Atchafalaya River Research Articles

Aquatic vegetation mediates the relationship between hydrologic connectivity and water quality in a managed floodplain

We used conservative isotope tracers (deuterium and oxygen-18) and biologically relevant water quality measurements to assess connectivity of the Atchafalaya River to other waterways in its floodplain during the rising limb, peak, and falling limb of the 2011 flood pulse. We compared isotope tracers and water quality (dissolved oxygen and specific conductance) in biweekly samples at 83 sites in two areas that differed in their connectivity. We also compared tracers to an 8-year dataset of water quality measurements from the same sites. Although tracers clearly described differences in connectivity between the two floodplain areas and were correlated with concurrent measures of water quality, relationships were mediated by a strong temporal component and site-level variation in aquatic vegetation. Our results suggest a delay in floodplain water quality response to water inputs, and a strong influence of aquatic vegetation that locally overwhelms connectivity as a primary driver of local water quality.

Predicting Floodplain Hypoxia in the Atchafalaya River, Louisiana, USA, a Large, Regulated Southern Floodplain River System

AbstractThe Atchafalaya River Basin Floodway (ARBF), a regulated river/floodplain distributary of the Mississippi River, experiences an annual flood pulse that strongly influences floodplain physicochemistry. We developed several metrics to investigate the relationship between the timing and magnitude of the flood pulse and floodplain hypoxia, which in most years is a spatially extensive and temporally prolonged problem in the lower ARBF. Principal components analysis of flood metrics from 2001 to 2009 revealed contrasting flood types (early cool and late warm), but component‐based general linear models were unable to predict the magnitude of hypoxia in ARBF water management areas (WMAs). Further analyses based on temperature and geographic information system‐determined WMA inundation with generalized additive models (GAMs) revealed WMA‐specific patterns of hypoxia, but the likelihood of hypoxia consistently increased when temperatures approached 20°C and inundation rose above 20–30%. Validation with held‐out data based on logistic regression indicated that the models constructed with the 2001–2009 temperature and inundation data were able to accurately predict the probabilities of hypoxia in two WMAs based on data collected from 2010 to 2013. The GAMs were an effective tool for visualizing and predicting the probability of hypoxia based on two easily generated parameters. Our analyses indicate that modification of the Atchafalaya River flood pulse could reduce the magnitude of hypoxia within the lower ARBF, subject to engineering (control structure operation) and economic (commercial fisheries production) constraints, by minimizing floodplain inundation after water temperatures reach 20°C. Copyright © 2015 John Wiley & Sons, Ltd.

Using Flow-Ecology Relationships to Evaluate Ecosystem Service Trade-Offs and Complementarities in the Nation's Largest River Swamp.

Large river systems are inextricably linked with social systems; consequently, management decisions must be made within a given ecological, social, and political framework that often defies objective, technical resolution. Understanding flow-ecology relationships in rivers is necessary to assess potential impacts of management decisions, but translating complex flow-ecology relationships into stakeholder-relevant information remains a struggle. The concept of ecosystem services provides a bridge between flow-ecology relationships and stakeholder-relevant data. Flow-ecology relationships were used to explore complementary and trade-off relationships among 12 ecosystem services and related variables in the Atchafalaya River Basin, Louisiana. Results from Indicators of Hydrologic Alteration were reduced to four management-relevant hydrologic variables using principal components analysis. Multiple regression was used to determine flow-ecology relationships and Pearson correlation coefficients, along with regression results, were used to determine complementary and trade-off relationships among ecosystem services and related variables that were induced by flow. Seven ecosystem service variables had significant flow-ecology relationships for at least one hydrologic variable (R (2)=0.19-0.64). River transportation and blue crab (Callinectes sapidus) landings exhibited a complementary relationship mediated by flow; whereas transportation and crawfish landings, crawfish landings and crappie (Pomoxis spp.) abundance, and blue crab landings and blue catfish (Ictalurus furcatus) abundance exhibited trade-off relationships. Other trade-off and complementary relationships among ecosystem services and related variables, however, were not related to flow. These results give insight into potential conflicts among stakeholders, can reduce the dimensions of management decisions, and provide initial hypotheses for experimental flow modifications.

Numerical analysis of the primary processes controlling oxygen dynamics on the Louisiana shelf

Abstract. The Louisiana shelf, in the northern Gulf of Mexico, receives large amounts of freshwater and nutrients from the Mississippi–Atchafalaya river system. These river inputs contribute to widespread bottom-water hypoxia every summer. In this study, we use a physical–biogeochemical model that explicitly simulates oxygen sources and sinks on the Louisiana shelf to identify the key mechanisms controlling hypoxia development. First, we validate the model simulation against observed dissolved oxygen concentrations, primary production, water column respiration, and sediment oxygen consumption. In the model simulation, heterotrophy is prevalent in shelf waters throughout the year, except near the mouths of the Mississippi and Atchafalaya rivers, where primary production exceeds respiratory oxygen consumption during June and July. During this time, efflux of oxygen to the atmosphere, driven by photosynthesis and surface warming, becomes a significant oxygen sink. A substantial fraction of primary production occurs below the pycnocline in summer. We investigate whether this primary production below the pycnocline is mitigating the development of hypoxic conditions with the help of a sensitivity experiment where we disable biological processes in the water column (i.e., primary production and water column respiration). With this experiment we show that below-pycnocline primary production reduces the spatial extent of hypoxic bottom waters only slightly. Our results suggest that the combination of physical processes (advection and vertical diffusion) and sediment oxygen consumption largely determine the spatial extent and dynamics of hypoxia on the Louisiana shelf.

Spatial and Temporal Patterns of Benthic Respiration and Net Nutrient Fluxes in the Atchafalaya River Delta Estuary

Benthic respiration and nutrient flux rates were quantified during four distinct periods (spring peak in river discharge, summer period of offshore hypoxia, autumn period after breakdown of offshore hypoxia, and the non-hypoxic winter period) over an annual cycle in the Atchafalaya River Delta Estuary (ARDE). The overall mean benthic respiration rate for the ARDE was 12.9 mmol O2 m−2 day−1 with the highest and lowest rates occurring in summer and winter (22.1 and 5.4 mmol O2 m−2 day−1), respectively, and surprisingly little variance in relation to distance offshore of the deltas. Bottom water temperature explained 94 % of the variance in the natural log of benthic respiration. The mean annual fluxes for ammonium, dissolved organic nitrogen (DON), phosphate, and silicate were from the sediments to the water column (0.91, 0.44, 0.03, and 1.04 mmol m−2 day−1, respectively), but nitrate had a mean annual flux (−0.90 mmol N m−2 day−1) from the water column into the sediments. Net nitrate fluxes were greatest in summer, were generally higher at delta than at offshore sites, and were most strongly correlated with benthic respiration. Net NH4 + fluxes from the sediments to the overlying water column were greater at offshore sites than at delta sites resulting in the delta and offshore sites behaving as net dissolved inorganic nitrogen (DIN) sinks and sources, respectively. The results from this study suggest that benthic processes in the ARDE greatly alter bottom water oxygen availability as well as the size and form of nutrient fluxes exported to the northern Gulf of Mexico.

Rethinking the Mississippi River diversion for effective capture of riverine sediments

Abstract. Many river deltas in the world are vibrant economic regions, serving as transportation hubs, population centres, and commercial hotspots. However, today, many of these deltaic areas face a tremendous challenge with land loss due to a number of factors, such as reduced riverine sediment supply, coastal land erosion, subsidence, and sea level rise. The development of the Mississippi River Deltaic Plain (MRDP) in southeast Louisiana, USA, over the past century is a good example. Since 1932, approximately 4877 km2 of the coastal land of MRDP has become submerged. The lower Mississippi River main channel entering the Gulf of Mexico has become an isolated waterway with both sides losing land. In contrast, large open water areas in the Mississippi River’s distributary basin, the Atchafalaya River basin, have been silted up over the past century, and the river mouth has developed a prograding delta feature at its two outlets to the Gulf of Mexico. The retrospective analysis of this paper makes it clear that the main cause of the land loss in the MRDP is not the decline of riverine sediment, but the disconnection of the sediment sources from the natural flood plains. Future sediment management efforts in the MRDP should focus on restoring the natural connection of riverine sediment supplies with flood plains, rather than solely using channelized river diversion. This could be achieved through controlled overbank flooding (COF) and artificial floods in conjunction with the use of a hydrograph-based sediment availability assessment.

Ecological Survey of a Dredged Material-supported Wetland in the Atchafalaya River, Louisiana: An Engineering with Nature Case Study

The geographical extent of wetlands continues to decrease at national and regional scales, including major wetland losses across southern Louisiana (Barras et al. 2003; USACE 2004). Wetland distribution and functional losses in coastal Louisiana have been linked to a lack of sediment inputs among other causes (Day et al. 2007). These losses have been accompanied by a decrease in wetland functions.

Mechanisms of surface wave energy dissipation over a high‐concentration sediment suspension

AbstractField observations from the spring of 2008 on the Louisiana shelf were used to elucidate the mechanisms of wave energy dissipation over a muddy seafloor. After a period of high discharge from the Atchafalaya River, acoustic measurements showed the presence of 20 cm thick mobile fluid‐mud layers during and after wave events. While total wave energy dissipation (D) was greatest during the high energy periods, these periods had relatively low normalized attenuation rates (κ = Dissipation/Energy Flux). During declining wave‐energy conditions, as the fluid‐mud layer settled, the attenuation process became more efficient with high κ and low D. The transition from high D and low κ to high κ and low D was caused by a transition from turbulent to laminar flow in the fluid‐mud layer as measured by a Pulse‐coherent Doppler profiler. Measurements of the oscillatory boundary layer velocity profile in the fluid‐mud layer during laminar flow reveal a very thick wave boundary layer with curvature filling the entire fluid‐mud layer, suggesting a kinematic viscosity 2–3 orders of magnitude greater than that of clear water. This high viscosity is also consistent with a high wave‐attenuation rates measured by across‐shelf energy flux differences. The transition to turbulence was forced by instabilities on the lutocline, with wavelengths consistent with the dispersion relation for this two‐layer system. The measurements also provide new insight into the dynamics of wave‐supported turbidity flows during the transition from a laminar to turbulent fluid‐mud layer.

The carbon dioxide system on the Mississippi River-dominated continental shelf in the northern Gulf of Mexico: 1. Distribution and air-sea CO2 flux.

River‐dominated continental shelf environments are active sites of air‐sea CO2 exchange. We conducted 13 cruises in the northern Gulf of Mexico, a region strongly influenced by fresh water and nutrients delivered from the Mississippi and Atchafalaya River system. The sea surface partial pressure of carbon dioxide (pCO2) was measured, and the air‐sea CO2 flux was calculated. Results show that CO2 exchange exhibited a distinct seasonality: the study area was a net sink of atmospheric CO2 during spring and early summer, and it was neutral or a weak source of CO2 to the atmosphere during midsummer, fall, and winter. Along the salinity gradient, across the shelf, the sea surface shifted from a source of CO2 in low‐salinity zones (0≤S<17) to a strong CO2 sink in the middle‐to‐high‐salinity zones (17≤S<33), and finally was a near‐neutral state in the high‐salinity areas (33≤S<35) and in the open gulf (S≥35). High pCO2 values were only observed in narrow regions near freshwater sources, and the distribution of undersaturated pCO2 generally reflected the influence of freshwater inputs along the shelf. Systematic analyses of pCO2 variation demonstrated the importance of riverine nitrogen export; that is, riverine nitrogen‐enhanced biological removal, along with mixing processes, dominated pCO2 variation along the salinity gradient. In addition, extreme or unusual weather events were observed to alter the alongshore pCO2 distribution and to affect regional air‐sea CO2 flux estimates. Overall, the study region acted as a net CO2 sink of 0.96 ± 3.7 mol m−2 yr−1 (1.15 ± 4.4 Tg C yr−1).

Assessing biophysical controls on Gulf of Mexico hypoxia through probabilistic modeling.

A mechanistic model was developed to predict midsummer bottom-water dissolved oxygen (BWDO) concentration and hypoxic area on the Louisiana shelf of the northern Gulf of Mexico, USA (1985-2011). Because of its parsimonious formulation, the model possesses many of the benefits of simpler, more empirical models, in that it is computationally efficient and can rigorously account for uncertainty through Bayesian inference. At the same time, the model incorporates important biophysical processes such that its parameterization can be informed by field-measured biological and physical rates. The model is used to explore how freshwater flow, nutrient load, benthic oxygen demand, and wind velocity affect hypoxia on the western and eastern sections of the shelf, delineated by the Atchafalaya River outfall. The model explains over 70% of the variability in BWDO on both shelf sections, and outperforms linear regression models developed from the same input variables. Model results suggest that physical factors (i.e., wind and flow) control a larger portion of the year-to-year variability in hypoxia than previously thought, especially on the western shelf, though seasonal nutrient loads remain an important driver of hypoxia, as well. Unlike several previous Gulf hypoxia modeling studies, results do not indicate a temporal shift in the system's propensity for hypoxia formation (i.e., no regime change). Results do indicate that benthic oxygen demand is a substantial BWDO sink, and a better understanding of the long-term dynamics of this sink is required to better predict how the size of the hypoxic zone will respond to proposed reductions in nutrient loading.

Photochemical production of CO and CO2 in the Northern Gulf of Mexico: Estimates and challenges for quantifying the impact of photochemistry on carbon cycles

The photochemical production of carbon dioxide (CO2) and carbon monoxide (CO), derived from marine colored dissolved organic matter (CDOM), is considered a significant oceanic loss process for the large and variable fluxes of terrigenous dissolved organic carbon (DOC) in river dominated coastal systems. Using samples collected over 4 seasons (2009–2010) in a grid centered on the Mississippi–Atchafalaya River system in the Northern Gulf of Mexico, we attempt to improve constraints for these direct photochemical impacts on DOC cycles by contributing the largest coherent set of photochemical apparent quantum yield (AQY) spectra for CO to date (n=99), 18 of which had CO2 AQY spectra determined in the same sample for use in calculating CO2:CO photoproduction ratios. Simple correlations, previously reported from much smaller data sets, between CO photoproduction and CDOM optical properties showed weak or no correlations within our much larger spatio-temporal study. However, grouping samples with an optical transition point (ag(320)=1.3m−1) allowed definition of two distinct inshore and offshore CO AQY spectra. Inputting these in regional photochemical models that use remotely sensed ocean color data and modeled water optical properties dramatically improves results over using a single CO AQY spectrum (modeled vs. measured r2=0.73 vs. 0.18 for single AQY), as has been done previously. Monthly average CO photoproduction rates ranged from 6.0 to 17.7μmolm−2d−1, amounting to a conservative estimate of 3.35GgCyr−1 for our study region. Our data also shows that the efficiency for CO2 photoproduction decreases over prolonged irradiations up to 48h. Consequently, no assessable method exists for direct determination of initial rates in clear waters with low CO2 photoproduction. Therefore CO2 photoefficiency in the Gulf of Mexico was inferred using CO photoproduction estimates and CO2:CO ratios. In agreement with previous studies, CO2:CO ratios determined here were also poorly constrained, ranging from ~6 to 66 providing a median value of 24.4. This approach estimates that direct photochemical production of CO2 plus CO can remineralize DOC on the order of 85GgC each year. This estimate provides an improvement over the use of a single inshore CO2 AQY spectrum for a largely blue water system but caution should be used when interpreting CO2 photochemical flux estimates that rely heavily on the selection of CO AQY spectra and/or poorly constrained relationships between CO2 and CO photoproduction. Our results stress that the continued expansion of the CO2:CO ratio database to new oceanic regimes is likely of very limited value and that assessable direct methods or better proxies to quantify CO2 photochemistry in clear marine waters are needed.

Winter storm-induced hydrodynamics and morphological response of a shallow transgressive shoal complex: Northern Gulf of Mexico

Using extended deployments during seasons of low and high discharge from the Atchafalaya River, meteorological, hydrodynamic and bottom boundary layer parameters were monitored from Tiger and Trinity Shoal complex, off Louisiana coast, USA. During winter storms, the surface current speed measured at both shoals exceeded 0.5 m/s and the entire water column followed the prevailing wind direction. The current speed close to the bottom exceeded 0.3 m/s during high energy northerly winds. The mean water level in the shoal complex increased during southerly winds and decreased during northerly winds, such that the difference between wind set-up and set-down exceeded 0.7 m in Tiger Shoal and 0.6 m in Trinity Shoal during high energy frontal passages. The swell height was inversely correlated with mean water level, and increased during pre-frontal phase and decreased during post-frontal phase of winter storms. The sea (short waves) height responded quickly to wind direction and speed; and within a few hours after the wind shifted and blowing from the north, the sea height increased during both deployments. Bimodal wave frequency spectrum was observed during wind veering from southerly to northerly, when both sea and swell intensities were significant. The Tiger Shoal bed sediment texture transformed drastically, from mud to shell and shell hash assemblage, within a period of two weeks during the December 2008 deployment. Backscatter signal intensity from a Pulse Coherent Acoustic Doppler Profiler (PCADP) and its velocity estimates were used to determine the vertical extend and timing of mud resuspension and their eventual flushing out from the shoal environment, when exposed to high energy winter storm passages. The computed time frame for a total transformation of bottom sediment texture (from muddy bottom to shell and shell hash assemblage) was supported by the combined wave and bottom current induced shear stress at shoal bed. The bed samples collected from Tiger Shoal before and after the deployment in spring 2009 consisted of more than 80% shell and shell hash, which again confirmed a stable bottom as predicted from the PCADP data. However, the fine sand and mud dominated bed at Trinity Shoal was highly dynamic and experienced a few cm of ephemeral sediment deposition during the passage of each cold front, as revealed from the analysis of acoustic backscatter data from the PCADP. Suspended sediment concentration estimated from Optical Backscatterance Sensors (OBS) and PCADP were in good agreement during low river discharge events in December 2008; but significantly diverged during the spring 2009 deployment, when a high suspended sediment load was discharged into the shelf from the Atchafalaya River, and subsequently pushed farther offshore into the deployment sites by wind-induced strong currents during the passage of cold fronts.

Effects of Agricultural Conservation Practices on N Loads in the Mississippi–Atchafalaya River Basin

A modeling framework consisting of a farm-scale model, Agricultural Policy Environmental Extender (APEX); a watershed-scale model, Soil and Water Assessment Tool (SWAT); and databases was used in the Conservation Effects Assessment Project to quantify the environmental benefits of conservation practices on cropland. APEX is used to simulate conservation practices on cultivated cropland and Conservation Reserve Program land to assess the edge-of-field water-quality benefits. Flow and pollutant loadings from APEX are input to SWAT. SWAT simulates the remaining noncultivated land and routes flow and loads generated from noncultivated land, point sources, and cropland to the basin outlet. SWAT is used for assessing the effects of practices on local and in-stream water-quality benefits. Each river basin is calibrated and validated for streamflow and loads at multiple gauging stations. The objectives of the current study are to estimate the effects of currently existing and additional conservation practices on total N (TN) loads in the Mississippi-Atchafalaya River Basin (MARB) and draw insights on TN load reductions necessary for reducing the hypoxic zone in the Gulf of Mexico. The effects of conservation practice scenarios on local and in-stream (riverine) water quality are evaluated. Model results indicate that conservation practices currently on cropland have reduced the TN losses to local waters between 20 and 59% in the six river basins within MARB and the TN load discharged to the Gulf by 17%. Further water-quality improvement can be obtained in the MARB with additional conservation treatment.

The Atchafalaya River Basin: history and ecology of an American wetland

The Atchafalaya River Basin: history and ecology of an American wetland

An Estimate of Basin‐Wide Denitrification Based on Floodplain Inundation in the Atchafalaya River Basin, Louisiana

AbstractMaximizing the reduction of nitrate to dinitrogen gas (denitrification) has been advocated as a means to decrease nitrate pollution that causes eutrophication and hypoxia in estuaries worldwide. Managing this flux in bottomland forest wetlands of the Mississippi River could potentially reduce the world's second largest hypoxic zone. We used published denitrification rates, geospatial data on habitat area and inundation frequency, water level records (1963–2011), and average monthly temperatures to estimate annual denitrification in the Atchafalaya River Basin, the principal distributary of the Mississippi River. Denitrification rates ranged from 5394 kg N year−1 (3.07 kg N km−2 year−1) in 1988 to 17 420 kg N year−1 (9.92 kg N km−2 year−1) in 1981, and rates were consistently higher in fall compared with those in spring. Total NO3− denitrified in the basin was negligible compared with total NO3− entering the Gulf of Mexico. If all N denitrified in the basin instead entered the Gulf, the hypoxic zone was predicted to increase only 5.07 km2 (0.06%). This negligible effect of the basin on N dynamics in the Gulf agrees with other mass balance and isotopic studies in the region. Copyright © 2014 John Wiley &amp; Sons, Ltd.

Current–wave interaction in the Mississippi–Atchafalaya river plume on the Texas–Louisiana shelf

Wave–current interaction over the Texas–Louisiana shelf, and its effects on the dispersal and mixing of the Mississippi–Atchafalaya river plume, have been investigated using the Coupled Ocean–Atmosphere–Wave–Sediment Transport (COAWST) Modeling System. The modeling system is driven by realistic wave and current conditions at the open boundaries and high frequency1-D wind measured from a nearby meteorological station. Skill analysis demonstrates that the model reproduces the wave and salinity fields reasonably well. Waves over the Texas–Louisiana shelf are dominated by locally forced wind seas, and generally propagate in the same direction as the winds. Investigation into the spatial differences in the effect of waves reveals two distinct dynamical regions: the Chenier shelf, the shelf region extending roughly offshore from Sabine Lake to Vermilion Bay, and the Louisiana Bight, the region between the Mississippi Delta and Terrebonne Bay. A variety of model runs are performed, where specific wave processes are either included or excluded, in order to isolate the processes acting in different regions. The Chenier shelf is mainly affected by wave enhanced bottom stress, whereas the Louisiana Bight is mostly affected by the surface wave induced mixing and 3-D wave forces. The wave enhanced bottom stress suppresses cross-shore exchange, and acts to trap more freshwater in the nearshore regions shallower than 50m over the Chenier shelf. Wave enhanced bottom stress plays only a minor role in the Louisiana Bight, where the surface-trapped Mississippi plume rarely feels the bottom. The surface intensified wave mixing and 3-D wave forces reduce the surface salinity and weaken the stratification in the region associated with the thin recirculating Mississippi plume in the Louisiana Bight. Model results indicate that the surface wave mixing, the 3-D wave forces, and the wave bottom stress exhibit little interaction over the Texas–Louisiana shelf. Finally, we have demonstrated that the one-way coupling is capable of resolving the majority of wave effects over the entire shelf if the seasonal scale is of interest.

Application of a one-dimensional model to explore the drivers and lability of carbon in the northern Gulf of Mexico

A one-dimensional water quality model, Gulf of Mexico Dissolved Oxygen Model (GoMDOM-1D), was developed to simulate phytoplankton, carbon, nitrogen and phosphorus in the northern Gulf of Mexico. The model was calibrated and corroborated against a comprehensive set of field observations and process measurements. Model results fit measured nutrients and phytoplankton concentrations well, but the model was unable to sustain dissolved organic carbon and water-column respiration at measured levels. The decline in modeled dissolved organic carbon (DOC) and respiration supports the notion that autochthonous production of DOC was insufficient to offset the respiratory demand from bacterial processes for the most of the Louisiana continental shelf. Transport of carbon from high production areas close to the Mississippi and Atchafalaya River plumes likely supplied a part of the respiratory demand of the water column. Modeling results showed that DOC in the study area on the Louisiana shelf was very labile with the highest lability during the month of June, and decreased lability in April. Primary production at the study sites was mainly nitrogen limited, although phosphorus limitation was found in the surface layers for the areas most impacted by the Mississippi River plume. The model also provided insights into the interplay between nutrients and light in regulating phytoplankton production, which is important when predicting the location and extent of hypoxia in the Gulf of Mexico.

A legacy of absence

The historical removal of accumulations of wood on medium to large rivers in the continental United States caused a fundamental change in river corridors that has received relatively little attention in the scientific literature. Although scientific literature discusses the natural wood rafts present on the Red and the Atchafalaya Rivers in the southeastern United States, there is little awareness that similar extensive masses of wood are documented in the historical record from forested river catchments as diverse and widespread as those in the northeast, southeast, Texas Gulf Coast, Pacific Northwest, and upper Great Lakes regions of the country. While present, these natural wood rafts decreased channel conveyance, increased channel-floodplain connectivity, and facilitated anastomosing channels and floodplain lakes. Removal of natural wood rafts began in the 17th century in the eastern United States and proceeded westward with the movement of European settlers, accelerating during the 19th-century era of steamboats and floating of cut timber. Removal of the natural wood rafts likely forced many rivers from a multi thread planform with high channel-floodplain connectivity into an alternative stable state of single-thread channels with substantially reduced overbank flow, sedimentation, and avulsions. There is now widespread recognition among the geomorphic community of how upland clearance increased sediment yields and floodplain aggradation. I propose that widespread removal of instream wood for steamboat routes, timber rafts, and flood control was equally significant in decreasing floodplain sedimentation and river complexity, and in causing a fundamental, extensive, and intensive change in forested river corridors throughout the United States.

Effects of environmental hypoxia on population characteristics of red swamp crayfish Procambarus clarkii in the Atchafalaya River Basin, Louisiana

Spatial and temporal heterogeneity in Atchafalaya River Basin (ARB) physical and chemical characteristics are associated with floodplain inundation and rising water temperatures, resulting in extensive areas of the ARB that experience hypoxia (dissolved oxygen [DO] ≤2.0 mg/L) for several weeks to months throughout the annual flood pulse. Although red swamp crayfish Procambarus clarkii can tolerate relatively low DO concentrations, effects of chronic hypoxic exposure on wild populations have not been extensively studied. Here we compare population characteristics of P. clarkii from normoxic and chronically hypoxic locations in the southeastern ARB that were sampled semi-monthly during the 2008 and 2009 crayfish harvest periods. Carapace length and catch per unit effort (CPUE) of P. clarkii from normoxic and hypoxic locations were similar prior to hypoxia development during both sample years. Hypoxia developed when mean water temperature reached 20°C, and diurnal DO concentrations at hypoxic locations remained below 2.0 mg/L on every sample date during the period of environmental hypoxia. During this period in both sample years, P. clarkii at hypoxic locations had a significantly smaller mean carapace length and mean CPUE was lower on every sample date.

Perirheic mixing and biogeochemical processing in flow‐through and backwater floodplain wetlands

AbstractInundation hydrology and associated processes control biogeochemical processing in floodplains. To better understand how hydrologic connectivity, residence time, and intrafloodplain mixing vary in floodplain wetlands, we examined how water quality of two contrasting areas in the floodplain of the Atchafalaya River—a flow‐through and a backwater wetland—responded to an annual flood pulse. Large, synoptic sampling campaigns occurred in both wetlands during the rising limb, peak, and falling limb of the hydrograph. Using a combination of conservative and reactive tracers, we inferred three dominant processes that occurred over the course of the flood pulse: flushing (rising limb), advective transport (peak), and organic matter accumulation (falling limb). Biogeochemistry of the two wetlands was similar during the peak while the river overflowed into both. However, during the rising and falling limbs, flow in the backwater wetland experienced much greater residence time. This led to the accumulation of dissolved organic matter and dissolved phosphorus. There were also elevated ratios of dissolved organic carbon to nitrate in the backwater wetland, suggesting nitrogen removal was limited by nitrate transported into the floodplain there. Collectively, our results suggest inclusion of a temporal component into the perirheic concept more fully describes inundation hydrology and biogeochemistry in large river floodplain.