceptual model would be unable to provide alone.

As sediment is carried through watersheds, it may be stored in floodplains and other alluvial deposits, remaining in place for hundreds to millions of years before being remobilized and transported farther downstream. Sediment routing models based on reservoir theory can account for time-varying sediment storage and predict lags in sediment delivery imposed by sediment storage, but observational data are needed to construct and validate these models. Because of the long timescales involved, direct observations are rarely useful, but stratigraphic observations coupled with sediment dating techniques can be used to quantify the amount of sediment stored through time and its associated age and storage (or transit) time distributions. To illustrate this approach, a meta-analysis of published geologic data is used to quantify river corridor storage through time associated with European colonization of the mid-Atlantic U.S. The history of floodplain growth from Holocene to the present is summarized by empirical distributions extracted from stratigraphic data; distributions were sampled to create thousands of synthetic age-depth curves. Deposits predating European colonization range in age from >18,000 yrs. to 225 yrs. B.P. and with a median thickness of 40% of the total accumulation; sedimentation rates for these deposits are low (median = 0.06 cm/yr). The median thickness of sediments deposited between 1750 and 1900 (“legacy sediments”) comprises 36% of the total; the median accumulation rate of legacy sediments is 0.32 cm/yr. The median thickness of sediments deposited after 1950 represents 11% of total accumulation, and the median contemporary sedimentation rate of 0.26 cm/yr is statistically indistinguishable from that of legacy sediments. Synthetic vertical sequences can be recast as age distributions, and when combined with geomorphic mapping and assessment of patterns of erosion through time, as storage time distributions as well. Age and storage time distributions at 1000 yrs. B.P., in 1900 A.D., and at present are highly variable, and could be represented by many different mathematical functions, though averaged data appear to be heavy-tailed. Records of mass accumulation through time and the present and past age and storage time distributions provide useful summaries of the history of sediment storage, and can be used to calibrate and verify watershed scale sediment routing models over millennial timescales.

We measured stratigraphic sections 1.91 km upstream of a 1-m high extant colonial mill dam on the W. Br. White Clay Creek in southeastern Pennsylvania. Exposed deposits can be divided into 4 characteristic facies: 1) matrix-supported (define roundness) cobbles and boulders; 2) clast-supported (define roundness) pebbles and cobbles; 3) Sand and mud with organic fragments (wood, leaves, stems); and 4) massive muddy sand and sandy mud. Matrix-supported gravels are interpreted as colluvium; these deposits are extensive, often overlie bedrock, and may be exhibit deformed strata suggesting transport by mass wasting. Clast-supported gravels are basal deposits exposed as thin (< 0.5 m) layers, often immediately overlying matrix-supported gravels (colluvium), occasionally preserved as triangular-shaped lenses ~10m long and < 1 m thick. These deposits are interpreted as channel gravels, while the triangular lenses represent buried bars. At two sections, basal gravels are overlain by decimeter-thick, laterally discontinuous layers of sand and mud with abundant organic fragments. Leaves and wood from these deposits yield radiocarbon dates of 308-473 and 918-1000 years B.P., indicating that these deposits pre-date European settlement and its associated watershed disturbance. Massive muddy sand and sandy mud represent the uppermost 0.5 - 2 m of nearly all the sections. Excavations of exposed tree roots and dendrochronology demonstrate that the upper decimeters are contemporary overbank deposits. Deeper layers have similar characteristics and are therefore also interpreted as overbank deposits. A decimeter-thick, dark-colored layer is exposed at a few locations at depths of ~ 1m within this unit; this is interpreted as a buried A horizon representing the elevation of a slowly aggrading floodplain before European settlement. Overbank deposits increase in thickness immediately upstream of the mill dam, but laminated deposits typical of subaqueous deposition in mill ponds are absent. Valley-fill deposits of our study area are a complex mosaic of colluvium (likely pre-Holocene in age), pre-Settlement alluvium consisting of channel and bar gravels, low-lying floodplain deposits, and small localized wetland deposits, and post-Settlement overbank and lateral accretion deposits.