High Flow Events Research Articles

Molecular Level Analysis Reveals Changes in Chemical Composition of Dissolved Organic Matter From South Texas Rivers After High Flow Events

Riverine dissolved organic matter (DOM) is a major source of reduced carbon exported from land to marine environments, and the inflow of riverine organic matter greatly affects biogeochemical cycling in estuaries and bays. Thus, any change in DOM composition, such as changes caused by flood waters as a result of storms and hurricanes, can subsequently affect estuarine environments. To investigate the impact of high flow events on riverine DOM, multidimensional molecular level information of DOM from four south Texas Rivers (Aransas, Lavaca, Mission, and Nueces Rivers) was acquired using high-resolution Ion Mobility Quadrupole Time of Flight Liquid Chromatography Mass Spectrometry (IM Q-TOF LCMS). Base-flow samples were collected in May, July and October of 2016, June of 2017, and March of 2018, while high-flow samples were collected in September of 2017, and June and September of 2018. Based on the molecular formula assigned from IM Q-TOF LCMS, H/C ratio decreased during high-flow event (1.52 to 1.51 in ESI+; 1.19 to 1.07 in ESI-), while O/C ratio increased (0.31 to 0.33 in ESI-). Furthermore, DOM shifted from a protein-like and lipid-like dominated community at base flow condition, to a lignin, tannin and condensed aromatic structure dominated one during high flow event based on MS and tandem MS data. These changes in high-flow riverine DOM indicate an increase of terrestrial signal, which likely is a result of mobilization of terrestrial organic matter from the watersheds by flooding. These mobilized DOM, though refractory at high-flow condition in rivers, could be reactive in coastal regions when condition changes, and thus potentially fuel microbial activities downstream. In addition, about 3.76 – 21.8% of DOM molecules contain structural isomers among different flow conditions. This low number of isomer percentages suggests that as the products of various enzymatic biochemical reactions, the number of isomers in DOM is constrained. Taken together, our study provides insights into structural changes of riverine DOM in response to extreme climate events in subtropical regions and have implications in understanding biogeochemical changes in estuaries under a changing climate.

Influences of a River Dam on Delivery and Fate of Sediments and Particulate Nutrients to the Adjacent Estuary: Case Study of Conowingo Dam and Chesapeake Bay

Dams impact the magnitude and nature of material transport through rivers to coastal waters, initially trapping much material in upstream reservoirs. As reservoirs fill, trapping decreases and bottom sediments can be scoured by high flows, increasing downstream delivery. This is the case for the Conowingo Dam, which historically has trapped much of the sediment and particulate nutrients carried by the Susquehanna River otherwise bound for Chesapeake Bay but has now reached dynamic equilibrium. While previous studies primarily focus on either delivery of river inputs or their fate in the Bay, this study synthesizes insights from field observations and modeling along the Reservoir-Bay continuum to evaluate potential impacts of infilling on Bay biogeochemistry. Results show most Susquehanna sediment and particulate nutrient loading occurs during high-flow events that occur only ~ 10% of the time. While loading during these events has increased since the late 1970s, consistent with a decreasing scour threshold for Reservoir sediments, loading during low-flow periods has declined. Loads entering the estuary are largely retained within the upper Bay but can be transported farther downstream during events. Reservoir sediments are highly refractory, and inputs of reservoir-like organic matter do not enhance modeled sediment-nutrient release in upper Bay sediments. These findings and an emerging literature highlight the Bay’s resilience to large sediment loads during events (e.g., Tropical Storm Lee in 2011), likely aided by ongoing restoration efforts and/or consistently low-moderate recent inflows (2012–2017). Thus, while events can have major short-term impacts, the long-term impact to Bay biogeochemistry is less severe.

Urbanization in Arid Central Arizona Watersheds Results in Decreased Stream Flashiness

AbstractThere is broad consensus that urbanization results in dramatic changes in stream hydrology, such as higher peak flows and greater flashiness. However, this has not been definitively tested for aridlands, which are characterized by these very same hydrograph properties. We analyzed streamflow records from 19 watersheds of central Arizona, USA, to determine how hydrograph characteristics varied with urban development. Using linear mixed effects models, which factored in imperviousness along with other watershed characteristics, we evaluated influences on streamflow regime metrics calculated from daily and subdaily flow data. We found that flashiness, coefficient of variation, zero‐flow days, and hydrograph rise and fall rates decreased with extent of imperviousness—the opposite pattern to that observed in previous studies primarily in humid regions. Engineered retention basins are one explanation for this observation though novel urban sources of dry weather flows are likely also playing a role. We also found strong relationships between these hydrologic metrics and mean area‐weighted discharge, watershed area, and annual precipitation. Like in humid systems, we did observe more high flow events in the urban desert streams compared to nonurban desert streams. However, this was only at the lower flood threshold; there was no increase in larger floods with urban development. Overall, the urban stream syndrome manifests differently in this arid system: Urbanization increases water retention and leads to less variable flows in stream ecosystems.

Morphometric diversity of supply-limited and transport-limited river systems in the Himalayan foreland

The Indo-Gangetic basin exhibits highly diverse hydro-geomorphic settings that influence the hydrology, sediment production, and transport rates of the rivers draining this region, and this, in turn, is manifested in morphometric diversity of river systems. The rivers draining the western Gangetic plains (WGP) show incised channels attributable to high stream power and low sediment yield. In contrast, the rivers draining the Eastern Gangetic Plains (EGP) have formed aggradational landforms such as megafans due to low stream power and high sediment yield. For a better process understanding of the causal factors of such spatial inhomogeneities in a river basin, a morphometric characterization of its hinterland is necessary. In this work, a set of morphometric parameters such as stream network, longitudinal profile, and hypsometric curve have been integrated with data on sediment connectivity and stream power index to establish the form-process relationships in two contrasting basins, the upper Ganga basin (UGB) and the upper Kosi basin (UKB). The results of this study suggest that high sediment flux in the UKB is attributed to high rainfall, steep topography, unstable channel longitudinal profiles, and high sediment connectivity. A large part of these sediments gets deposited within the channel belt in the alluvial reaches and is remobilized during high flow events. However, the UGB produces fewer sediments due to low rainfall, mild topography, stable channel longitudinal profile, and low sediment connectivity. Also, the presence of a major dam at Tehri further reduces the functional sediment connectivity in the Upper Ganga Basin.

Long run optimization of landscape level irrigation through managed aquifer recharge or expanded surface reservoirs

Decades of efforts to reduce groundwater overdraft through investment in irrigation efficiency has, in many cases, failed to reduce aquifer depletion. This is one reason for greater attention to water supply augmentation efforts through the capture of surface water during high flow events for managed aquifer recharge (MAR) or additional surface storage. We use a landscape level model to examine the optimal interaction among MAR, surface reservoir storage, crop choice, and groundwater conservation policies in Eastern Arkansas. We find the cost of MAR water must be less than half the average groundwater pumping cost to increase the aquifer level. Increases in irrigation intensive crops mean that only a fraction of the recharged water raises groundwater levels. We observe that policies to conserve groundwater, such as a pumping cap or tax, are more cost-effective for society with a lower cost of MAR water.

Contrasting behavior of nitrate and phosphate flux from high flow events on small agricultural and urban watersheds

Soluble nutrient flux is increasingly implicated in the degradation of receiving water quality. With high-frequency sensors, river discharge along with nitrate and orthophosphate concentrations were collected over 2 years. We examined how storms (16 events) affected the dissolved nutrient flux in two watersheds with contrasting landcover—urban and agricultural. The adjacent watersheds, both < 7 km2, had very similar soil, slope, relief, and physiography. Wastewater is conveyed outside of the watersheds minimizing this nutrient source. Cumulative nitrate and orthophosphate fluxes in the agricultural watershed were substantially higher than the urban watershed. In both watersheds, a disproportionate amount of discharge, higher orthophosphate concentrations and flux occurred during the highest flow events—approximately 50–70% of flux occurring in the highest 5% of daily discharge. The nitrate flux was dampened compared to discharge during storm events. Baseflow accounted for 66% of the nitrate flux and had higher or comparable concentrations than storm events. The pattern of flux and concentration within storm events also differed for the two solutes. Nitrate exhibited positive hysteresis (higher concentrations on the rising limb of the hydrograph) and magnification of flux during the first flush of storm runoff. In contrast, orthophosphate concentrations were lower on the rising limb and flux during the first flush of storm runoff was dampened. In addition to targeting source reductions, orthophosphate flux may require runoff reducing strategies (i.e., enhanced infiltration) from large runoff events. First flush stormwater management practices combined with enhancement/protection of groundwater nitrate sinks (e.g., riparian wetlands) may help reduce nitrate fluxes.

Atmospheric circulation amplifies shift of winter streamflow in southern Ontario

Flooding is a major concern for Canadian society as it is the costliest natural disaster type in Canada. Southern Ontario, which houses one-third of the Canadian population, is located in an area of high vulnerability for floods. The most significant floods in the region have historically occurred during the months of March and April due to snowmelt coupled with extreme rain events. However, during the last three decades, there has been a shift of flooding events to earlier months. The aim of this study was to understand the impacts of atmospheric circulation on the temporal shift of streamflow and high flow events observed in southern Ontario over 1957–2013 period. Predominant weather regimes over North America, corresponding to recurrent meteorological situations, were identified using a discretization of daily geopotential height at 500HpA level (Z500). A regime-normalized hypothetical temperature and precipitation dataset was constructed to quantify the contribution of atmospheric circulation on streamflow response. The hypothetical dataset was used as input in the Precipitation Runoff Modeling System (PRMS), a rainfall-runoff semi-distributed hydrological model, and applied to four watersheds in southern Ontario. The results showed an increase in the temporal frequency of the regime identified here as High Pressure (HP) close to eight occurrences per decade. Regime HP, characterized by a northern position of the polar vortex, is correlated with a positive phase of the NAO and is associated with warm and wet conditions over southern Ontario during winter. The temporal increase in HP contributed more than 40% of the increase in streamflow in winter and 30–45% decrease in streamflow in April. This atmospheric situation also contributed to increase the number of high flows by 25–50% in January. These results are important to improve the seasonal forecasting of high flows and to assess the uncertainty in the temporal evolution of streamflow in the Great Lakes region.

River Bed Elevation Variability Reflects Sediment Supply, Rather Than Peak Flows, in the Uplands of Washington State

AbstractRiver channel beds aggrade and incise through time in response to temporal variation in the upstream supply of water and sediment. However, we lack a thorough understanding of which of these is the dominant driver of channel bed elevation change. This lack hampers flood hazard prediction, as changes to the bed elevation can either augment or reduce flood heights. Here, we explore the drivers of channel change using multidecadal time series of river bed elevation at 49 United States Geological Survey (USGS) gage sites in the uplands of Washington State, USA. We find that channel bed elevations at many of the gages change remarkably little over &gt;80 years, while others are highly unstable. Despite regionally synchronous decadal fluctuations in flood intensity, there is a lack of regional synchrony of channel response at the decadal scale. At the monthly scale, the magnitude of antecedent high flow events between gage measurements does not predict either the direction or magnitude of shift in channel bed elevation. That variations in flood magnitude are insufficient to explain changes in bed elevation suggests that fluctuations in sediment supply, rather than variation in peak flows, are the primary driver of change to river bed elevation. In this region, channels downstream from glaciers have statistically significantly greater variability in bed elevation compared to those lacking upstream glaciers. Together, these findings suggest that aggradation and incision signals in this region predominately reflect fluctuations in sediment supply, commonly associated with glaciogenic sources, rather than response to high flow events.

Unclogging improvement based on interdate and interreach comparison of water release monitoring (Durance, France)

AbstractLarge‐scale flow release experiments are becoming common for improving aquatic habitat quality downstream of dams. Because of the naturally high fine sediment load in the Durance River, France due to inputs from torrential tributaries draining badlands, unpredictable high flow events with dam overspill are not always sufficient to prevent clogging, which can lead to habitat degradation (especially spawning habitats) in bypassed reaches. Therefore, large‐scale flow experiments were conducted on four reaches to test the efficacy of clear‐water releases to improve aquatic habitat conditions. Continuous measurements of water depth, suspended sediment concentrations, and turbidity were conducted during twelve releases and compared on nine. Before and after each release, superficial clogging was measured. The study shows that there is a link between the volume of suspended sediments carried by water releases and the initial clogging. The volumes carried were low compared with the river's annual transport. The effect on clogging can vary significantly from one release to another. In particular, the hydrological context surrounding the releases has a significant influence on clogging. Comparisons of monitoring showed that releases are more effective on reaches that are more severely regulated (high hydrological control) than on those that are less well‐controlled. The areas with the highest initial clogging tend to unclog more than those with lower initial clogging; however, the latter zones are most impacted by ineffective releases. Performing a release on environments with low initial clogging can therefore be environmentally damaging. To ensure that releases are successful and that intervention is warranted, initial clogging should be measured in the field and releases should only be performed if the clogging is judged to be unfavourable.

On the choice of calibration metrics for “high-flow” estimation using hydrologic models

Abstract. Calibration is an essential step for improving the accuracy of simulations generated using hydrologic models. A key modeling decision is selecting the performance metric to be optimized. It has been common to use squared error performance metrics, or normalized variants such as Nash–Sutcliffe efficiency (NSE), based on the idea that their squared-error nature will emphasize the estimates of high flows. However, we conclude that NSE-based model calibrations actually result in poor reproduction of high-flow events, such as the annual peak flows that are used for flood frequency estimation. Using three different types of performance metrics, we calibrate two hydrological models at a daily step, the Variable Infiltration Capacity (VIC) model and the mesoscale Hydrologic Model (mHM), and evaluate their ability to simulate high-flow events for 492 basins throughout the contiguous United States. The metrics investigated are (1) NSE, (2) Kling–Gupta efficiency (KGE) and its variants, and (3) annual peak flow bias (APFB), where the latter is an application-specific metric that focuses on annual peak flows. As expected, the APFB metric produces the best annual peak flow estimates; however, performance on other high-flow-related metrics is poor. In contrast, the use of NSE results in annual peak flow estimates that are more than 20 % worse, primarily due to the tendency of NSE to underestimate observed flow variability. On the other hand, the use of KGE results in annual peak flow estimates that are better than from NSE, owing to improved flow time series metrics (mean and variance), with only a slight degradation in performance with respect to other related metrics, particularly when a non-standard weighting of the components of KGE is used. Stochastically generated ensemble simulations based on model residuals show the ability to improve the high-flow metrics, regardless of the deterministic performances. However, we emphasize that improving the fidelity of streamflow dynamics from deterministically calibrated models is still important, as it may improve high-flow metrics (for the right reasons). Overall, this work highlights the need for a deeper understanding of performance metric behavior and design in relation to the desired goals of model calibration.

An experimental and theoretical analysis of floating wood diffusion coefficients

Wood transport during high flow events is here treated as an advective–diffusive phenomenon. A theoretical definition of the governing equation is first provided, highlighting the dependence of floating wood diffusion on the adoption of adequate diffusion coefficients. To estimate these coefficients, an experimental investigation on large wood debris was carried out in a channel presenting a weakly sinusoidal plant, employing regular cylinders with various sizes under different flow conditions. For each configuration, a consistent number of trajectories for the floating wood were acquired and processed through imaging techniques, allowing for a statistical analysis of the wood dynamics. The tests showed that floating logs travel at a velocity lower than the water surface one, not completely aligned to the flow direction and tend to disperse streamwise and transversely. Under the assumption that wood dispersion can be derived from the analysis of wood trajectories fluctuations, the longitudinal, transversal and angular diffusion coefficients were computed. Finally, a preliminary dimensional analysis is presented discussing the relevant spatial and temporal scales for these coefficients.

Application of continuous turbidity sensors to supplement estimates of total phosphorus concentrations in the Grand River, Ontario, Canada

Accurate estimates of total phosphorus (TP) loadings to eastern Lake Erie are critical for developing load reduction targets and for determining if commitments are being met under the Great Lakes Water Quality Agreement, 2012 (GLWQA). Currently, loading calculations from Canadian priority tributaries are supported by year-round event-focused water quality sampling using automated samplers and laboratory water quality measurements. Here we evaluate the suitability of continuously-measured parameters, namely turbidity and flow, to supplement or enhance knowledge about TP concentrations in the Grand River, ON, by providing continuous data alongside event-focused sample measurements. A series of simple and multiple linear regression models were evaluated and compared with respect to their ability to predict TP water concentrations as a function of different combinations of explanatory variables. Explanatory variables included turbidity, flow, season and flow condition (i.e. hysteresis). The models that performed best explained 63–65% of the variation of TP which is comparable to surrogate model applications in the U. S and elsewhere. Additional model calibration work is needed due to gaps in turbidity data particularly during high flow events. We emphasize the need for continued automated, event-focused water quality sampling. However, provided that operational challenges are overcome, our results indicate that sensor-derived water quality parameters to predict TP concentrations is a promising technique that may supplement and improve nutrient loading estimates in the Grand River into the future and provides guidance for the utilization of this method in other tributaries.

Channel geomorphology differences between stream reaches with grass- or tree-dominated riparian vegetation in southern Manitoba

Removal of trees for pasture or crop production is common along the stream reaches in the Canadian Prairies, resulting in a patchwork of forested and nonforest riparian vegetation along most streams. The effect of vegetation type on channel geomorphology and potential to influence sediment dynamics was studied using eight paired reaches (forested and nonforest) within agricultural watersheds in southern Manitoba, Canada. High potential for bank erosion was observed at all sites (bank erosion hazard index scores), but Pfankuch channel stability scores were significantly higher for forested reaches compared with nonforested reaches. Furthermore, forested reaches had higher width to depth ratios, but flood-prone widths did not differ significantly, resulting in lower entrenchment ratios. Reduced channel width and cross-sectional area in nonforested reaches created an overall reduction of in-stream habitat, increased velocity, and increased potential for exceedance of channel capacity and floodplain access during high-flow events. Channel widening in response to riparian afforestation efforts has been observed in a variety of other locations globally and the results of this study suggest that widening with afforestation can still be anticipated in this region where stream gradients are low, hydrology is dominated by snowmelt, and forest cover is minimal.

Enterolithic folds in evaporites as microbially induced sedimentary structures: New model of formation and interpretation in the geological record

AbstractEnterolithic structures are stratigraphically localized folds in gypsum beds found in certain saline evaporitic sedimentary units in a wide variety of basins. Different models of formation have been proposed, all related to inorganic processes. These models include: diagenetic transformation of gypsum beds producing either displacive growth of crystals or volume changes; mechanical folding caused by compressional stress; and folding produced by slumping. The analysis of three Cenozoic evaporite sequences in Spain reveals that none of the previous models explains their origin and existence. In these outcrops, gypsum enterolithic structures occur in horizontal beds with parallel troughs and crests of the folds. They appear in shoreline facies of lacustrine environments and did not undergo major diagenetic transformations after the primary lithification of the original sediment. Based on these observations, together with the study of a modern analogue in Minorca, Spain, a new model is proposed for the genesis of enterolithic structures. This new model is based on the existence of a microbial mat exposed to brine concentration–dilution cycles and strong wind events. The high wind flow events enhanced folding of the microbial mat that became subaerially exposed and lithified due to subsequent evaporation. Therefore, the presence of enterolithic structures could be used as an indicator of shallow water environmental conditions subject to variations in brine concentration in areas with strong wind flow events. Previous studies of some evaporitic successions should be revisited, taking into account the proposed model, which would imply new depositional environment interpretations. At the same time, the proposed model could explain the existence of Kinneyia‐type structures, also known as wrinkle structures, formed beneath microbial mats in peritidal zones. Moreover, considering enterolithic structures as microbially induced sedimentary structures could be useful as evidence of microbial life in the ancient geological record and on other planets such as Mars.

Assessment of large-scale patterns of hydrological alteration caused by dams

Nowadays, alteration of the natural flow regime is considered one the most widespread and damaging impacts for river ecosystems. Hence, increasing our understanding of large-scale hydrological alteration patterns would help us design more effective water use policies. The present study aims to establish general patterns of hydrological alteration caused by dams on a national level, with Spain as a case study. First, we developed a classification of the natural flow regime of the Spanish river network, which served as the reference to assess the degree of hydrological alteration of 139 altered-river gauges. In addition, using the flow series of the altered-river gauges we defined a set of 7 types of altered regimes (TARs), which allowed the stratification of the analyses. The results revealed that the magnitude and direction of hydrological alteration depended on the natural flow class of the altered rivers. In this regard, major effects of dams on Spanish rivers were related to the modification of the intra-annual variability of daily flow, the magnitude of seasonal maximum and minimum flows and the patterns of high flow events. Our results also showed that the distribution of the TARs partially followed a geographic order, but associations between TARs and natural flow classes were not straightforward. In addition, we highlighted that the nature of the hydrological alteration was independent of the registered dam uses.

Occurrence, fate, and confounding influence of ghost passive integrated transponder tags in an intensively monitored watershed

Over the last three decades, passive integrated transponder (PIT) tags have been widely used to study fish populations. Interpretation of PIT tag detections, however, can be confounded by the presence of ghost tags, tags liberated when a fish dies. We used a combination of mobile antenna surveys, stationary antenna detections, and multistate mark–recapture modeling to assess the abundance and fate of ghost tags in a coastal California watershed. Accumulation of ghost tags from released hatchery-origin coho salmon (Oncorhynchus kisutch) smolts was substantial during California’s recent drought, with 2224 ghost tags identified during mobile reader surveys. Between surveys, PIT tags moved downstream a median distance of 346 m and a maximum distance of 1982 m. Stationary antenna array detections indicated that these movements occurred during high-flow events, concurrent with live fish movement. The multistate model estimated that, during winter, approximately 40% of tags were buried in the substrate beyond the read range of mobile readers. Failure to account for transport and burial dynamics of ghost tags can lead to biased estimates of fish abundance, survival, and movement.

The influence of multiple controls on structural and functional characteristics of macroinvertebrate community in a regulated Alpine river

AbstractIn the past 30 years, several studies have proved that the river flow regime is fundamental in structuring biotic communities. The condition of the ecosystem results not only from the occurrence of extreme events, such as floods and droughts but also from habitat availability and from its temporal variation. Describing the relationships between macroinvertebrate community characteristics and environmental gradients of flow, temperature, nutrient supplies, and habitat conditions is fundamental to understand ecological dynamics. This is the basis for predicting changes within the communities and in ecosystem functions and ultimately to properly manage and conserve the riverine ecosystems. Seven sites, along a 20‐km river sector, were surveyed for macroinvertebrates and water chemistry seasonally, from 2012 to 2016. Habitat conditions were assessed along a 500‐m stretch in each site. The river discharge was continuously monitored by two water level recorders and used to reconstruct various hydrological indices specific for each sampling location. During the sampling period, numerous high flow events and some prolonged periods of low flow were observed. Quantile regression was used to describe the effects of potential limiting factors on macroinvertebrate community, which were primarily driven by antecedent flow conditions and season whereas habitat conditions and water chemistry played only a minor role. Quantitative models have been developed to predict structural and functional characteristics of macroinvertebrate community as a function of antecedent flow conditions, habitat, and physicochemical water characteristics. Those models allow to identify the main drivers and predict the effect of different water management strategies to riverine ecosystem.

Using geochemistry to discern the patterns and timescales of groundwater recharge and mixing on floodplains in semi-arid regions

Floodplains may alternate between discharge zones for regional groundwater and areas of recharge from river water during high-flow events. Understanding the mechanisms and timescales of recharge on floodplains is important for their management and for the protection of fragile ecosystems. The floodplains of the River Murray host important ecosystems, particularly remnant eucalypt forests that are vulnerable to changes in inundation, rising regional water tables, and salt accumulation. This study addresses floodplain recharge and groundwater mixing on the Pike and Katarapko floodplains of South Australia. At Pike, 3H activities of groundwater in the low hydraulic conductivity Coonambidgal Formation that crops out on the floodplain are ∼0.25 TU. 3H activities of groundwater in the underlying higher hydraulic conductivity Monoman Formation decrease from ∼1 TU near the contact with the Coonambidgal Formation to <0.02 TU at >15 m depth. Groundwater 14C activities are between 40 and 95 pMC but are less well correlated with depth. The 3H or 14C activities do not vary systematically with distance from the surface water channels on the floodplains. These observations imply that groundwater recharge at Pike is dominantly through the floodplain rather than through the channel banks. In contrast to the regional groundwater where total dissolved solids (TDS) concentrations are commonly >35,000 mg/L, the TDS of groundwater on the floodplain is locally <500 mg/L. A correlation between 3H activities and TDS and the presence of groundwater with relatively low 14C activities but above detection 3H activities implies that recently recharged waters have mixed with regional groundwater in the floodplain sediments. The 3H activities in these mixed waters implies that mixing occurs over a few years. By contrast, at Katarapko, the highest 3H activities in the Monoman Formation groundwater (up to 2.35 TU) are closer to the Murray River, implying that recharge through the bank may occur. These contrasting patterns of recharge probably reflect local topographic controls. Understanding the recharge-discharge relationships are vital for managing proposed floodplain inundation programs aimed at improving ecosystem health.

Terrestrial laser scanning and structure-from-motion photogrammetry concordance analysis for describing the surface layer of gravel beds

Terrestrial laser scanning (TLS) and structure-from-motion photogrammetry (SfMp) offer rapid, non-invasive surveying of in situ gravels. Numerous studies have used the point clouds derived from TLS or SfMp to quantify surface layer characteristics, but direct comparison of the methods for grain-scale analysis has received relatively little attention to date. Comparing equivalent products of different data capture methods is critical as differences in errors and sampling biases between the two methods may produce different outputs, effecting further analysis. The sampling biases and errors related to SfMp and TLS lead to differences in the point clouds produced by each method. The metrics derived from the point clouds are therefore likely to differ, potentially leading to different inputs for entrainment threshold models, different trends in surface layer development being identified and different trajectories for physical processes and habitat quality being predicted. This paper provides a direct comparison between TLS and SfMp surveys of an exposed gravel bar for three different survey periods following inundation and reworking of the bar surface during high flow events. The point clouds derived from the two methods are used to describe changes in the character of the surface layer between bar inundation events, and comparisons are made with descriptions derived from conventional pebble counts. The results found differences in the metrics derived using each method do exist, but the grid resolution used to detrend the surfaces and identify spatial variations in surface layer characteristics had a greater impact than survey method. Further research is required to understand the significance of these variations for quantifying surface texture and structure and for predicting entrainment thresholds and transport rates.

Evaluation of a simple, inexpensive, in situ sampler for measuring time‐weighted average concentrations of suspended sediment in rivers and streams

AbstractThe accurate measurement of suspended sediment (&lt;200 μm) in aquatic environments is essential to understand and effectively manage changes to sediment, nutrient, and contaminant concentrations on both temporal and spatial scales. Commonly used sampling techniques for suspended sediment either lack the ability to accurately measure sediment concentration (e.g., passive sediment samplers) or are too expensive to deploy in sufficient number to provide landscape‐scale information (e.g., automated discrete samplers). Here, we evaluate a time‐integrated suspended sediment sampling technique, the pumped active suspended sediment (PASS) sampler, which collects a sample that can be used for the accurate measurement of time‐weighted average (TWA) suspended sediment concentration and sediment particle size distribution. The sampler was evaluated against an established passive time‐integrated suspended sediment sampling technique (i.e., Phillips sampler) and the standard discrete sampling method (i.e., manual discrete sampling). The PASS sampler collected a sample representative of TWA suspended sediment concentration and particle size distribution of a control sediment under laboratory conditions. Field application of the PASS sampler showed that it collected a representative TWA suspended sediment concentration and particle size distribution during high flow events in an urban stream. The particle size distribution of sediment collected by the PASS and Phillips samplers were comparable and the TWA suspended sediment concentration of the samples collected using the PASS and discrete sampling techniques agreed well, differing by only 4% and 6% for two different high flow events. We should note that the current configuration of the PASS sampler does not provide a flow‐weighted measurement and, therefore, is not suitable for the determination of sediment loads. The PASS sampler is a simple, inexpensive, and robust in situ sampling technique for the accurate measurement of TWA suspended sediment concentration and particle size distribution.