Seasonal, tidal, and geomorphic controls on sediment export to Amazon River tidal floodplains

Abstract

Mainstem–floodplain material exchange in the tidal freshwater reach of major rivers may lead to significant sequestration of riverine sediment, but this zone remains understudied compared to adjacent fluvial and marine environments. This knowledge gap prompts investigation of floodplain‐incising tidal channels found along the banks of tidal rivers and their role in facilitating water and suspended‐sediment fluxes between mainstem and floodplain. To evaluate this role, and how it evolves along the tidal river and with time, we measured water level, flow velocity, temperature, and suspended‐sediment concentration (SSC) in four tidal channels along the tidal Amazon River, Brazil. Eleven deployments were made during low, rising, high, and falling seasonal Amazon discharge. Generally, channels export high‐SSC water from the mainstem to the tidal floodplain on flood tides and transfer low‐SSC water back to the mainstem on ebbs. Along the length of the tidal river, the interaction between tidal and seasonal water‐level variations and channel–floodplain morphology is a primary control on tidal‐channel sediment dynamics. Close to the river mouth, where tides are large, this interaction produces transient flow features and current‐induced sediment resuspension, but the importance of these processes decreases with distance upstream. Although the magnitude of the exchange of water and sediment between mainstem and floodplain via tidal channels is a small percentage of the total mainstem discharge in this large tidal‐river system, tidal channels are important conduits for material flux between these two environments. This flux is critical to resisting floodplain submergence during times of rising sea level. © 2019 John Wiley & Sons, Ltd.

Morphology and dynamics of the intertidal floodplain along the Amazon tidal river

“…There are no discernible levees along the lower Amazon tidal river; instead, there are planar surfaces with well‐developed networks of dendritic tidal channels (tens to hundreds of meters wide) through which most exchange of water and sediment between the river and floodplain occurs (Nowacki et al. ). The close agreement between the elevation of the land surface and the water‐level datum downstream of Cajari highlights the degree of floodplain infilling along this reach (Figures and ).…”

“…Near Macapá, for example, the floodplain is elevated to the point that a combination of seasonally high water and high tide is necessary to inundate it (Nowacki et al. ). Because the water‐level datum defined in this paper represents neap tidal conditions during the seasonal flood, the banks surveyed near Macapá are up to 50 cm above the datum.…”

“…Observations of river‐bank morphology made during earlier fieldwork along the Amazon tidal river (see Fricke et al, ; Nowacki et al, ) suggest that the river banks change form over the length of the tidal river (Figure B,C). Levees appear to be prominent in the upper tidal river and absent from the lower reach.…”

Sustainable Management, Conservation, and Restoration of the Amazon River Delta and Amazon-Influenced Guianas Coast: A Review

“…The tidal range increases to 1-2 m in the central tidal river [41] and sediment accumulation rates commonly exceed 1 cm/year on the floodplain [29]. The lower tidal river is dominated by floodplains where turbid tidal flows move through dendritic networks of channels, presently exchanging about 2% of the Amazon sediment load with the floodplain [46] and building vegetated tidal flats composed of fine silt and clay (5-20 µm, <30% sand, [45]). The Amazon delta accounts for 75% of Brazil's mangroves [47].…”

Connecting Sediment Retention to Distributary‐Channel Hydrodynamics and Sediment Dynamics in a Tide‐dominated Delta: The Ayeyarwady Delta, Myanmar

“…Starting in the fresh, tidal river, sediment export tends to dominate within channels, while regular overbank flows deposit sediment on floodplain surfaces (Fricke et al., 2019; Hoitink & Jay, 2016; Nowacki et al., 2019). During flood events, sediment delivery can scale non‐linearly with fluvial discharge while sediment export scales linearly, leading to net sediment retention (Ralston & Geyer, 2017).…”