High Flow Events Research Articles

Observations of bedload transport in a gravel bed river during high flow using fiber‐optic DTS methods

AbstractThe question: ‘how does a streambed change over a minor flood?’ does not have a clear answer due to lack of measurement methods during high flows. We investigate bedload transport and disentrainment during a 1.5‐year flood by linking field measurements using fiber optic distributed temperature sensing (DTS) cable with sediment transport theory and an existing explicit analytical solution to predict depth of sediment deposition from amplitude and phase changes of the diurnal near‐bed pore‐water temperature. The method facilitates the study of gravel transport by using near‐bed temperature time series to estimate rates of sediment deposition continuously over the duration of a high flow event coinciding with bar formation. The observations indicate that all gravel and cobble particles present were transported along the riffle at a relatively low Shields Number for the median particle size, and were re‐deposited on the lee side of the bar at rates that varied over time during a constant flow. Approximately 1–6% of the bed was predicted to be mobile during the 1.5‐year flood, indicating that large inactive regions of the bed, particularly between riffles, persist between years despite field observations of narrow zones of local transport and bar growth on the order ~3–5 times the median particle size. In contrast, during a seven‐year flood approximately 8–55% of the bed was predicted to become mobile, indicating that the continuous along‐stream mobility required to mobilize coarse gravel through long pools and downstream to the next riffle is infrequent. Copyright © 2017 John Wiley & Sons, Ltd.

Quantitative hydrological preferences of benthic stream invertebrates in Germany

Current knowledge regarding the flow preferences of benthic stream invertebrates is mostly based on qualitative data or expert knowledge and literature analysis. These established flow preferences are difficult to use in predictions of the effects of global change on aquatic biota. To complement the existing categories, we performed a large-scale analysis on the distribution of stream invertebrates at stream monitoring sites in order to determine their responses to various hydrological conditions.We used 325 invertebrate surveys from environmental agencies at 238 sites paired to 217 gauges across Germany covering a broad range of hydrological conditions. Based on these data, we modelled the respective probabilities of occurrences for 120 benthic invertebrate taxa within this hydrological range using hierarchical logistic regression models.Our analyses revealed that more than one-third of the taxa (18–40%) can be considered as ubiquitous and having a broad hydrological tolerance. Furthermore, 22–41% of the taxa responded to specific ranges of flow conditions with detectable optima. “Duration high flow event” represented the flow parameter that correlated best with the abundance of individual taxa, followed by “rate of change average event”, with 41 and 38% of the taxa showing a peak in their probability of occurrence at specific ranges of these metrics, respectively. The habitat suitability for these taxa may be potentially affected by global change-induced hydrological changes.Quantified hydrological traits of individual taxa might therefore support stream management and enable the prediction of taxa responses to flow alteration. The hydrological traits of stream benthic invertebrates may be used in forecasting studies in central Europe, and the methods used in this study are suitable for application in other regions with different flow regimes.

Quantifying habitat benefits of channel reconfigurations on a highly regulated river system, Lower Missouri River, USA

We present a quantitative analysis of habitat availability in a highly regulated lowland river, comparing a restored reach with two reference reaches: an un-restored, channelized reach, and a least-altered reach. We evaluate the effects of channel modifications in terms of distributions of depth and velocity as well as distributions and availability of habitats thought to be supportive of an endangered fish, the pallid sturgeon (Scaphirhynchus albus). It has been hypothesized that hydraulic conditions that support food production and foraging may limit growth and survival of juvenile pallid sturgeon. To evaluate conditions that support these habitats, we constructed two-dimensional hydrodynamic models for the three study reaches, two located in the Lower Missouri River (channelized and restored reaches) and one in the Yellowstone River (least-altered reach). Comparability among the reaches was improved by scaling by bankfull discharge and bankfull channel area. The analysis shows that construction of side-channel chutes and increased floodplain connectivity increase the availability of foraging habitat, resulting in a system that is more similar to the reference reach on the Yellowstone River. The availability of food-producing habitat is low in all reaches at flows less than bankfull, but the two reaches in the Lower Missouri River – channelized and restored – display a threshold-like response as flows overtop channel banks, reflecting the persistent effects of channelization on hydraulics in the main channel. These high lateral gradients result in punctuated ecological events corresponding to flows in excess of bankfull discharge. This threshold effect in the restored reach remains distinct from that of the least-altered reference reach, where hydraulic changes are less abrupt and overbank flows more gradually inundate the adjacent floodplain. The habitat curves observed in the reference reach on the Yellowstone River may not be attainable within the channelized system on the Missouri River, but the documented hydraulic patterns can be used to inform ongoing channel modifications. Although scaling to bankfull dimensions and discharges provides a basis for comparing the three reaches, implementation of the reference reach concept was complicated by differences in flow-frequency distributions among sites. In particular, habitat availability in the least-altered Yellowstone River reach is affected by increased frequency of low-flow events (less than 0.5 times bankfull flow) and moderately high-flow events (greater than 1.5 times bankfull flow) compared to downstream reaches on the Lower Missouri River.

Spatial distribution, total length frequencies and otolith morphometry as tools to analyse the effects of a flash flood on populations of roach (Rutilus rutilus)

AbstractFloods are known to be the major source of natural variability and disturbance in stream ecosystems. However, the management and channelisation of large rivers have impacted the fluvial geomorphology and disconnected the main channel and floodplains used as nurseries by many species of fish. This study examines the influence of a first autumnal flood event of the Ardèche River on the roach population, Rutilus rutilus (Linnaeus, 1758), in the Caderousse reach on the Rhône River. This influence was evaluated through an analysis of the spatial distribution for the abundance and size of juvenile roach and an analysis of otolith shape. Juvenile roach from the three sections of the reach formed well‐defined local populations before and after the flood event. Juvenile roach were always more abundant in the less modified section (BPS) of the river than in the other sections (RES and TAIL). While the original primary river channel was directly impacted by the flash flood, no significant difference in otolith shape was observed before or after the flash flood. Our results suggest that this part of the reach provides refuge habitats used by fish during high flow events.

SPM response to tide and river flow in the hyper-turbid Ems River

In this paper, we analyse the behaviour of fine sediments in the hyper-turbid Lower Ems River, with focus on the river’s upper reaches, a stretch of about 25 km up-estuary of Terborg. Our analysis is based on long records of suspended particulate matter (SPM) from optical backscatter (OBS) measurements close to the bed at seven stations along the river, records of salinity and water level measurements at these stations, acoustic measurements on the vertical mud structure just up-estuary of Terborg and oxygen profiles in the lower 3 m of the water column close to Leerort and Terborg. Further, we use cross-sectionally averaged velocities computed with a calibrated numerical model. Distinction is made between four timescales, i.e. the semi-diurnal tidal timescale, the spring–neap tidal timescale, a timescale around an isolated peak in river flow (i.e. about 3 weeks) and a seasonal timescale. The data suggest that a pool of fluid/soft mud is present in these upper reaches, from up-estuary of Papenburg to a bit down-estuary of Terborg. Between Terborg and Gandersum, SPM values drop rapidly but remain high at a few gram per litre. The pool of fluid/soft mud is entrained/mobilized at the onset of flood, yielding SPM values of many tens gram per litre. This suspension is transported up-estuary with the flood. Around high water slack, part of the suspension settles, being remixed during ebb, while migrating down-estuary, but likely not much further than Terborg. Around low water slack, a large fraction of the sediment settles, reforming the pool of fluid mud. The rapid entrainment from the fluid mud layer after low water slack is only possible when the peak flood velocity exceeds a critical value of around 1 m/s, i.e. when the stratified water column seems to become internally supercritical. If the peak flood velocity does not reach this critical value, f.i. during neap tide, fluid mud is not entrained up to the OBS sensors. Thus, it is not classical tidal asymmetry, but the peak flood velocity itself which governs the hyper-turbid state in the Lower Ems River. The crucial role of river flow and river floods is in reducing these peak flood velocities. During elongated periods of high river flow, in e.g. wintertime, SPM concentrations reduce, and the soft mud deposits consolidate and possibly become locally armoured as well by sand washed in from the river. We have no observations that sediments are washed out of the hyper-turbid zone. Down-estuary of Terborg, where SPM values do not reach hyper-turbid conditions, the SPM dynamics are governed by classical tidal asymmetry and estuarine circulation. Hence, nowhere in the river, sediments are flushed from the upper reaches of the river into the Ems-Dollard estuary during high river flow events. However, exchange of sediment between river and estuary should occur because of tide-induced dispersion.

Modeling sediment mobilization using a distributed hydrological model coupled with a bank stability model

AbstractIn addition to surface erosion, stream bank erosion and failure contributes significant sediment and sediment‐bound nutrients to receiving waters during high flow events. However, distributed and mechanistic simulation of stream bank sediment contribution to sediment loads in a watershed has not been achieved. Here we present a full coupling of existing distributed watershed and bank stability models and apply the resulting model to the Mad River in central Vermont. We fully coupled the Bank Stability and Toe Erosion Model (BSTEM) with the Distributed Hydrology Soil Vegetation Model (DHSVM) to allow the simulation of stream bank erosion and potential failure in a spatially explicit environment. We demonstrate the model's ability to simulate the impacts of unstable streams on sediment mobilization and transport within a watershed and discuss the model's capability to simulate watershed sediment loading under climate change. The calibrated model simulates total suspended sediment loads and reproduces variability in suspended sediment concentrations at watershed and subbasin outlets. In addition, characteristics such as land use and road‐to‐stream ratio of subbasins are shown to impact the relative proportions of sediment mobilized by overland erosion, erosion of roads, and stream bank erosion and failure in the subbasins and watershed. This coupled model will advance mechanistic simulation of suspended sediment mobilization and transport from watersheds, which will be particularly valuable for investigating the potential impacts of climate and land use changes, as well as extreme events.

Capturing Excess Nutrients from Waterways

Capturing Excess Nutrients from Waterways

Capacity of semi-parametric regression models to predict extreme-event water quality in the Northeastern US

This study assessed the capacity of semi-parametric regression models to predict riverine solute concentrations during extreme high-flow hydrologic events, when such events are absent from the models' calibration data. Using a large dataset from 459 monitoring stations across the US Northeast, the models showed a tendency to overpredict extreme-event concentrations, with increasing bias and variance for increasingly extreme hydrologic conditions. The validation framework in this study effectively compared model performance across disparate hydrologic regimes and constituents, yet can be used to estimate individual model performance under an unobserved extreme-flow condition, regardless of whether any extreme-flow data are available for that model. The validation procedure can further be generalized to explore model performance in an arbitrarily defined extreme condition for a broad range of model types. Despite an overall increase in uncertainty for extreme-event concentration estimates, estimates under extreme hydrologic conditions could be improved by taking into account the observed bias in the aggregated regional database.

Legacy of contaminant N sources to the NO3\u2212 signature in rivers: a combined isotopic (\u03b415N-NO3\u2212, \u03b418O-NO3\u2212, \u03b411B) and microbiological investigation

Nitrate content of surface waters results from complex mixing of multiple sources, whose signatures can be modified through N reactions occurring within the different compartments of the whole catchment. Despite this complexity, the determination of nitrate origin is the first and crucial step for water resource preservation. Here, for the first time, we combined at the catchment scale stable isotopic tracers (δ15N and δ18O of nitrate and δ11B) and fecal indicators to trace nitrate sources and pathways to the stream. We tested this approach on two rivers in an agricultural region of SW France. Boron isotopic ratios evidenced inflow from anthropogenic waters, microbiological markers revealed organic contaminations from both human and animal wastes. Nitrate δ15N and δ18O traced inputs from the surface leaching during high flow events and from the subsurface drainage in base flow regime. They also showed that denitrification occurred within the soils before reaching the rivers. Furthermore, this study highlighted the determinant role of the soil compartment in nitrate formation and recycling with important spatial heterogeneity and temporal variability.

Estimation of Amazon River discharge based on EOF analysis of GRACE gravity data

River discharge is a critical component for understanding hydrological processes and sustainable management of water resources. The importance of discharge observation has increased due to its potential extreme variation resulting from the projected climate change and stronger variability of precipitation and temperature in some large basins. However, inherent difficulties in ground-based observations and decreasing number of gauge stations hinder accurate measurement of global river discharge and its spatio-temporal variations. Various remote sensing methods have been examined as alternatives, however, they require ground measurements to convert their proxy measurements into the actual river discharge. In this study, we estimate the discharge at the Óbidos station and the mouth of the Amazon basin using the water storage variations derived from GRACE gravity data without relying on any auxiliary ground observations. We extract the water mass signal along the main stem of the river by applying the Empirical Orthogonal Function (EOF) for water storage variations over the basin. The relative water storage variations along the main stem derived from the EOF decomposition are highly correlated with in-situ discharge at the Óbidos. However, in high water season, the GRACE-based discharge is estimated larger than the in-situ observations, and the difference is particularly significant during the 2009 extreme flood season. We argue that the in-situ river discharge in 2009 was underestimated due to the missed water volume for the flow detouring around the Óbidos gauge station during the high-flow event. Net river discharge of the Amazon Basin to Atlantic Ocean is also estimated, and its annual discharge is about 23% larger than that of the Óbidos. In particular, 2009 river discharge to Atlantic Oceans is estimated as 1050Gton.

Release from Streambed to Water Column during Baseflow Periods: A Modeling Study.

Streambed sediments can harbor large populations that are released into the water column during high-flow events. Few studies have been conducted on the rates of transfer from streambed sediment to water column in low-flow conditions in natural streams. The aim of this work was to apply the watershed-scale model SWAT (Soil and Water Assessment Tool) to a natural stream to evaluate the need to account for the release from streambed sediments during baseflow periods and to compare the results of simulating such a release by assuming predominantly passive transport, driven by groundwater influx, against simulations assuming predominantly active transport of random or chemotaxis-driven bacteria movement. concentrations in water during baseflow periods were substantially underestimated when release from the streambed was attributed only to streambed sediment resuspension. When considered in addition to the release due to sediment resuspension at high flows, the active and passive release assumptions provided 42 and 4% improvement, respectively, in the RMSE of logarithms of concentrations. Estimated fluxes to water column during the baseflow periods from June to November ranged from 3.3 × 10 colony-forming units (CFU) m d in the game land area to 1.4 × 10 CFU m d in the mixed pasture and cropland. Results demonstrate that release of from streambed sediments during baseflow periods is substantial and that water column concentrations are dependent on not only land management practices but also on in-stream processes.

Land use, soil properties and weather conditions influence nutrient fluxes into a deep oligotrophic lake

Many streams and rivers in upland watersheds in southern New Zealand drain into deep, oligotrophic lakes with major aesthetic, recreational and conservation values. We examined the effects of increasing pasture cover and weather-related variables on nitrogen (N), phosphorus (P) and dissolved organic carbon (DOC) concentrations in eight streams draining into Lake Wanaka and hypothesised that (1) DOC and N concentrations would increase with increasing land development, but soil characteristics in the watershed would mediate P input and (2) weather-related factors that increase hydrological connectivity in the landscape would enhance the influx of N and DOC. Agricultural development correlated positively with N and DOC concentrations in stream water, but temperature and soil moisture mitigated the influence of pasture cover on surface-water DOC concentration under very dry or very wet conditions. Weather-related factors did not influence N concentrations in streams. Neither land use nor weather-related conditions correlated with concentrations of dissolved P, possibly reflecting good P-binding in soils and lack of sampling during high-flow events. Our results provided evidence that agricultural development in grassland watersheds increases the concentrations of N and DOC entering Lake Wanaka, and weather and soil conditions mediate the amount of DOC transferred from soils into streams.

Design and monitoring of woody structures and their benefits to juvenile steelhead (Oncorhynchus mykiss) using a net rate of energy intake model

Despite substantial effort and resources being invested in habitat rehabilitation for stream fishes, mechanistic approaches to designing and evaluating how habitat actions influence the fish populations they are intended to benefit remain rare. We used a net rate of energy intake (NREI) model to examine expected and observed changes in energetic habitat quality and capacity from woody debris additions in a 40 m long study reach being treated as part of a restoration experiment in Asotin Creek, Washington. We simulated depths, velocities, and NREI values for pre-treatment, expected, and post-treatment habitat conditions, and we compared pre-treatment versus expected and pre-treatment versus post-treatment simulation results. The pre-treatment versus expected topography simulations suggested treatment would increase energetically favorable area, mean NREI, and capacity in the study reach. Pre-treatment versus post-treatment comparisons yielded similar predictions, though to smaller magnitudes, likely due to the short time span and single high-flow event between pre- and post-treatment data collection. We feel the NREI modelling approach is an important tool for improving the efficacy of habitat rehabilitation actions for stream fishes.

Repeated flood disturbance enhances rotifer dominance and diversity in a zooplankton community of a small dammed mountain pond

<p>The zooplankton community in a relatively small and mountain pond was studied during the spring growing season. To investigate which factors operate in the community structure, we explored several physical conditions, such as high inflows, and the biotic dynamics of the main zooplankton groups (i.e., rotifers, cladocerans and copepods). Two extreme flood events occurred during the investigated period and caused dramatic changes in physical conditions and reduction of the planktonic community abundances. The short period between both high-flow events was enough for the recovery of microplankton, but not for the metazoan zooplankton. Our results are in agreement with the common situation in which high flood events commonly favour rotifers over crustaceans, likely due to rotifer species have great colonization ability and grow faster. However, we found that the dominance of rotifers over crustaceans in our system is evidenced by an extremely, unusual high ratio between their abundances. We observed that, at the time of the great floods, crustacean abundances as well as rotifer populations notably decreased until near zero values. Although rotifer abundance began declining before high floods, the decrease was particularly notable when the great flood happened. Our results evidenced that i) dilution rate and temperature were the main drivers which are operating in the structure of the zooplankton community; and ii) no negative biotic interactions were detected between large and small cladocerans and rotifers. Additionally, we found surprisingly that a repeated disturbance caused by high flood events does increase the species diversity of rotifers. Finally, our study also detected some cues which may indicate that diapausing egg bank is also playing an important role in the zooplankton community, favouring the dominance of rotifers; however, this phenomenon deserves further studies. </p>

Sediment source identification and load prediction in a mixed-use Piedmont watershed, South Carolina

Many streams in the Piedmont region of the southeastern United States transport a disproportionately large amount of suspended sediment in response to moderately increased streamflows. Transport and deposition of excess sediment affect the stability of the channel and the health of the biological community; therefore, identifying the main source(s) of sediment and assessing the relationships between source, transport, and streamflow are critical to aquatic life and habitat management, dynamic equilibrium preservation, and development of feasible mitigation scenarios. The objectives of this study were to: (1) predict the annual suspended sediment yield and (2) identify significant contributing upland sources of sediment in the Lawsons Fork Creek basin, a 217 km2 mixed-use watershed in the South Carolina Piedmont. A regularly monitored cross-section located in the downstream reach was equipped with a passive sediment sampler, gage-height recorder, and sediment tiles. Streamflow and sediment concentration were measured over a 24-month period under variable hydrologic regimes. Results indicated that the average annual sediment yield (168 t/km2/yr) is significantly higher than yields documented in Piedmont watersheds of comparable size. To identify and prioritize sources of sediment contribution, stable isotopes of nitrogen (δ15N) and carbon (δ13C) were used as tracers. Source material was compared with suspended sediment near the watershed outlet (target material) and SIAR, a Bayesian Inference model, was used to estimate source apportionment. Results of this source study indicate that approximately 60% of the total sediment load in the water column during high flow events is derived from stream bank erosion. Findings are consistent with observed unstable stream bank conditions in the watershed. This study supports the use of a dual-isotopic fingerprinting approach in tandem with traditional sediment monitoring as a cost-effective method to identify and target sediment sources in a mixed-use watershed.

Response of freshwater macroinvertebrates to rainfall-induced high flows: A hydroecological approach

Hydraulic-habitat and biological data were integrated within a twofold-purpose study: (i) to investigate the response of freshwater macroinvertebrates to a rainfall-induced high flow event of moderate magnitude and (ii) to derive hydroecological relationships between habitat variability and macroinvertebrate microdistribution. 142 microhabitats (unique combinations of flow velocity, water depth and substrate type) allocated in four sites of no or very minor anthropogenic influence were sampled and analyzed, before and after the event. Freshwater macroinvertebrates were additionally collected and specific community metrics were derived. To identify possible pre- and post- impact benthic community differences, independent sample t-tests were applied, while Boosted Regression Tree models were developed to quantify the response of macroinvertebrates to flow alteration. Macroinvertebrate abundance, taxonomic richness, EPT richness and diversity decreased significantly by 90%, 60%, 50% and 25% respectively between the pre- and post- impact microhabitats. The relative abundance of macroinvertebrate predators and passive filter feeders increased after the event, mainly in specific substrate types (boulders and large stones), which served as flow refugia, maintaining less degraded (compared to finer substrates), still heavily impacted, benthic communities. According to the hydroecological analysis, the high flow event exerted the strongest impact on all macroinvertebrate metrics. Optimal (suitable) ranges of the hydraulic-habitat variables for benthic macroinvertebrates were identified (optimal flow velocity from 0.3m/s to 0.7m/s, optimal water depth at 0.2m), while boulders and large stones were the most suitable substrate types. The aforementioned data provide valuable information for the provisioning of biologically-derived environmental flows and an essential input of hydrodynamic habitat models to facilitate the selection of the optimal environmental flow scenario towards ensuring the integrity of aquatic ecosystems downstream of anthropogenic activities provoking hydrological alteration.

Identification of long-term high-flow regime changes in selected stations along the Danube River

Abstract The aim of the paper is to study spatial and temporal changes in the magnitude, duration and frequency of high flows in the Danube basin. A hydrological series of the mean daily discharges from 20 gauging stations (operated minimally since 1930) were used for the analysis of changes in the daily discharges. The high flow events were classified into three classes: high flow pulses, small floods, and large floods. For each year and for each class, the means of the peak discharges, the number and duration of events, and the rate of changes of the rising and falling limbs of the waves were determined. The long-term trends of the annual time series obtained were analyzed and statistically evaluated. The long-term high flow changes were found to be different in three individual high flow classes. The duration of the category of high flow pulses is decreasing at 19 stations on the Danube and is statistically significant at the Linz, Vienna, Bratislava and Orsova stations. The frequency of the high flow pulses is increasing in all 20 stations. Also, the rising and falling rates of the high flow pulse category are increasing at the majority of the stations. The long-term trends of the selected characteristics of the small floods are very similar to the trends of the high flow pulses, i.e., the duration of small floods is decreasing, and their mean number per year is increasing. In the category of large floods the changes were not proved.

A Sub-Catchment Based Approach for Modelling Nutrient Dynamics and Transport at a River Basin Scale

The prediction of nutrient pollution at realistic details is difficult due to lack of proper description of inherent processes in modelling tools. To overcome that this study has adopted a process based approach to build a semi-distributed model to simulate nutrient pollution in changing environment. The model was built to describe: (1) nutrient generation process in the catchment with consideration of different aspects of external and internal sources, (2) nutrient release from surface to the waterways via runoff and soil erosion, and (3) in-stream transport and chemical reaction process. The key novelty of this research is the linking of the nutrient generation process with transport mechanism for modelling nutrient dynamics at a basin scale. A flow capacity based approach was introduced to determine nutrient export from catchment to the waterways, which was useful to achieve the high resolution outputs from the model. The model performed reasonably well to represent the behaviour of nutrient in high flow events as well as in seasonal flow in two catchments located in distinct hydro-climatic regions. The study has shown that the nutrient model is suitable for predicting actual nutrient pollution in rivers for both high flow and seasonal flow under different hydro-climatic conditions. By simulating organic and inorganic nutrients separately, the model allows to estimate river water quality status in detail.

A multi-model approach to evaluating target phosphorus loads for Lake Erie

Abstract In response to water quality changes in the Great Lakes since implementing the 1978 Amendment to the Great Lakes Water Quality Agreement, the US and Canada renegotiated the agreement in 2012, requiring the governments to review and revise phosphorus (P) load targets, starting with Lake Erie. In response, the governments supported a multi-model team to evaluate the existing objectives and P load targets for Lake Erie and provide the information needed to update those targets. Herein, we describe the process and resulting advice provided to the binational process. The collective modeling effort concluded that avoiding severe Western Basin (WB) cyanobacteria blooms requires: 1) focusing on reducing total P loading from the Maumee River, with an emphasis on high-flow events during March–July, 2) focusing on dissolved reactive P load alone will not be sufficient because there is significant bioavailable P in the particulate phosphorus portion of the load, and 3) loading from the Detroit River is not a driver of cyanobacteria blooms. Reducing Central Basin (CB) hypoxia requires a CB + WB load reduction greater than what is needed to reach the WB cyanobacteria biomass goal. Achieving Cladophora thresholds will be challenging without site-specific load reductions, and more research is needed.

Phosphorus Retention in Stormwater Control Structures across Streamflow in Urban and Suburban Watersheds

Recent studies have shown that stormwater control measures (SCMs) are less effective at retaining phosphorus (P) than nitrogen. We compared P retention between two urban/suburban SCMs and their adjacent free-flowing stream reaches at the Baltimore Long-Term Ecological Study (LTER) site, and examined changes in P retention in SCMs across flow conditions. Results show that, when compared with free-flowing stream reaches, the SCMs had significantly lower dissolved oxygen (%DO) and higher P concentrations, as well as lower mean areal retention rates and retention efficiencies of particulate P (PP). In all the SCMs, concentrations of total dissolved phosphorus (TDP) consistently exhibited inverse correlations with %DO that was lower during summer base flows. Particulate phosphorus (PP) concentrations peaked during spring high flow period in both streams and in-line pond/SCMs, but they were also higher during summer base flows in suburban/urban SCMs. Meanwhile, PP areal retention rates and retention efficiencies of the SCMs changed from positive (indicating retention) during high flows to negative (indicating release) during low flows, while such changes across flow were not observed in free-flowing stream reaches. We attribute the changing roles of SCMs from a PP sink to a PP source to changes in SCM hydrologic mass balances, physical sedimentation and biogeochemical mobilization across flows. This study demonstrates that in suburban/urban SCMs, P retained during high flow events can be released during low flows. Cultivation of macrophytes and/or frequent sediment dredging may provide potential solutions to retaining both P and nitrogen in urban SCMs.