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1. The Influence of Tides on Coastal Plain Channel Geomorphology: Altamaha River, Georgia, USA

   https://doi.org/10.1029/2020JF005839  

   Sulaiman, Z. A. ; Viparelli, E. ; Torres, R. ; Yankovsky, A. ; Grego, J. ( July 2021 , Journal of Geophysical Research: Earth Surface)

   Rivers that traverse the terrestrial‐marine interface may have lower reaches that are influenced by both terrestrial and marine processes. However, only a handful of studies have focused on how the interactions of fluvial and tidal processes translate to channel geomorphology, and those are largely from delta/distributary systems. Here we quantify channel properties along the fluvial‐tidal transition reach of a coastal plain river and provide insight into their origins. The study site is a 47 km long tidal, single‐thread freshwater section of a river at 29 to 76 river kilometers inland of the estuary mouth, upstream of the delta/distributary system, and with average riverbed slope of 10⁻⁴. Results show that a tidal wave approaching the study reach loses 15%–17% of its incident energy (per horizontal area) per kilometer of channel, and at 51 km upstream of the mouth the incident energy is reduced to <1%. Also, at or near 51 km we observed breaks in along‐channel trends of channel cross‐section geometry, bed grain size, sinuosity, channel bed and water surface slopes. We propose that fluvial‐tidal flow processes and corresponding geomorphic feedbacks are apparent as abrupt changes in channel properties that highlight the influence of tides, and these discontinuities may be endemic to fluvial‐tidal transition zones in general. How these transition reaches self‐adjust in response to climate change remains largely unexplored but these reaches are likely to become important geomorphic hotspots.

    

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2. Otolith microchemistry highlights the importance of extensive connectivity for conservation of an iconic migratory fish in a large tropical river basin

   https://doi.org/10.1002/aqc.4003  

   Pease, Allison A. ; Jacobs, Gregory R. ; Mendoza‐Carranza, Manuel ; Rodiles‐Hernández, Rocío ; Wenger, Seth J. ; Capps, Krista A. ( August 2023 , Aquatic Conservation: Marine and Freshwater Ecosystems)

   Migratory fishes exert important influences on tropical river food webs, but these species are often most vulnerable to environmental change. Movement patterns of fishes in tropical rivers are also generally poorly understood, hindering conservation efforts.

   Common snook,Centropomus undecimalis,depend on connected coastal habitats for reproduction and growth and can migrate extensively up rivers when not hindered by barriers to movement. In southern Mexico, they are frequently captured 300 river km or more upstream from the delta of the Usumacinta River. The Usumacinta is a large river system with no mainstem dams, providing an expansive network of connected river, stream and wetland habitats which form a much larger migratory domain that exists in many other systems where common snook have been studied.

   To assess whether fish captured in different zones were associated with different natal habitats or distinct migratory patterns, variation in common snook otolith chemical signatures was examined in the Usumacinta River from the Gulf of Mexico coast to as far as 600 river km upriver.

   Otolith microchemistry was useful for characterizing migratory histories of individuals, but there was no clear evidence that fish captured in different river zones used different types of natal habitats. Based on lifetime Sr:Ca in otoliths, a diverse array of movement patterns was evident in the Usumacinta system, with 97% of common snook showing evidence of freshwater habitat use.

   Prevalent use of riverine habitats by common snook far from the coast reinforces the need to preserve connectivity in the Usumacinta River and other undammed systems supporting migratory species. The natural flow regime and lack of barriers allow for longitudinal and lateral connectivity in this system, providing pathways for migratory species to move extensively and have access to an array of habitats, including productive floodplain lagoons.

    

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3. Groundwater–surface water interactions in a river estuary and the importance of geomorphology: Insights from hydraulic, thermal and geophysical observations

   https://doi.org/10.1002/hyp.14372  

   Zamora, Peter B. ; Cardenas, M. Bayani ; Cook, Perran L. M. ( October 2021 , Hydrological Processes)

   This study presents the groundwater flow and salinity dynamics along a river estuary, the Werribee River in Victoria, Australia, at local and regional scales. Along a single reach, salinity across a transverse section of the channel (~80 m long) with a point bar was monitored using time‐lapse electrical resistivity (ER) through a tidal cycle. Groundwater fluxes were concurrently estimated by monitoring groundwater levels and temperature profiles. Regional porewater salinity distribution was mapped using 6‐km long longitudinal ER surveys during summer and winter. The time‐lapse ER across the channel revealed a static electrically resistive zone on the side of the channel with a pronounced cut bank. Upward groundwater flux and steep vertical temperature gradients with colder temperatures deeper within the sediment suggested a stable zone of fresh groundwater discharge along this cut bank area. Generally, less resistive zones were observed at the shallow portion of the inner meander bank and at the channel center. Subsurface temperatures close to surface water values, vertical head gradients indicating both upward and downward groundwater flux, and higher porewater salinity closer to that of estuary water suggest strong hyporheic circulation in these zones. The longitudinal surveys revealed higher ER values along deep and sinuous segments and low ER values in shallow and straighter reaches in both summer and winter; these patterns are consistent with the local channel‐scale observations. This study highlights the interacting effects of channel morphology, broad groundwater–surface water interaction, and hyporheic exchange on porewater salinity dynamics underneath and adjacent to a river estuary.

    

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4. River management response to multi‐decade changes in timing of reservoir inflows, Columbia River Basin, USA

   https://doi.org/10.1002/hyp.13910  

   Jones, Julia A. ; Hammond, John C. ( October 2020 , Hydrological Processes)

   Around the world, long‐term changes in the timing and magnitude of streamflow are testing the ability of large managed water resource systems constructed in the 20th century to continue to meet objectives in the 21st century. Streamflow records for unregulated rivers upstream of reservoirs can be combined with records downstream of reservoirs using a paired‐watershed framework and concepts of water resource system performance to assess how reservoir management has responded to long‐term change. Using publicly available data, this study quantified how the intra‐annual timing of inflows and outflows of 25 major reservoirs has shifted, how management has responded, and how this has influenced reliability and vulnerability of the water resource system in the 668,000 km²Columbia River basin from 1950 to 2012. Reservoir inflows increased slightly in early spring and declined in late spring to early fall, but reservoir outflows increased in late summer from 1950 to 2012. Average inflows to reservoirs in the low flow period exceeded outflows in the1950s, but inflows are now less than outflows. Reservoirs have increased hedging, that is, they have stored more water during the spring, in order to meet the widening gap between inflows and outflows during the summer low flow period. For a given level of reliability (the fraction of time flow targets were met), vulnerability (the maximum departure from the flow target) was greater during periods with lower than average inflows. Thus, the water management system in this large river basin has adjusted to multi‐decade trends of declining inflows, but vulnerability, that is, the potential for excess releases in spring and shortfalls in summer, has increased. This study demonstrates the value of combining publicly available historical data on streamflow with concepts from paired‐watershed analyses and metrics of water resource performance to detect, evaluate, and manage water resource systems in large river basins.

    

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5. Sea Level, Tidal, and River Flow Trends in the Lower Columbia River Estuary, 1853–present

   https://doi.org/10.1029/2019JC015656  

   Talke, S. A. ; Mahedy, A. ; Jay, D. A. ; Lau, P. ; Hilley, C. ; Hudson, A. ( March 2020 , Journal of Geophysical Research: Oceans)

   Few tidal records are available pre‐1900 for the Pacific Ocean. We improve data coverage by recovering historical tabulations and digitizing analog tide rolls from Astoria, Oregon, for 1853–1876. Nearly 13,500 overlapping images of tides from 1855–1870 were digitized at a 6 min resolution using a line‐finding algorithm. Available hourly and high/low tabulations were also digitized, as were nearby hourly records from 1933 to 1943. Uncertainty was assessed by evaluating manual staff measurements, historical documents, and leveling surveys. Results suggest that uncertainty in mean sea level varies from ±0.07 m (early 1850s) to ±0.03 m (1867–1876) and is driven primarily by datum and benchmark uncertainty, rather than measurement precision, data reduction procedures, or hydrodynamic changes. We also corrected an up‐to 0.05 m error in the 1925–1960 tidal datum at Astoria. Harmonic analysis shows that major tidal constituents increased by up to 7% between 1855 and 2018. Mean tidal range increased by 0.1 m (5%), with more change occurring in July (0.17 m larger) than winter (0.07 m larger). By contrast, sea level increased most in winter and least in spring/summer. Tidally based estimates of river discharge suggest that these observations are caused by a ~50% reduction in peak spring discharge and a 30–60% increase in winter discharge. No evidence of altered upwelling is found. Overall, Astoria relative sea level (RSL) increased by 0.06 m ± 0.04 m since the 1858–1876 epoch or, after accounting for vertical land motion, 0.11 ± 0.09 m. Consistent with GNSS measurements, RSL has dropped near the estuary mouth since 1905, indicating a strong tectonic influence.

    

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