High‐resolution topography reveals that floodplains along meandering rivers in Indiana commonly contain intermittently flowing channel networks. We investigated how the presence of floodplain ...channels affects lateral surface‐water connectivity between a river and floodplain (specifically exchange flux and timescales of transport) as a function of flow stage in a low‐gradient river‐floodplain system. We constructed a two‐dimensional, surface‐water hydrodynamic model using Hydrologic Engineering Center's River Analysis System (HEC‐RAS) 2D along 32 km of floodplain (56 km along the river) of the East Fork White River near Seymour, Indiana, USA, using lidar elevation data and surveyed river bathymetry. The model was calibrated using land‐cover specific roughness to elevation‐discharge data from a U.S. Geological Survey gage and validated against high‐water marks, an aerial photo showing the spatial extent of floodplain inundation, and measured flow velocities. Using the model results, we analyzed the flow in the river, spatial patterns of inundation, flow pathways, river‐floodplain exchange, and water residence time on the floodplain. Our results highlight that bankfull flow is an oversimplified concept for explaining river‐floodplain connectivity because some stream banks are overtopped and major low‐lying floodplain channels are inundated roughly 19 days per year. As flow increased, inundation of floodplain channels at higher elevations dissected the floodplain, until the floodplain channels became fully inundated. Additionally, we found that river‐floodplain exchange was driven by bank height or channel orientation depending on flow conditions. We propose a conceptual model of river‐floodplain connectivity dynamics and developed metrics to analyze quantitatively complex river‐floodplain systems.
Key Points
Floodplain inundation occurs roughly 19 days per year, which obfuscates the notion of bankfull flow
Highly connected, meandering river‐floodplain systems have three flow‐dependent regimes: single thread, multithread, and fully inundated
We quantify the surface‐water connectivity of a river‐floodplain system with metrics of residence time and river‐floodplain exchange
Field studies of hyporheic exchange in mountain systems are often conducted using short study reaches and a limited number of observations. It is common practice to assume these study reaches ...represent hyporheic exchange at larger scales or different sites and to infer general relationships among potential causal mechanisms from the limited number of observations. However, these assumptions of representativeness are rarely tested. In this study, we develop numerical models from four segments of mountain streams in different geomorphologic settings and extract shorter reaches to test how representative exchange metrics are in shorter reaches compared to their reference segments. We also map the locations of the representative reaches to determine if a pattern exists based on location. Finally, we compare variance of these shorter within‐site reaches to 29 additional reaches across the same basin to understand the impacts of inferring causal mechanisms, for example, the expectation that wide and narrow valley bottoms will yield different hyporheic exchange patterns. Our results show that the location and length strategy of the study reach must be considered before assuming an exchange metric to be representative of anything other than the exact segment studied. Further, it is necessary to quantify within and between site variations before making causal inferences based on observable characteristics, such as valley width or stream morphology. Our findings have implications for future field practices and how those practices are translated into models.
Plain Language Summary
Hydrology models are used to generalize processes and make predictions about how water moves through the environment. For example, models are used to study how water moves in and out of streambeds because processes occurring in the streambed can significantly impact the water quality. These models assume that data and observations from short sections of streams can represent longer portions of a stream or similar streams in different places. However, short sections of the stream may not accurately represent the conditions of the whole site that is being modeled, meaning that the assumptions and ultimately the models are misrepresenting the process. To test this, we compared how water was exchanged between a long segment of a stream and its subsurface to exchanges occurring in a shorter section of the stream and its subsurface. We found that only some of the shorter sections of the stream we modeled accurately represented the longer segment of the stream from which it was taken. These findings are important for future field studies as they demonstrate how scientists need to consider several aspects of the stream before they choose a section of a stream as the basis of their assumption for a model.
Key Points
Reach length strategy and location both determine how representative a reach is compared to the larger stream segment in which it is located
Researchers must clarify, test, and quantify assumptions of within‐ and between‐reach variation
Assumed behavior cannot be inferred from visual inspection and iteration between field and modeling studies minimize uncertainties in hyporheic exchange
Land use within a watershed impacts stream channel morphology and hydrology and, therefore, in‐stream solute transport processes and associated transient storage mechanisms. This study evaluated ...transport processes in two contrasting stream sites where channel morphology was influenced by the surrounding land use, land cover, climate and geologic controls: Como Creek, CO, a relatively undisturbed, high gradient, forested stream with a gravel bed and complex channel morphology, and Clear Creek, IA, an incised, low‐gradient stream with low‐permeability substrate draining an agricultural landscape. We performed conservative stream tracer injections at these sites to address the following questions: (1) How does solute transport vary between streams with differing morphologies? and (2) How does solute transport at each stream site change as a function of discharge? We analysed in‐stream tracer time series data and compared results quantifying solute attenuation in surface and subsurface transient storage zones. Significant trends were observed in these metrics with varying discharge conditions at the forested site but not at the agricultural site. There was a broad range of transport mechanisms and evidence of substantial exchange with both surface and hyporheic transient storage in the relatively undisturbed, forested stream. Changing discharge conditions activated or deactivated different solute transport mechanisms in the forested site and greatly impacted advective travel time. Conversely in the simplified agricultural stream, there was a narrow range of solute transport behaviour across flows and predominantly surface transient storage at all measured discharge conditions. These results demonstrate how channel simplification inhibits available solute transport mechanisms across varying discharge conditions.
Land use within a watershed impacts stream channel morphology and hydrology and, therefore, in‐stream solute transport processes and associated transient storage mechanisms. In the relatively undisturbed, forested stream, there was a broad range of transport mechanisms and evidence of exchange with both surface and hyporheic transient storage that varied at different discharge conditions. Conversely in the simplified agricultural stream, there was a narrow range of solute transport behaviour across flows and predominantly surface transient storage at all measured discharge conditions.
Recent studies have demonstrated that compartmentalized pools of water preferentially supply either plant transpiration (poorly mobile water) or streamflow and groundwater (highly mobile water) in ...some catchments, a phenomenon referred to as ecohydrologic separation. The omission of processes accounting for ecohydrologic separation in standard applications of hydrological models is expected to influence estimates of water residence times and plant water availability. However, few studies have tested this expectation or investigated how ecohydrologic separation alters interpretations of stores and fluxes of water within a catchment. In this study, we compare two rainfall‐runoff models that integrate catchment‐scale representations of transport, one that incorporates ecohydrologic separation and one that does not. The models were developed for a second‐order watershed at the H.J. Andrews Experimental Forest (Oregon, USA), the site where ecohydrologic separation was first observed, and calibrated against multiple years of stream discharge and chloride concentration. Model structural variations caused mixed results for differences in calibrated parameters and differences in storage between reservoirs. However, large differences in catchment storage volumes and fluxes arise when considering only mobile water. These changes influence interpreted residence times for streamflow‐generating water, demonstrating the importance of ecohydrologic separation in catchment‐scale water and solute transport.
Representing ecohydrologic separation in hydrologic models alters interpretations of water storage and the transport of water and solutes at the catchment scale. These changes in internal catchment functioning influence plant water availability as well as the potential for biogeochemical reactions, demonstrating the relevance our evolving conceptual model has on ecological processes.
Over half of global rivers and streams lack perennial flow, and understanding the distribution and drivers of their flow regimes is critical for understanding their hydrologic, biogeochemical, and ...ecological functions. We analyzed nonperennial flow regimes using 540 U.S. Geological Survey watersheds across the contiguous United States from 1979 to 2018. Multivariate analyses revealed regional differences in no‐flow fraction, date of first no flow, and duration of the dry‐down period, with further divergence between natural and human‐altered watersheds. Aridity was a primary driver of no‐flow metrics at the continental scale, while unique combinations of climatic, physiographic and anthropogenic drivers emerged at regional scales. Dry‐down duration showed stronger associations with nonclimate drivers compared to no‐flow fraction and timing. Although the sparse distribution of nonperennial gages limits our understanding of such streams, the watersheds examined here suggest the important role of aridity and land cover change in modulating future stream drying.
Plain Language Summary
A majority of global streams are nonperennial, flowing only part of the year, and are critical for sustaining flow downstream, providing habitat for many organisms, and regulating chemical and biological processes. Using long‐term U.S. Geological Survey measurements for 540 watersheds across the contiguous United States, we mapped patterns and examined the causes of no‐flow fraction, the fraction of each climate year with no flow, no‐flow timing, the date of the climate year on which the first recorded no flow takes place, and length of the dry‐down period, the average number of days from a local peak in daily flow to the first occurrence of no flow. We found differences in patterns of no‐flow characteristics between regions, with higher no‐flow fraction, earlier timing, and shorter dry‐down duration in the western United States. No‐flow fractions were greater and less variable in natural watersheds, while no‐flow timing was earlier and dry‐down duration was shorter in human‐modified watersheds. Aridity had the greatest effect on intermittence across the United States, but unique combinations of climate, biophysical, and human impacts were important in different regions. The number of gages measuring streamflow in nonperennial streams is small compared to perennial streams, and increased monitoring is needed to better understand drying behavior.
Key Points
Three metrics reveal regional and human‐driven patterns of nonperennial flow: no‐flow fraction, day of first no flow, and dry‐down duration
Streams with human modifications generally dry more quickly than unmodified streams, especially in California and the Southern Great Plains
Climate strongly influences no‐flow fraction and timing, but physiographic variables are more important for the duration of dry down
Schistosomiasis is an important fresh-water-borne parasitic disease caused by trematode worms of the genus Schistosoma. With > 250 million people infected worldwide and approximately 800 million ...people at risk, the World Health Organization considers schistosomiasis to be the most important human helminth infection. Several prophylactic non-living vaccines are in pre-clinical and clinical development, but only one has been assessed for therapeutic effect in an animal model with modest results. Live attenuated Salmonella have multiple potential advantages as vaccine vectors. We have engineered an attenuated Salmonella enterica Typhimurium strain (YS1646) to produce a vaccine that targets the parasite digestive enzyme Cathepsin B (CatB). A multi-modality immunization schedule was used in chronically infected mice that included three oral (PO) doses of this CatB-bearing YS1646 strain on days one, three, and five as well as an intramuscular (IM) dose of recombinant CatB on day one. Parasite burden (worm count, intestinal and liver egg numbers) were 46.5 – 50.3% lower than in control animals 1 month post-vaccination and relative reductions further increased to 63.9 – 73.3% at 2 months. Serum anti-CatB IgG increased significantly after vaccination with the development of a more balanced TH1/TH2 pattern of response (ie: a shift in the IgG1:IgG2c ratio). Compared to control animals, a broad and robust CatB-specific cytokine/chemokine response was seen in splenocytes isolated 1 month post-vaccination. A vaccine that has both prophylactic and therapeutic activity would be ideal for use in conjunction with mass treatment campaigns with praziquantel in schistosome-endemic countries.
The spatial and temporal scales of hyporheic exchange within the stream corridor are controlled by stream discharge and groundwater inflow interacting with streambed morphology. While decades of ...study have resulted in a clear understanding of how morphologic form controls hyporheic exchange at the feature scale, we lack comparable predictive power related to stream discharge and the spatial structure of groundwater inflows at the reach scale, where spatial heterogeneity in both geomorphic setting and hydrologic forcing are present. In this study, we simulated vertical hyporheic exchange along a 600 m mountain stream reach under high, medium, and low stream discharge while considering groundwater inflow as negligible, spatially uniform, or proportional to upslope accumulated area. Most changes to hyporheic flow path residence time or length in response to stream discharge were small (<5%), suggesting that discharge is a secondary control relative to morphologically driven hyporheic exchange. Groundwater inflow was a primary control and mostly caused decreases in hyporheic flow path residence time and length. This finding generally agrees with expectations from the literature; however, flow path response was not consistent across the study reach. Instead, we found that flow paths driven by large hydraulic gradients coinciding with large morphologic features were less sensitive to changes in groundwater inflow than those driven by hydraulic gradients similar to the valley gradient. Our results indicate that consideration of heterogeneous arrangement of morphologic features is necessary to differentiate between hyporheic flow paths that persist in time and those that are sensitive to changing hydrologic conditions.
Key Points
Hyporheic flow paths do not respond uniformly to different stream discharge and groundwater inflow conditions
Hyporheic flow paths with hydraulic gradients similar to the valley gradient are the most sensitive to changing hydrologic conditions
Hyporheic flow paths with hydraulic gradients set by large morphologic features are the least sensitive to changing hydrologic conditions
Excess nitrogen (N) impairs inland water quality and creates hypoxia in coastal ecosystems. Agriculture is the primary source of N; agricultural management and hydrology together control aquatic ...ecosystem N loading. Future N loading will be determined by how agriculture and hydrology intersect with climate change, yet the interactions between changing climate and water quality remain poorly understood. Here, we show that changing precipitation patterns, resulting from climate change, interact with agricultural land use to deteriorate water quality. We focus on the 2012–2013 Midwestern U.S. drought as a “natural experiment”. The transition from drought conditions in 2012 to a wet spring in 2013 was abrupt; the media dubbed this “weather whiplash”. We use recent (2010–2015) and historical data (1950–2015) to connect weather whiplash (drought-to-flood transitions) to increases in riverine N loads and concentrations. The drought likely created highly N-enriched soils; this excess N mobilized during heavy spring rains (2013), resulting in a 34% increase (10.5 vs. 7.8 mg N L⁻¹) in the flow-weighted mean annual nitrate concentration compared to recent years. Furthermore, we show that climate change will likely intensify weather whiplash. Increased weather whiplash will, in part, increase the frequency of riverine N exceeding E.P.A. drinking water standards. Thus, our observations suggest increased climatic variation will amplify negative trends in water quality in a region already grappling with severe impairments.
PDF
