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1. Intermittent Channel Systems of a Low‐Relief, Low‐Gradient Floodplain: Comparison of Automatic Extraction Methods

   https://doi.org/10.1029/2020WR027603  

   Xu, H. ; van der Steeg, S. ; Sullivan, J. ; Shellye, D. ; Cely, J. E. ; Viparelli, E. ; Lakshmi, V. ; Torres, R. ( September 2020 , Water resources research)

   null (Ed.)

   Intermittent floodplain channels are low‐relief conduits etched into the floodplain surface and remain dry much of the year. These channels comprise expansive systems and are important because during low‐level inundation they facilitate lateral hydraulic connectivity throughout the floodplain. Nevertheless, few studies have focused on these floodplain channels due to uncertainty in how to identify and characterize these systems in digital elevation models (DEMs). In particular, their automatic extraction from widely available DEMs is challenging due to the characteristically low‐relief and low‐gradient topography of floodplains. We applied three channel extraction approaches to the Congaree River floodplain DEM and compared the results to a channel reference map created through numerous field excursions over the past 30 years. The methods that we tested are based on flow accumulation area, topographic curvature, and mathematical morphology, or the D8, Laplacian, and bottom‐hat transform (BHT), respectively. Of the 198 km of reference channels the BHT, Laplacian, and D8 extracted 83%, 71%, and 23%, respectively, and the BHT consistently had the highest agreement with the reference network at the local (5 m) and regional (10 km) scales. The extraction results also include commission “error”, augmenting the reference map with about 100 km of channel length. Overall, the BHT method provided the best results for channel extraction, giving over 298 km in 69 km2 with a detrended regional relief of 1.9 m. Further, these analyses allow us to shed light on the meaning and use of the term “low‐relief landscapes”. 

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2. Vertical surface water–groundwater exchange processes within a headwater floodplain induced by experimental floods

   https://doi.org/10.1002/hyp.10884  

   Hester, Erich T. ; Guth, Christopher R. ; Scott, Durelle T. ; Jones, Charles N. ( June 2016 , Hydrological Processes)

   Restoring hydrologic connectivity between channels and floodplains is common practice in stream and river restoration. Floodplain hydrology and hydrogeology impact stream hydraulics, ecology, biogeochemical processing, and pollutant removal, yet rigorous field evaluations of surface water–groundwater exchange within floodplains during overbank floods are rare. We conducted five sets of experimental floods to mimic floodplain reconnection by pumping stream water onto an existing floodplain swale. Floods were conducted throughout the year to capture seasonal variation and each involved two replicate floods on successive days to test the effect of varying antecedent moisture. Water levels and specific conductance were measured in surface water, soil, and groundwater within the floodplain, along with surface flow into and out of the floodplain. Vegetation density varied seasonally and controlled the volume of surface water storage on the floodplain. By contrast, antecedent moisture conditions controlled storage of water in floodplain soils, with drier antecedent moisture conditions leading to increased subsurface storage and slower flood wave propagation across the floodplain surface. The site experienced spatial heterogeneity in vertical connectivity between surface water and groundwater across the floodplain surface, where propagation of hydrostatic pressure, preferential flow, and bulk Darcy flow were all mechanisms that may have occurred during the five floods. Vertical connectivity also increased with time, suggesting higher frequency of floodplain inundation may increase surface water–groundwater exchange across the floodplain surface. Understanding the variability of floodplain impacts on water quality noted in the literature likely requires better accounting for seasonal variations in floodplain vegetation and antecedent moisture as well as heterogeneous exchange flow mechanisms. Copyright © 2016 John Wiley & Sons, Ltd.

    

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3. Scalable Flux Metrics at the Channel‐Floodplain Interface as Indicators of Lateral Surface Connectivity During Flood Events

   https://doi.org/10.1029/2019WR026080  

   Byrne, C. F. ; Stone, M. C. ; Morrison, R. R. ( November 2019 , Water Resources Research)

   Lateral surface connectivity (LSC), defined here as the surficial component of hydrologic exchange flows perpendicular to longitudinal flow, is vital to biogeochemical, geomorphological, and ecological processes on floodplains. Because rivers throughout much of the world have been subjected to anthropogenic manipulation, LSC has been greatly altered. While qualitative descriptions of LSC exist, only a limited number of studies quantify this process. The objective of this study was to quantify mass and momentum flux as proxies for LSC. This objective was met through the analysis of hydrodynamic model outputs along a 10‐km reach of the Middle Rio Grande near Albuquerque, New Mexico, USA. Mass and momentum fluxes were quantified for two synthetic floods across three spatial scales: geomorphic sub‐units, units, and reaches. In addition, three floodplain surface elevations were defined: inset, restored, and historical. Mass and momentum fluxes displayed distinct hydrodynamic signatures at each floodplain surface elevation. Inset floodplains were subjected to the greatest and most widely distributed flux magnitudes, an indication of the anthropogenic alteration along the reach. Flux heterogeneity on inset surfaces was also greatest, and increased flood magnitude did not result in comparable heterogeneity at higher floodplain elevations. Unsteady results showed that peak flux magnitudes can occur on the rising and falling limbs of a flood hydrograph, providing evidence of hysteresis in channel‐floodplain connections. Ultimately, the results are indicative of complexities associated with LSC processes along geomorphologically altered channel‐floodplain systems. The scalable methods are applicable to a wide variety of river‐floodplain systems and are complementary to empirical flux measurements.

    

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4. Future regulated flows of the Colorado River in Grand Canyon foretell decreased areal extent of sediment and increases in riparian vegetation

   https://doi.org/10.1088/1748-9326/abc9e4  

   Kasprak, Alan ; Sankey, Joel B. ; Butterfield, Bradley J. ( January 2021 , Environmental Research Letters)

   Sediment transfer, or connectivity, by aeolian processes between channel-proximal and upland deposits in river valleys is important for the maintenance of river corridor biophysical characteristics. In regulated river systems, dams control the magnitude and duration of discharge. Alterations to the flow regime driven by dams that increase the inundation duration of sediment, or which drive the encroachment of vegetation into areas formerly composed of labile sediment and result in channel narrowing, may reduce sediment transfer from near-channel deposits to uplands via aeolian processes. Employing spatial methods developed by Kaspraket al(2018Prog. Phys. Geogr.), here we use data describing the areal extent of bare (i.e. subaerially exposed and non-vegetated) sediment along 168 km of the Colorado River downstream from Glen Canyon Dam in Grand Canyon, USA, in conjunction with inundation extent modeling to forecast how future flows of this highly regulated river will drive changes in the areal extent of sediment available for aeolian transport. We also compare modern bare sediment area to that which presumably would have existed under pre-dam hydrographs. Over the next two decades, the planned flow regime from Glen Canyon Dam will result in slight decreases in bare sediment area (−1%) on an annual scale. This is in contrast to pre-dam years, when unregulated low flows led to marked increases in bare sediment area as compared to the current discharge regime. Our findings also indicate that ∼75% of bare sediment in the study reach is inundated continuously at present, owing to increased baseflows in the post-dam flow regime; consequently, any reductions in flows below modern-day low discharges have the potential to expose large areas of bare sediment. We use vegetation modeling to quantify areas susceptible to vegetation encroachment under future flows, finding that 80% of bare sediment area is suitable for colonization by invasive tamarisk under the current flow regime. Our findings imply that the Colorado River in Grand Canyon, a system marked by widespread erosion of sediment resources and encroachment of riparian vegetation in the post-dam period, is likely to continue to see decreasing bare sediment extent over the coming decades in the absence of direct intervention through flow regime modification or widespread vegetation removal.

    

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5. Infilling Abandoned Deltaic Distributary Channels Through Landward Sediment Transport

   https://doi.org/10.1029/2019JF005254  

   Carlson, B. N. ; Nittrouer, J. A. ; Moodie, A. J. ; Kineke, G. C. ; Kumpf, L. L. ; Ma, H. ; Parsons, D. R. ; Wang, H. ( February 2020 , Journal of Geophysical Research: Earth Surface)

   Upon avulsion, abandoned deltaic distributary channels receive water and sediment delivered by a tie channel, overbank flow, and by tidal inundation from the receiving basin. The transport and deposition of sediment arising from this latter input have important impacts on delta development yet are not well constrained from field observations or numerical models. Herein, the Huanghe (Yellow River) delta, China, is used as a case study to evaluate how marine‐sourced sediment impacts abandoned channel morphology. For this system, artificial deltaic avulsions occur approximately decadally; the abandoned channels are inundated by tides, and deposition of sediment transforms the channel into a mudflat. Field data were collected from a channel abandoned 20 yr ago and included cores that penetrated the tidally deposited mud and antecedent fluvial channel sediment, topography, bathymetry surveys, and detailed time series monitoring of hydrodynamic conditions within the tidal channel and adjacent mudflat. These data are used to validate a model that predicts the rate of accumulation and grain size of sediment delivered from the tidal channel to the mudflat. The thickness of the marine‐sourced mud differs spatially by an order of magnitude and is primarily impacted by antecedent channel topography. Sediment has aggraded to an elevation approaching mean high tide, which is likely the limit of fill. As this elevation is below antecedent levees, assuming stationary relative sea level, the abandoned channel will remain a topographic low on the delta landscape and is therefore susceptible to reoccupation during future avulsions.

    

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