Sediment dynamics and deposits along the fluvial–marine transition: Tidal river to mangrove coast
Fricke, Aaron T
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Rivers supply the vast majority of sediment that reaches the global ocean. As many rivers approach the sea, they experience tidal influence in the absence of salinity, along a reach known as the tidal river. As a result, a significant fraction of the sediment discharged by rivers around the world passes through a tidal river before entering the ocean. Within the tropics, these tidal rivers also supply sediment to coastal mangrove forests near the river mouths. Although common, the deposits and dynamics associated with tidal rivers and the coastal mangrove forests they nourish remain poorly understood. Processes acting within tidal-river environments, as well as between tidal rivers and adjacent mangrove forests, are governed by a combination of fluvial and tidal processes, which are a focus of this work. The Amazon River is the largest fluvial source of freshwater and sediment to the global ocean and has the longest tidally-influenced reach in the world. Two major rivers, the Tapajós and Xingu, enter the Amazon along its tidal reach. However, unlike most fluvial confluences, these are not one-way conduits through which water and sediment flow downstream toward the sea. The drowned river valleys (rias) at the confluences of the Tapajós and Xingu with the Amazon River experience water-level fluctuations associated not only with the seasonal rise and fall of the river network, but also with semidiurnal tides that propagate as far as 800 km up the Amazon River. Superimposed seasonal and tidal forcing, distinct sediment and temperature signatures of Amazon and tributary waters, and antecedent geomorphology combine to create mainstem–tributary confluences that act as sediment traps rather than sources of sediment. Hydrodynamic measurements are combined with data from sediment cores to determine the distribution of tributary- and Amazon-derived sediment within the ria basins, characterize the sediment-transport mechanisms within the confluence areas, and estimate rates of sediment accumulation within both rias. The Tapajós and Xingu ria basins trap the majority of the sediment carried by the tributaries themselves in addition to ~20 Mt y-1 of sediment sourced from the Amazon River. These findings have implications for the interpretation of stratigraphy associated with incised-valley systems, such as those that dominated the transfer of sediment to the oceans during low-stands in sea level. The estimates of water and sediment discharged by the Amazon River are based on data from the lowermost non-tidal gauging station at Óbidos, ~800 km upstream of the Atlantic Ocean. Depositional environments along the lengthy tidal river downstream of Óbidos have been proposed as important sinks for up to a third of the reported sediment discharge from the Amazon River. However, the morphology and dynamics of the intertidal floodplain have yet to be described. River-bank surveys in five areas along the Amazon tidal river reveal a distinct evolution in bank morphology between the upper, central, and lower reaches of the tidal river. The upper tidal-river floodplain is defined by prominent natural levees that strongly control the transfer of water and sediment between the mainstem Amazon River and its floodplain. Increased tidal influence in the central tidal river suppresses levee development, and tidal currents increase sediment transport into the distal parts of the floodplain. The floodplain morphology in the lower tidal river closely resembles marine intertidal environments (e.g., mud flats, salt marshes), with dendritic tidal channels incising elevated vegetated flats. Theory, morphology, and geochronology suggest that the dynamics of sediment delivery to the intertidal floodplain of the Amazon tidal river vary along its length due to the relative dominance of fluvial and tidal influence. The interplay between fluvial and marine influence is similarly felt in coastal mangrove forests that are nourished by tidal rivers. Mangrove forests are an important means of coastal protection along many shorelines in the tropics, and are often associated with large rivers there. The mangrove forest at the seaward end of Cù Lao Dung, an island in the Mekong Delta, includes areas with progradation rates of 10s of meters per year, and areas that have experienced little to no progradation in recent decades. The physical proximity (<12 km) of these two environments allows detailed hydro- and sediment-dynamic measurements to be related directly to morphologic change and century-scale stratigraphy. Contrary to conventional understanding, the region of mangrove forest prograding most rapidly is subject to the greatest wave attack, while progradation is slowest in the most quiescent area. Limited progradation here is the product of a reduction in the supply of sediment to certain parts of the mangrove forest due to estuarine dynamics operating on spring–neap timescales. Measurements of sediment flux show transport into the rapidly prograding part of the forest, and transport out of the area with minimal progradation. Century-scale rates of sediment accumulation determined using Pb-210 geochronology are consistent with in-situ dynamical measurements and geomorphic evolution of the mangrove forest. Where progradation is most rapid, sediment accumulation rates (3–5.1 cm y-1) exceed the rate of local sea-level rise (~1.5 cm y-1). In contrast, sediment-accumulation rates in the area of minimal progradation (0.8–2.8 cm y-1) barely keep pace with local sea-level rise, if at all. Physical stratification is well preserved in cores from areas of rapid progradation, consistent with energetic transport processes and an ample sediment supply. Greater impact from bioturbation and episodic sediment delivery produce more chaotic bedding where progradation is less rapid. The presence of a supply-limited mangrove forest adjacent to a major sediment source highlights the complexity of sediment-supply pathways in coastal mangrove environments.
- Oceanography