Small streams dominate US tidal reaches and will be disproportionately impacted by sea-level rise

Biogeochemical control points of connectivity between a tidal creek and its floodplain

“…We therefore suggest the lateral connectivity events identified by sensor data represent "export" control points (Bernhardt et al 2017), where hydraulic gradients drive short, rapid, and biogeochemically significant pulses, as is observed in other tidal systems (Regier and Jaffé 2016;Bogard et al 2020) and soil incubations performed at Beaver Creek (Sengupta et al 2021). Carbon and nutrients mobilized from groundwater into the creek may represent outwelling events that support productivity of downstream receiving waters, or, alternatively instigate eutrophication events downstream (Seitzinger and Phillips 2017;Bogard et al 2020;Tagestad et al 2021), with potential impacts to coastal oceans (Doney 2010). GHGs mobilized via lateral inputs into the creek, as observed in other tidal systems (Sadat-Noori et al 2016;Trifunovic et al 2020), may reduce the efficacy of the floodplain to sequester carbon, while creating an additional source of GHGs.…”

Seasonal Changes in the Drivers of Water Physico-Chemistry Variability of a Small Freshwater Tidal River

“…The stretch of the Union River monitored in this study is an example of a freshwater tidal system where freshwater discharge is high enough to prevent extensive estuarine influence (i.e., there is very little seawater intrusion), yet not so much to initiate floodplain dynamics. Tidal systems across the United States predominantly consist of small coastal watersheds like the Union River, representing the majority (66%) of total tidal stream length, yet are poorly researched (Tagestad et al, 2021). These loworder coastal streams are particularly vulnerable to sea level rise (Tagestad et al, 2021), as sea level rise increases both the extent of rivers experiencing tidal influence (Ensign and Noe, 2018) and increases estuarine mixing in previously freshwater tidal systems.…”

“…Tidal systems across the United States predominantly consist of small coastal watersheds like the Union River, representing the majority (66%) of total tidal stream length, yet are poorly researched (Tagestad et al, 2021). These loworder coastal streams are particularly vulnerable to sea level rise (Tagestad et al, 2021), as sea level rise increases both the extent of rivers experiencing tidal influence (Ensign and Noe, 2018) and increases estuarine mixing in previously freshwater tidal systems. In the Union River, for instance, there is currently a clear seasonality to the drivers of river biogeochemistry in this system, and only a small tidal influence.…”

“…Water chemistry dynamics of rivers (e.g., Kämäri et al, 2018) and estuaries (e.g., Baumann and Smith, 2017;Nelson et al, 2017;Proum et al, 2018) have been more frequently examined than freshwater tidal rivers, yet approximately 2,850 km of tidal freshwater rivers exist in the eastern United States alone (Ensign and Noe, 2018;Tagestad et al, 2021). One study modeling pH in the tidal freshwater portion of the Potomac River estuary found that hydrology, specifically low discharge, was the predominant influence on pH by increasing residence time and thus algal production (Cerco et al, 2013).…”

“…Physical and chemical properties in smaller freshwater tidal rivers may differ from larger freshwater tidal rivers such as the Amazon River, and therefore constitute a distinct type of tidal system likely with different drivers. Additionally, these low-order coastal streams represent the majority (66%) of total tidal stream length in the United States (Tagestad et al, 2021). The dynamics of nutrient, carbon, and lignin export in the Union River, a river with a freshwater tidal stretch in western Washington state, United States, during storm rainfall events were previously examined (Ward et al, 2012).…”