Abstract
Stream confluences are ubiquitous interfaces in freshwater networks and serve as junctions of previously independent landscapes. However, few studies have investigated how confluences ...influence the transport, mixing, and fate of organic matter (OM) and inorganic nutrients at the scale of river networks. To understand how network biogeochemical fluxes may be altered by confluences, we conducted two sampling campaigns at five confluences in summer and fall 2021 spanning the extent of a mixed land use stream network. We sampled the confluence mainstem and tributary reaches as well as throughout the mixing zone downstream. We predicted that biologically reactive solutes would mix non‐conservatively downstream of confluences and that alterations to downstream biogeochemistry would be driven by differences in chemistry and size of the tributary and upstream reaches. In our study, confluences were geomorphically distinct (e.g., wider, deeper, unique erosional, and depositional features) downstream compared to reaches upstream of the confluence. Dissolved OM and nutrients mixed non‐conservatively downstream of the five confluences. Biogeochemical patterns downstream of confluences were only partially explained by contributing reach chemistry and drainage area. We found that the relationship between geomorphic variability, water residence time, and microbial respiration differed between reaches upstream and downstream of confluences. The lack of explanatory power from network‐scale drivers suggests that non‐conservative mixing downstream of confluences may be driven by biogeochemical processes within the confluence mixing zone. The unique geomorphology, non‐conservative biogeochemistry, and ubiquity of confluences highlights a need to account for the distinct functional role of confluences in water resource management in freshwater networks.
Plain Language Summary
The confluences of two streams merging together are features fundamental to the structure of freshwater networks. Confluences are often sites of physical disturbance where water draining different landscapes meet and mix, but how processes within confluences alter organic matter (OM) and nutrient export downstream remains unknown. To test how water chemistry and microbial activity (i.e., carbon metabolism) may change at confluences, we sampled the upstream and downstream reaches of five confluences across a stream network draining heterogeneous land cover. We found that OM and nutrients did not mix as we would predict downstream of confluences, and that alterations to downstream water chemistry could not be explained by differences in chemistry or stream size. Stream channels in our study were typically wider and deeper at the confluence mixing zone, and the relationship between channel width, flow and microbial activity differed between reaches upstream and downstream of confluences. Our findings suggest that localized differences in the physical environment downstream of confluences may result in altered biogeochemistry. The prevalence of confluences and their potential impacts on the function of aquatic ecosystems highlights a need to understand the role of stream confluences within landscapes.
Key Points
Stream reaches downstream of confluences are geomorphically distinct from upstream reaches and have unique biogeochemical signatures
Differences in upstream and tributary reach chemistry or drainage area do not explain non‐conservative mixing of biologically reactive solutes at confluences
Confluence geomorphic heterogeneity and changes to water residence time downstream may drive differences in biogeochemical processes in confluence mixing zones
Floodplain inundation poses both risks and benefits to society. In this study, we characterize floodplain inundation across the United States using 5800 stream gages. We find that between 4% and ...12.6% of a river's annual flow moves through its floodplains. Flood duration and magnitude is greater in large rivers, whereas the frequency of events is greater in small streams. However, the relative exchange of floodwater between the channel and floodplain is similar across small streams and large rivers, with the exception of the water-limited arid river basins. When summed up across the entire river network, 90% of that exchange occurs in small streams on an annual basis. Our detailed characterization of inundation hydrology provides a unique perspective that the regulatory, management, and research communities can use to help balance both the risks and benefits associated with flooding.
Scientists and policymakers increasingly recognize that headwater regions contain numerous temporary streams that expand and contract in length, but accurately mapping and modeling dynamic stream ...networks remain a challenge. Flow intermittency sensors offer a relatively new approach to characterize wet stream length dynamics at high spatial and temporal resolutions. We installed 51 flow intermittency sensors at an average spacing of 40 m along the stream network of a high-relief, headwater catchment (33 ha) in the Valley and Ridge of southwest Virginia. The sensors recorded the presence or absence of water every 15 min for 10 months. Calculations of the wet network proportion from sensor data aligned with those from field measurements, confirming the efficacy of flow intermittency sensors. The fine temporal scale of the sensor data showed hysteresis in wet stream length: the wet network proportion was up to 50% greater on the rising limb of storm events than on the falling limb for dry antecedent conditions, at times with a delay of several hours between the maximum wet proportion and peak runoff at the catchment outlet. Less stream length hysteresis was evident for larger storms with higher event and antecedent precipitation that resulted in peak runoff > 15 mm/day. To assess spatial controls on stream wetting and drying, we performed a correlation analysis between flow duration at the sensor locations and common topographic metrics used in stream network modeling. Topography did not fully explain spatial variation in flow duration along the stream network. However, entrenched valleys had longer periods of flow on the rising limbs of events than unconfined reaches. In addition, large upslope contributing areas corresponded to higher flow duration on falling limbs. Future applications that explore the magnitude and drivers of stream length variability may provide further insights into solute and runoff generation processes in headwater regions.
Stream restoration efforts in the United States are increasingly aimed towards water quality improvement, yet little process-based guidance exists to compare pollutant removals from different ...restoration techniques for variable site conditions. Excess nitrate (NO3−) is a frequent pollutant of concern due to eutrophication in downstream waterbodies such as the Chesapeake Bay. We used MIKE SHE to simulate hydraulics and NO3− removal in a 90m restored reach of Stroubles Creek, a second-order stream in Blacksburg, Virginia. Site specific geomorphic, hydrologic, and hydraulic data were used to calibrate the model. We evaluated in-stream structures that induce hyporheic zone denitrification during baseflow and inset floodplains that remove NO3− during storm flows. We varied hydraulic conditions (winter baseflow, summer baseflow, storm flow), biogeochemical parameters (literature hyporheic zone denitrification rates and newly available inset floodplain removal rates) and boundary conditions (upstream NO3− concentration), sediment conditions (hydraulic conductivity), and stream restoration design parameters (inset floodplain length). Our results indicate that NO3− removal rates within the 90m reach were minimal. Structure-induced hyporheic zone denitrification did not exceed 3.1% of mass flowing in from the upstream channel, was achieved only during favorable background groundwater hydraulic conditions (i.e. summer baseflow), and was transport-limited such that non-trivial removal rates were achieved only when the streambed hydraulic conductivity (K) was at least 10−4m/s. Inset floodplain nitrogen removal was limited by floodplain residence time and NO3− removal rate, and did not exceed 1% of inflowing mass. Summing these removals for both restoration practices over the course of the year based on the frequency of storm and summer baseflow conditions yielded ∼2.1% annual removal. Achieving 30% NO3− removal required increasing the length of stream reach restored to 0.9km–819km (depending on hydraulic conductivity) and 3.8–46km (depending on inset floodplain length and nitrogen removal rate) for in-stream structures during baseflow and inset floodplains during storm flow, respectively. In one of the first comparisons of process-based modeling to the Chesapeake Bay Program stream restoration guidance, we found that the guidance overestimated hyporheic NO3− removal for our modeled reach, but correctly estimated inset floodplain removal. Overall, our results indicate that in-stream structures and inset floodplains can improve water quality, but overall required level of effort may be high to achieve desired results.
Understanding the quantity and quality of dissolved organic matter (DOM) in potential watershed sources is critical for explaining and quantifying the exports of DOM in stream runoff. Here, we ...examined the concentration and quality of DOM for ten watershed sources in a 12 ha forested catchment over a two-year period. DOM composition was evaluated for: throughfall, litter leachate, soil water (zero and tension), shallow and deep groundwater, stream water, hyporheic zone, and groundwater seeps. DOM quality was measured using a suite of optical indices including UV-visible absorbance and PARAFAC modeling of fluorescence excitationemission matrices (EEMs). DOM concentrations and quality displayed a pronounced trend across watershed sources. Surficial watershed sources had higher DOM concentrations and more humic-like DOM with higher molecular weight whereas deeper groundwater sources were rich in % protein-like fluorescence. The greater % contribution of protein-like fluorescence in groundwater suggested that a larger fraction of groundwater DOM may be bioavailable. DOM for wetland groundwater was more aromatic and humic-like than that at the well-drained riparian location. Principal component analyses (PCA) revealed that the differences in surficial watershed compartments were dictated by humic-like components while groundwater sources separated out by % protein-like fluorescence. Observations from optical indices did not provide any conclusive evidence for preferential association of dissolved organic carbon (DOC) or dissolved organic nitrogen (DON) with any particular DOM quality pools.
Excess nutrient pollution and eutrophication are widespread, and stream restoration is increasingly implemented as a solution. Yet few studies evaluate the cumulative effects of multiple individual ...restoration projects on watershed-scale nutrient loading. We developed a new modeling approach linking the U. S. Army Corps of Engineers Hydrologic Engineering Center's River Analysis System (HEC-RAS) to an auxiliary R script that simulates hyporheic exchange. We used the modeling approach to simulate hyporheic enhancement by in-stream restoration features (e.g., structures, pool-riffles, gravel bars) implemented throughout a generic 4th-order gaining watershed in the eastern USA. We assumed groundwater was widely impacted by nitrate, thus the primary pollutant source was baseflow gaining. Model results indicated that hyporheic restoration throughout all streams of our 4th-order watershed would reduce nitrate loading to downstream waterbodies by ~83%. This percentage assumes removal of all nitrate that enters the hyporheic zone and is for a gravel/sand bed, so reductions would be smaller with finer sediments or incomplete removal. For example, when we reduced the hyporheic exchange rate by an order of magnitude, the maximum watershed nitrate load reduction decreased to ~25%. The relationship between the percent of watershed stream channels that have been restored and percent nitrate load reduction at the watershed outlet was nonlinear. This relationship was exponential in smaller streams (1st- and 2nd-order) due to efficient removal of all incoming nitrate, but became linear in larger streams (3rd- and 4th-order) due to “recycling” of channel flow through the hyporheic zone more than once. Yet restoration was more effective at overall nitrate load reduction in larger (e.g., 3rd-4th order) streams because the majority of nitrate enters the watershed through groundwater gaining in those larger streams. Thus, the location of restoration projects within a watershed is important in determining their effect on nitrate loads at the watershed outlet. Overall, our results indicate hyporheic restoration can significantly reduce watershed nitrate loading to downstream waterbodies, yet watersheds must be viewed as a whole to understand the potential impacts of any particular project under consideration.
River flooding impacts human life and infrastructure, yet provides habitat and ecosystem services. Traditional flood control (e.g., levees, dams) reduces habitat and ecosystem services, and ...exacerbates flooding elsewhere. Floodplain restoration (i.e., bankfull floodplain reconnection and Stage 0) can also provide flood management, but has not been sufficiently evaluated for small frequent storms. We used 1D unsteady Hydrologic Engineering Center's River Analysis System to simulate small storms in a 5 km‐long, second‐order generic stream from the Chesapeake Bay watershed, and varied % channel restored (starting at the upstream end), restoration location, restoration bank height (distinguishes bankfull from Stage 0 restoration), and floodplain width/Manning's n. Stream restoration decreased (attenuated) peak flow up to 37% and increased floodplain exchange by up to 46%. Floodplain width and % channel restored had the largest impact on flood attenuation. The incremental effects of new restoration projects on flood attenuation were greatest when little prior restoration had occurred. By contrast, incremental effects on floodplain exchange were greatest in the presence of substantial prior restoration, setting up a tradeoff. A similar tradeoff was revealed between attenuation and exchange for project location, but not bank height or floodplain width. In particular, attenuation and exchange were always greater for Stage 0 than for bankfull floodplain restoration. Stage 0 thus may counteract human impacts such as urbanization.
The concentrations and quality of dissolved organic matter (DOM) and their sources were studied for multiple storm events collected over a three‐year period (2008–10) in a forested headwater (12 ha) ...catchment in the mid‐Atlantic Piedmont region of the USA. DOM constituents were characterized using a suite of indices derived from ultraviolet absorbance and PARAFAC modeling of fluorescence excitation emission matrices. Runoff sources and hydrologic flow paths were identified using an end‐member mixing model, stable isotope data, and groundwater elevations from valley‐bottom saturated areas. DOM constituents and their sources differed dramatically between base flow and storm‐event conditions. The aromatic and humic DOM constituents in stream water increased significantly during storm events and were attributed to the contributions from surficial sources such as throughfall, litter leachate and soil water. Groundwater sources contributed a large fraction of the DOM constituents during base flow and were responsible for the high % protein‐like fluorescence observed in base flow. Hydrologic flow paths and runoff sources were critical for explaining the differences in DOM among the storm events. This study underscored the value of studying multiple storm events across a range of hydrologic and seasonal conditions. Summer events produced the highest concentrations for humic and aromatic DOM while the corresponding response for winter events was muted. A large event following summer drought produced a complex DOM response which was not observed for the other events. These extreme events provided important insights into how DOM quality may change for future changes in climate and water quality implications for sensitive coastal ecosystems.
Key Points
Storm event patterns of DOM
DOM quality from optical indices
Seasonal patterns of DOM
Riverine organic matter supports of the order of one-fifth of estuarine metabolism. Coastal ecosystems are therefore sensitive to alteration of both the quantity and lability of terrigenous dissolved ...organic matter (DOM) delivered by rivers. The lability of DOM is thought to vary with age, with younger, relatively unaltered organic matter being more easily metabolized by aquatic heterotrophs than older, heavily modified material. This view is developed exclusively from work in watersheds where terrestrial plant and soil sources dominate streamwater DOM. Here we characterize streamwater DOM from 11 coastal watersheds on the Gulf of Alaska that vary widely in glacier coverage (0-64 per cent). In contrast to non-glacial rivers, we find that the bioavailability of DOM to marine microorganisms is significantly correlated with increasing (14)C age. Moreover, the most heavily glaciated watersheds are the source of the oldest ( approximately 4 kyr (14)C age) and most labile (66 per cent bioavailable) DOM. These glacial watersheds have extreme runoff rates, in part because they are subject to some of the highest rates of glacier volume loss on Earth. We estimate the cumulative flux of dissolved organic carbon derived from glaciers contributing runoff to the Gulf of Alaska at 0.13 +/- 0.01 Tg yr(-1) (1 Tg = 10(12) g), of which approximately 0.10 Tg is highly labile. This indicates that glacial runoff is a quantitatively important source of labile reduced carbon to marine ecosystems. Moreover, because glaciers and ice sheets represent the second largest reservoir of water in the global hydrologic system, our findings indicate that climatically driven changes in glacier volume could alter the age, quantity and reactivity of DOM entering coastal oceans.
Excess nutrients commonly lead to eutrophication and harmful algal blooms. Stream restoration is increasingly popular for nutrient removal enhancing exchange with the reactive hyporheic zone. ...Hyporheic reactions such as denitrification are often transport-limited and instream restoration structures have been proposed to enhance hyporheic exchange and nutrient removal. However, the comparative effects of instream structure types and watershed setting (i.e. environmental characteristics such as sediment hydraulic conductivity, stream slope) are still poorly understood. Here we used MIKE SHE to model groundwater and surface water interaction and nitrate removal (denitrification) in a 200 m second order stream reach. We simulated various in-stream structures (channel-spanning weirs, partially spanning structures such as cross veins, buried structures) and investigated the effect of controlling environmental characteristics that vary with watershed setting. We found that the environmental characteristics had the greatest effect on surface water-groundwater exchange and therefore denitrification, including streambed hydraulic conductivity, natural or background stream topography and slope, and groundwater levels. Type and number of instream structures also influenced surface water-groundwater exchange and denitrification, but to a lesser degree. Human effects at the watershed scale from agriculture and urbanization likely play a role in whether reach-scale restoration practices succeed in achieving water quality goals both through effect on exchange itself (e.g., altering bed sediment texture) and on nitrate sources. More broadly, restoration efforts at the watershed scale itself, such as reducing fertilizer use or improving stormwater management, may be necessary to achieve ambitious water quality goals. Nevertheless, reach-scale restoration efforts such as in-stream structures may play a useful role in certain watershed settings, for example where groundwater conditions induce neither strong gaining nor strong losing conditions. The interaction of reach-scale modifications and watershed setting must be understood to optimize nutrient removal from stream restoration through enhanced hyporheic exchange.
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