Challenges in Estuarine and Coastal Science: Estuarine and Coastal Sciences Association 50th Anniversary Volume

CHAPTER 1

Morphodynamics of Tropical Atlantic River Mouths and their Adjacent Shorelines*

HELENE BURNINGHAM, SILVIA PALOTTI POLIZEL and AWA BOUSSO DRAMÉ

Abstract

The tropical, coastal plain hinterlands of the Atlantic comprise large rivers, with plentiful sediment supply, that meet swell-dominated wave climates at the coast, capable of modifying river mouth deposits into a range of deltaic sedimentary frameworks. Land-use, land-cover changes and other anthropogenic activities across the river basins at these latitudes (e.g. deforestation, agriculture, large-scale water management) have impacted shorelines through modifications to the discharge and sediment supply to the coast. The tropical coastal river systems of West Africa and east Brazil encompass a range of morphologies that reflect the relative roles that marine and fluvial processes play. Along both the west and east oceanic margins, these river mouth shorelines are sensitive to climate change influencing fluvial discharge and wave climate variability. Modifications in wave climate and hydrological management (i.e. dam activities) can affect the biogeomorphology of river mouths through changes in sediment supply to the coast, and sediment redistribution along the adjacent deltaic shorelines. In future decades, climate change and sea-level rise have the potential to trigger a range of changes in these processes that would impact the morphodynamics of river mouths and associated delta shorelines. Understanding the hydro- and morphodynamics associated with these coastal landforms is crucial to deal with such future challenges and evaluate adaptation measures.

Keywords: river mouth, hydrological regime, shoreline change analysis, delta, estuary, inlet, catchment water management

Correspondence: [email protected]

Introduction

River mouths are some of the most productive environments in the world, where the interplay of marine and fluvial processes supply sediments and nutrients from different sources, encouraging the development of a heterogeneous framework of sedimentary environments that support a range of habitats and species. The depositional systems that develop in these contexts can also be very important in providing several ecosystem services to local coastal and hinterland communities, particularly in terms of regulating services (e.g. protection) and provision services (e.g. food).

The geomorphology of river mouths can be highly varied and is determined by the relative influence of river, wave and tide processes (Wright 1977), which may lead to tides extending far into river valleys or wave modification of a river course, for example. The potential for complex interactions between these processes has underpinned much of the work classifying the depositional systems that develop in river mouth contexts (e.g. Galloway 1975; Boyd et al. 1992). Deltas and estuaries are the key sedimentary and geomorphological frameworks that develop around river mouths. Although there are several different definitions that address specific geological, hydrological and biochemical features and processes, at a simple geomorphological level, the differentiation between deltas and estuaries reflects the scale of the zone of interaction and relative dominance of tidal and river currents. Estuarine river mouths are those where tides extend landward into the river valley, where currents exert some control on sediment transport and where there is some degree of mixing of the fresh (river) and saline (marine) water bodies. Deltaic river mouths extend seaward of the former or structural river valley through the accumulation of riverine sediment, and tidal interactions are largely limited to only the most seaward inlet of the deltaic system. Wave processes are largely confined to the seaward-most part of the river mouth and are more important in the shaping of mouth morphology and supply of sediments to adjacent shorelines. These systems therefore have the potential to vary significantly in both time and space.

Accommodation space (the potential sedimentary infill zone contained by the antecedent valley or coastal plain topography) and sediment supply are also key factors in influencing the morphology and dynamics of depositional structures formed, both at the river mouth and across the wider coast. For example, in coastal plain environments and catchments delivering large sediment fluxes, river mouth deposits can extend far across the shoreface (cross-shore and alongshore). As such, the role of catchment management, and specifically the construction of dams, has been highlighted by many as a key driver of river mouth and coastal erosion over recent decades (Syvitski et al. 2005; Fagherazzi et al. 2015) and a fundamental control on coastal sediment budgets (Warrick 2020). Wider human impacts include enhanced subsidence linked to groundwater abstraction and petroleum mining, and when combined with sea-level rise and shifts in weather patterns associated with climate change, there is a clear concern over the sustainability of river mouths in the future, bringing significant potential risks to millions of people (Syvitski 2008).

These risks and the sensitivity of coastal communities to future environmental change are more pronounced within the tropics where there is a greater proportion of less economically developed countries. Climate change and climate variability are the significant drivers of societal challenges in the tropics, where the increased frequency of hurricanes, droughts and floods impacts shoreline populations in both direct (storm surge and destructive wind) and indirect (changes in rainfall patterns and delivery of water across catchments) ways. In the tropical Atlantic, the Inter Tropical Convergence Zone (ITCZ) atmospheric system that connects South America to West Africa moves north and south each year and drives the wet and dry seasons on each continent, meaning that the climate of the east and west Atlantic is interlinked. Decadal variability in this, driven by the North–South Atlantic thermohaline gradient, impacts rainfall most significantly in South America and West Africa (Foltz et al. 2019). Several of the tropical river systems entering the Atlantic Ocean contribute significant freshwater that drives the salinity variability, influencing the large-scale Atlantic Meridional Overturning Circulation that controls much of the climate around the North Atlantic Ocean, but significantly also controls the Atlantic Multidecadal Variability and hence rainfall in West Africa (Jahfer, Vinayachandran and Nanjundiah 2020). Tropical river systems are also particularly important geomorphologically, in delivering relatively more sediment to the oceans than their temperate counterparts, and thereby performing a significant denudation function across continental land masses (Milliman and Farnsworth 2013).

The climate interlinkage across the tropical Atlantic has important implications on the future sustainability of river mouth systems in relation to the way that water is managed across catchments, and the associated controls on sediment flux, particularly when combined with the additional pressures of increasing coastal populations and sea-level rise. In this chapter, we review the key geomorphological characteristics of tropical Atlantic river mouths to understand more specifically the possible sensitivities to climate and climate change. We also present an analysis of morphology, hydrology and shoreline dynamics of two case examples from east and west Atlantic – the São Francisco in Brazil, South America, and the Senegal on the Senegal–Mauritania border, Africa. We illustrate the impact that anthropogenic interventions have had on these systems, and reflect on the future challenges for communities in these regions.

Overview of the approach

River mouths of the tropical Atlantic were assessed in terms of their current geomorphology. The Caldwell et al. (2019) global coastal and river delta dataset was subsampled to consider just those river mouths located within the tropics entering the Atlantic. A systematic review of these was undertaken in the context of high-resolution satellite imagery (ESA Sentinel-2 and through Google Earth Pro) to identify the presence of key geomorphological features within the river mouth environment (including depositional features such as beach ridges and spits, and structural control such as reefs or engineering). Within this database, Caldwell et al. (2019) identified deltaic river mouths based on the presence of distributary networks and/or extension of a subaerial deposit from the main shoreline. Here, we extend this to identify estuarine river mouths, but owing to the lack of clear visible criteria for this, we used the literature to inform this classification; that is, if the river is described anywhere in the published literature as an estuary, it is recorded as such in this analysis. This database was also joined to the global delta and river mouth dataset of Nienhuis et al. (2020) to supplement the marine process context (wave height, tidal range, sea-level rise, bathymetric slope) derived by Caldwell et al. (2019) with fluvial process context (basin area and river discharge).

The two specific river mouths of São Francisco (Brazil) and Senegal (West Africa) are reviewed in more detail to evaluate the relative change in fluvial and marine contexts over recent decades on either side of the Atlantic. Coastal dynamics were analysed following a change analysis of shorelines derived from satellite imagery between the 1980s and 2020, and compared with change since 2010. Catchment water level and discharge data, and the European Re-Analysis (ERA5) hindcast wave data were evaluated over the same timeframe to capture river and marine processes.

Tropical Atlantic river mouths

The catchments of rivers entering the tropical Atlantic lie almost entirely within tropical climate regions (Fig 1.1a); despite arid climates being present across much of the north and south of the tropical zone of Africa, it is perhaps not surprising that these areas do not comprise significant river catchments. The arid, hot, steppe of Eastern Brazil covers a significant area of the São Francisco catchment, overlapping part of an area known as Polígono das Secas, which experiences irregular rainfall and is characterised by socioeconomic underdevelopment (Soares, 2013). Tropical South America and western Africa are both dominated by near continental-scale basins (the Amazon and the Congo (Fig 1.1b)) that exert well-established controls on tropical Atlantic circulation and climate variability (Materia et al. 2012; Jahfer, Vinayachandran and Nanjundiah 2020). These two basins therefore indirectly exert significant controls over all tropical Atlantic river systems through their function in controlling climate variability. In terms of marine processes, annual average wave heights (Fig 1.1c) are an equivalent magnitude to the median tidal range (Fig 1.1d). Both are minimum in the Caribbean and Gulf of Mexico, but this does not capture the storm-wave climate associated with the prevalence for hurricanes in this region. For much of the west coast of Africa, annual average wave heights are less than 1.5 m, but rise to 2–2.5 m at the north and south extents of the tropics. The east coast of South America experiences a more spatially consistent annual average of 1.5–2 m except in the vicinity of the more shallow and sheltered mouth region of the Amazon. Tidal range is for the most part microtidal throughout the tropics but increases to low mesotidal in West Africa (Guinea and Guinea-Bissau) and meso/macrotidal in the Amazon region.

Figure 1.1 Tropical Atlantic contexts for river mouths: (a) Köppen-Geiger climate classification, (b) delineation of drainage basins and their relative river discharge, (c) mean annual significant wave height (1980–2020), and (d) median tidal range. Data sourced from Beck et al. (2018) [a], Nienhuis et al. (2020) [b], ERA5 [c], Caldwell et al. (2019) [d]

Given the discrete nature of spatial variation in tidal range, wave climate and river discharge across the tropics (beyond some specific regions (e.g. Amazon, Congo, Guinea) there are large stretches of coastline experiencing very similar conditions), it could be assumed that river mouths would exhibit similar geomorphology. River mouths previously identified as deltas and estuaries (Fig 1.2a/b) are found throughout the tropics; around 39% of river mouths have been described as deltas, 28% as estuaries and 54% as neither. But interestingly, around 13% of river mouths are concurrently referred to as both deltaic and estuarine (Fig 1.3a). For instance, the Senegal and the São Francisco river mouths are recognised in the literature as both estuary and delta. This duality often reflects the presence of delta-front estuary systems (Fairbridge 1980) – estuaries that develop within the seaward reaches of large-river delta frameworks (e.g. Bianchi and Allison 2009). But in some cases, this might highlight an Anthropocene shift from regressive to transgressive systems, incorporating (a) human intervention in catchment hydrology and sediment flux, reducing the potential for progradational processes (Syvitski et al. 2009; Nienhuis et al. 2020) and (b) sea-level rise and coastal subsidence, leading to flooding of river valleys and development of transgressive coastal sequences (Dalrymple et al. 1992).

Figure 1.2 Presence of specific geomorphological features at tropical Atlantic river mouths: (a) delta (Caldwell et al. 2019), (b) estuary (derived from a literature review), (c) beach ridges, (d) beaches, barriers and spits, (e) structural control at the mouth (reef, rock and engineering) and (f) those that comprise distributaries, show evidence of being ephemeral and where mangroves are present. The small black dots in each sub-figure are river mouth locations (Caldwell et al. 2019) where the features are not present.

The presence of specific landforms and landform features at river mouths, including barriers and distributaries, can provide some basis for understanding the recognition of a river mouth as deltaic or estuarine. Systematic review of the presence of such features is shown in Fig 1.2c–f, which again displays widespread occurrence of a range of river mouth characteristics. There is limited direct association between the presence of wave-formed landforms such as spits and beach ridges and higher energy wave climates, and the analysis supports the view that it is the relative, not absolute strength of different processes, in combination with sediment supply, that determines the development of specific landforms. River mouths illustrating evidence of being ephemeral are largely confined to Central America and parts of the African coast. But even this feature is a more complex reflection of river flow variability and persistence in the wave climate. Multivariate analysis of the presence and absence of river mouth landforms relative to key process parameters such as wave height and river discharge (Fig 1.3b) shows that there are no dominant gradients driving the distribution of this geomorphology and no specific geographic pattern. Despite a lack of evidence in the geographic distribution, wave height is more strongly associated with the presence of wave-formed landforms such as beaches, barriers and spits, but is also associated with the presence of ephemeral river mouths and a lack of developed mangrove system. Tidal range is more strongly associated with the presence of distributaries. River mouths that have neither been defined as estuaries nor deltas are more strongly associated with smaller basins, lower river discharge and the presence of bedrock control. River mouths defined as estuaries and/or deltas tend to be linked to larger river systems; interestingly, no specific landforms are associated with the river mouth definitions, suggesting that this geomorphology is not an underpinning factor in the classification of river mouths within the tropics.

Figure 1.3 (A) Frequency distribution of tropical Atlantic river mouths classified as deltas, estuaries, both (dual) or neither, (B) principal component (PC) analysis of different geomorphological features at river mouths explored in the context of marine and river variables (wave height, bathymetric slope, tidal range, sea-level rise derived from Caldwell et al. (2019); river basin area and discharge from Nienhuis et al. (2020)), quoting percent explained and component scores for the first two components

Tropical coastal river systems of east Brazil and West Africa

As already demonstrated, there is a strong climate link between South America and West Africa, and a broad agreement that climate change impacts will be significant for both regions in terms of changes in rainfall patterns. Coupled with the legacy of successive catchment management interventions, rivers and their mouth environments are already responding to forcing associated with the Anthropocene (Brown et al. 2017).

Case example: São Francisco River

The São Francisco River’s source is in the State of Minas Gerais at an altitude of 1,800 m (Guimarães 2010; Rangel and Dominguez 2019), and the river flows for 2,486 km until it reaches the Atlantic Ocean, in north-east Brazil. The São Francisco watershed occupies an area of 639,219 km2, representing 7.5% of the national territory (Pereira et al. 2007; Medeiros et al. 2014). Owing to its size and diversity of environments, this watershed is divided into four physiographic regions: upper, middle, lower-middle and lower (Bettencourt et al. 2016) (Fig 1.4a). The São Francisco River is an important and strategic river for the north-east region, as it provides 70% of the superficial waters available for the region, supplying a population of 18.2 million (Castro and Pereira 2019). As noted previously, São Francisco watershed covers different climatic zones, transitioning from humid to arid, meaning that both evapotranspiration (c. 900 mm yr–1) and precipitation (c. 1,000 mm yr–1) play important roles in catchment hydrology; rainfall is mainly controlled by the South American Monsoon System and the ITCZ (Dominguez and Guimarães 2021) and varies from 1,400 mm in the upper reaches to 350 mm in the coastal valley (Castro and Pereira 2019). Human interventions in the São Francisco valley started early in the seventeenth century with the development of routes to connect the coast to the interior. Intensification of intervention in the mid-1940s reflected increased need for water management for irrigated agriculture and energy production (IBGE, 2009).

Figure 1.4 (a) Location of the São Francisco River in Brazil and its respective watershed with subdivisions (upper, middle, lower-middle and lower), also showing the location of dams and hydrometric stations; (b) wave rose for the 1984–2020 wave climate for the area adjacent to the São Francisco River mouth; (c) monthly discharges of the São Francisco River for the period 1976–2020 at the Piranhas (bright blue line), Traipu (grey line) and Propriá (cyan line) hydrometric stations, located, respectively, 176.8 km, 87.7 km and 55.7 km upstream from the river mouth. Red dashed lines indicate the beginning of operation of each hydroelectric plant constructed along the São Francisco River in this period. Discharge data from ANA for 1976–2020 (National Water Agency – Hidroweb – https://www.snirh.gov.br/hidroweb/serieshistoricas )

The São Francisco River delta is wave dominated, and is one of the most prominent delta complexes on the Brazilian coast. Covering over 900 km2, the delta region is not densely populated (Piaçabuçu is the largest settlement with only 17,203 inhabitants (in 2010; IBGE 2021). The delta was formed during the Quaternary period with the construction of beach ridges and associated development of secondary sedimentary environments including mangroves and dunes (Dominguez 1996; Bittencourt et al. 2007). The deltaic shoreline is mainly formed of sand, with a predominance of medium sands in the beaches near the São Francisco River mouth (Dominguez et al. 2016); beach, spit, dune, mangrove, barrier and beach ridge environments are located around the river mouth, and all demonstrate dynamic behaviour over the recent multi-decadal timescale. The system experiences a semidiurnal microtidal regime (1.74 m spring tidal range) (Bittencourt et al. 2007; Dominguez and Guimarães 2021), and the wave climate is characterised by more frequent (60%) larger waves (1–3 m) from the south-east and less frequent (~35%) smaller (1–2 m) waves from east-south-east (Fig 1.4b).

The hydrological regime of the São Francisco watershed is illustrated at three hydrometric stations (Fig 1.4c). Dams were first constructed in the 1940s for the purpose of generating electricity (Castro and Pereira 2019), and over the decades since, a series of dams working in cascade have been installed: Paulo Afonso (1949), Três Marias (1962), Moxotó (1977), Sobradinho (1979), Itaparica (1988) and Xingó (1994) (Dominguez and Guimarães 2021). These dams control 98% of the watershed and in combination have reduced the discharge to the river mouth by 35%, regulating the original seasonal discharge to a steady flow of around 2,000 m3 s–1 (Knoppers et al. 2005). There was a significant reduction in the magnitude of discharge at Piranhas and Traipu following completion of the Xingó dam in 1994; discharge control has been intensified since 2012 with a critical decrease of the flows during droughts (Melo, Araújo Filho and Carvalho 2020), and this is evident downstream at Propriá. Discharge reduction has also impacted the circulation and salt balance in the estuarine reaches of the São Francisco. With less freshwater reaching the coast, saline intrusion has migrated upstream, meaning seawater influences the river up to 12 km from the mouth on a spring high tide (Paiva and Schettini 2021).

The delta shoreline has experienced significant changes in recent decades, with large stretches of coast experiencing either persistent advance or retreat (Fig. 1.5). Shoreline dynamics directly around the river mouth dominate the overall picture, where significant reshaping of this shoreline has generated shoreline change rates of over 50 m yr–1. Over the multi-decadal timescale, erosional behaviour is exhibited in both margins of the mouth, but over the recent decade, the south margin has experienced notable progradation. Scales of shoreline change decrease as you move away from the river mouth, but increase again around the Parapuca channel (a back-barrier distributary) mouth that has migrated southward by 10 km over the 36-year period analysed. The stretch of shoreline between the São Francisco and Parapuca mouths, exhibiting retreat at the north end and advance at the south end, implies shoreline rotation. It is possible that sediment eroded from the shoreline south of the São Francisco mouth has been supplied to the north of the Parapuca mouth, facilitating this anticlockwise rotation. The orientation of the shoreline on this southern aspect of the delta fan allows waves, predominately from the south-east, to arrive at the shoreline with a slight onshore tendency to the south-west, meaning that they can facilitate alongshore sediment transport southward away from the main river mouth.

Figure 1.5 Shoreline change analysis for the São Francisco River delta, showing (a) the São Francisco delta shoreline and key locations; (b) End Point Rate (EPR) calculations for a long-term (1984–2020) and a short-term (2010–20) period of analysis

Over the last ten years, shoreline change has been more variable, illustrating a predominance of shoreline advance along the beach-dune coasts beyond the river and channel mouth regions. The greater magnitudes of change over the short term suggest that changes occur episodically, and that over the longer term the net effect is smaller, meaning there is some cyclicity in behaviour. The north/east shoreline of the delta shows less alongshore variability over the shorter term, with a prevalence for shoreline advance; to the south/west, the patterns over multi-decadal and multi-annual timescales are similar, beyond the shift from retreat to advance on the south margin of the mouth, but the magnitudes are notably different, particularly in the more recent enhanced progradation of Paraíso Escondido Beach. The shoreline rotation between the São Francisco and Parapuca mouths is enhanced, but over a shorter alongshore distance as the shoreline advanced immediately to the south of the São Francisco mouth, in response to the onshore welding of river mouth bars. Interestingly the pivot of anticlockwise shoreline rotation between the São Francisco and Parapuca mouths does not change between the two timescales; a 1.5 km stretch of coastal barrier here, 250–500 m wide, comprising transverse dunes backed by mangrove and the Parapuca channel, has experienced only minor change in shoreline position over the last 40 years, irrespective of timescale within that period. But the rotation of the São Francisco and Parapuca stretch of coast is more extreme over the recent decade, largely because of increased accretion north of the Parapuca mouth.

This increase in magnitude of change around the river and channel mouths over the short term likely reflects the tendency for localised spit development and sediment bypassing around the mouth through bar development, migration and shoreline attachment. It is possible that these dynamics in the recent decade have increased with reduction in the São Francisco discharge following 2010, which has been consistently suppressed since then. Dominguez and Guimarães (2021) point out that river flow regulation with the maintenance of constant discharge (below 2,000 m3/s) and absence of peak flows might not be sufficient to maintain the delta shoreline. The national water agency also decreed in 2017 to significantly reduce the minimum discharge volumes from the dams located in the lower parts of the catchment (Sobradinho and Xingó) (ANA 2017), which are now delivering less than a third of the discharge than was expected to maintain the shoreline and contain erosion processes considering the river inputs.

Case example: Senegal River

Among the largest estuaries in Africa, the Senegal River has a cross-border watershed between Senegal, Mauritania and Mali (Fig 1.6a), sourced initially in the Fouta Djallon mountains in Guinea (Descroix et al. 2020). The river flows through different subtropical climates from Guinean (>1,000 mm of rainfall), through Sudanian (900–1,000 mm) to reach the Atlantic with a Sahelian climate (c. 350 mm), at the confluence between harmattan winds and Atlantic trade winds (Ministère de l’agriculture 1995). Like the São Francisco, the Senegal river mouth is described as both delta and estuary, and comprises an integrated suite of beach, dune, barrier and spit environments. The delta system was primarily constructed during the post-Nouakchottian period (5,500 years ago) (Michel 1973), and reshaping, valley infilling and meander development occurred during the Tafolien marine transgression (4,000–2,500 years ago) (Ameryckx et al. 1967; Thiam 2012). The shift toward estuarine functioning may have started with the Sahelian drought of the 1960s–80s that significantly lowered water levels in the Senegal River, and reduced its competency to transport sediments downstream. Human interventions such as damming policies undertaken to mitigate water availability in the valley, followed by breaching in the early 2000s of the Langue de Barbarie barrier that fronts the estuary and delta system, have possibly reinforced the development of estuarine processes. Tidal regime in the Senegal estuary is semidiurnal and microtidal. The wave climate is predominately from the north-north-east (Fig