Deltaic islands are distinct hydro-environmental zones with global significance in food security, biodiversity conservation, and fishery industry. These islands are the fundamental building blocks of many river deltas. However, deltaic islands are facing severe challenges due to intensive anthropogenic activities, sea level rise, and climate change. In this study, dynamic changes of deltaic islands in Wax Lake Delta (WLD) and Atchafalaya Delta (AD), part of the Atchafalaya River Delta Complex (ARDC) in Louisiana, USA, were quantified based on remote sensing images from 1991 to 2019 through a machine learning method. Results indicate a significant increase in deltaic islands area for the whole ARDC at a rate of 1.29 km2/yr, with local expansion rates of 0.60 km2/yr for WLD and 0.69 km2/yr for AD. All three parts of the WLD naturally prograded seaward, with the western part (WP) and central part (CP) expanding southwestward to the sea, while the eastern part (EP) prograding southeastwards. Differently from WLD, the three parts of AD irregularly expanded seaward under the impacts of anthropogenic activities. The WP and CP of the AD expanded respectively northwestwards and southwestwards, while the EP remained stable. Different drivers dominate the growth of deltaic islands in the WLD and AD. Specifically, fluvial suspended sediment discharge and peak flow events were responsible for the shift in the spatial evolution of WLD, while dredging and sediment disposal contributed to the expansion of AD. Tropical storms with different intensity and landing locations caused short-term deltaic island erosion or expansion. Tropical storms mainly generated erosion on the deltaic islands of the WLD, while causing transient erosion or siltation on the deltaic islands of the AD. In addition, high-intensity hurricanes that made landfall east of the deltas caused more erosion in the AD. Finally, sea level rise, at the current rate of 8.17 mm/yr, will not pose a threat to the deltaic island of WLD, while the eastern part of AD may be at risk of drowning. This study recognizes the complexity of factors influencing the growth of deltaic islands, suggesting that quantitative studies on the deltaic island extent are of critical for the restoration and sustainable management of the Mississippi River Delta and other deltas around the world.
more »
« less
Co-evolution of wetland landscapes, flooding, and human settlement in the Mississippi River Delta Plain
Abstract River deltas all over the world are sinking
beneath sea-level rise, causing significant threats to natural
and social systems. This is due to the combined effects of
anthropogenic changes to sediment supply and river flow,
subsidence, and sea-level rise, posing an immediate threat
to the 500–1,000 million residents, many in megacities that
live on deltaic coasts. The Mississippi River Deltaic Plain
(MRDP) provides examples for many of the functions and
feedbacks, regarding how human river management has
impacted source-sink processes in coastal deltaic basins,
resulting in human settlements more at risk to coastal
storms. The survival of human settlement on the MRDP is
arguably coupled to a shifting mass balance between a
deltaic landscape occupied by either land built by the
Mississippi River or water occupied by the Gulf of Mexico.
We developed an approach to compare 50 % L:W isopleths
(L:W is ratio of land to water) across the Atchafalaya and
Terrebonne Basins to test landscape behavior over the last
six decades to measure delta instability in coastal deltaic
basins as a function of reduced sediment supply from river
flooding. The Atchafalaya Basin, with continued sediment
delivery, compared to Terrebonne Basin, with reduced
river inputs, allow us to test assumptions of how coastal
deltaic basins respond to river management over the last
75 years by analyzing landward migration rate of 50 %
L:W isopleths between 1932 and 2010. The average landward
migration for Terrebonne Basin was nearly 17,000 m
(17 km) compared to only 22 m in Atchafalaya Basin over
the last 78 years (p\0.001), resulting in migration rates of
218 m/year (0.22 km/year) and\0.5 m/year, respectively.
In addition, freshwater vegetation expanded in Atchafalaya
Basin since 1949 compared to migration of intermediate
and brackish marshes landward in the Terrebonne Basin.
Changes in salt marsh vegetation patterns were very distinct
in these two basins with gain of 25 % in the Terrebonne
Basin compared to 90 % decrease in the Atchafalaya
Basin since 1949. These shifts in vegetation types as L:W
ratio decreases with reduced sediment input and increase in
salinity also coincide with an increase in wind fetch in
Terrebonne Bay. In the upper Terrebonne Bay, where the
largest landward migration of the 50 % L:W ratio isopleth
occurred, we estimate that the wave power has increased
by 50–100 % from 1932 to 2010, as the bathymetric and
topographic conditions changed, and increase in maximum
storm-surge height also increased owing to the landward
migration of the L:W ratio isopleth. We argue that this
balance of land relative to water in this delta provides a
much clearer understanding of increased flood risk from
tropical cyclones rather than just estimates of areal land
loss. We describe how coastal deltaic basins of the MRDP
can be used as experimental landscapes to provide insights
into how varying degrees of sediment delivery to coastal
deltaic floodplains change flooding risks of a sinking delta
using landward migrations of 50 % L:W isopleths. The
nonlinear response of migrating L:W isopleths as wind
fetch increases is a critical feedback effect that should
influence human river-management decisions in deltaic
coast. Changes in land area alone do not capture how
corresponding landscape degradation and increased water
area can lead to exponential increase in flood risk to human
populations in low-lying coastal regions. Reduced land
formation in coastal deltaic basins (measured by changes in
the land:water ratio) can contribute significantly to
increasing flood risks by removing the negative feedback
of wetlands on wave and storm-surge that occur during
extreme weather events. Increased flood risks will promote
population migration as human risks associated with living
in a deltaic landscape increase, as land is submerged and
coastal inundation threats rise. These system linkages in
dynamic deltaic coasts define a balance of river management
and human settlement dependent on a certain level of
land area within coastal deltaic basins (L).
more »
« less
- NSF-PAR ID:
- 10017028
- Date Published:
- Journal Name:
- Sustainability Science
- ISSN:
- 1862-4065
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Complete transformations of land cover from prairie, wetlands, and hardwood forests to row crop agriculture and urban centers are thought to have caused profound changes in hydrology in the Upper Midwestern US since the 1800s. In this study, we investigate four large (23 000–69 000 km2) Midwest river basins that span climate and land use gradients to understand how climate and agricultural drainage have influenced basin hydrology over the last 79 years. We use daily, monthly, and annual flow metrics to document streamflow changes and discuss those changes in the context of precipitation and land use changes. Since 1935, flow, precipitation, artificial drainage extent, and corn and soybean acreage have increased across the region. In extensively drained basins, we observe 2 to 4 fold increases in low flows and 1.5 to 3 fold increases in high and extreme flows. Using a water budget, we determined that the storage term has decreased in intensively drained and cultivated basins by 30–200 % since 1975, but increased by roughly 30 % in the less agricultural basin. Storage has generally decreased during spring and summer months and increased during fall and winter months in all watersheds. Thus, the loss of storage and enhanced hydrologic connectivity and efficiency imparted by artificial agricultural drainage appear to have amplified the streamflow response to precipitation increases in the Midwest. Future increases in precipitation are likely to further intensify drainage practices and increase streamflows. Increased streamflow has implications for flood risk, channel adjustment, and sediment and nutrient transport and presents unique challenges for agriculture and water resource management in the Midwest. Better documentation of existing and future drain tile and ditch installation is needed to further understand the role of climate versus drainage across multiple spatial and temporal scales.more » « less
-
more » « less
Abstract Coastal rivers that build deltas undergo repeated avulsion events—that is, abrupt changes in river course—which we need to understand to predict land building and flood hazards in coastal landscapes. Climate change can impact water discharge, flood frequency, sediment supply, and sea level, all of which could impact avulsion location and frequency. Here we present results from quasi‐2D morphodynamic simulations of repeated delta‐lobe construction and avulsion to explore how avulsion location and frequency are affected by changes in relative sea level, sediment supply, and flood regime. Model results indicate that relative sea‐level rise drives more frequent avulsions that occur at a distance from the shoreline set by backwater hydrodynamics. Reducing the sediment supply relative to transport capacity has little impact on deltaic avulsions, because, despite incision in the upstream trunk channel, deltas can still aggrade as a result of progradation. However, increasing the sediment supply relative to transport capacity can shift avulsions upstream of the backwater zone because aggradation in the trunk channel outpaces progradation‐induced delta aggradation. Increasing frequency of overbank floods causes less frequent avulsions because floods scour the riverbed within the backwater zone, slowing net aggradation rates. Results provide a framework to assess upstream and downstream controls on avulsion patterns over glacial‐interglacial cycles, and the impact of land use and anthropogenic climate change on deltas.
-
more » « less
Abstract Lowland deltas experience natural diversions in river course known as avulsions. River avulsions pose catastrophic flood hazards and redistribute sediment that is vital for sustaining land in the face of sea‐level rise. Avulsions also affect deltaic stratigraphic architecture and the preservation of sea‐level cycles in the sedimentary record. Here, we present results from an experimental lowland delta with persistent backwater effects and systematic changes in the rates of sea‐level rise and fall. River avulsions repeatedly occurred where and when the river aggraded to a height of nearly half the channel depth, giving rise to a preferential avulsion node within the backwater zone regardless of sea‐level change. As sea‐level rise accelerated, the river responded by avulsing more frequently until reaching a maximum frequency limited by the upstream sediment supply. Experimental results support recent models, field observations, and experiments, and suggest anthropogenic sea‐level rise will introduce more frequent avulsion hazards farther inland than observed in recent history. The experiment also demonstrated that avulsions can occur during sea‐level fall—even within the confines of an incised valley—provided the offshore basin is shallow enough to allow the shoreline to prograde and the river to aggrade. Avulsions create erosional surfaces within stratigraphy that bound beds reflecting the amount of deposition between avulsions. Avulsion‐induced scours overprint erosional surfaces from sea‐level fall, except when the cumulative drop in sea‐level is greater than the channel depth and less than the basin depth. Results imply sea‐level signals outside this range are removed or distorted in delta deposits.
-
more » « less
The lower Usumacinta–Grijalva River Basin contains one of the richest biodiversity landscapes of the Maya region. Our research is based on (1) an integrative literature review of the geomorphological and archaeological papers published about the lower Usumacinta–Grijalva River Basin and (2) topographic analysis of digital elevation models using a geographical information system to explore the relationship between past human settlement and landscape accessibility along the coastal plain of Tabasco. This work provides a new synthesis of previous research and proposes new models for the geomorphic evolution of the lower Usumacinta–Grijalva River Basin in the context of four millennia of human land use and settlement. For the evolution of the strand-plain of the Usumacinta and Grijalva rivers, there are two published geochronological models that provide different chronologies. We discuss here how both geochronological models encompass Pre-Columbian human settlement in the delta. Interestingly, we notice that one of them overlaps a possible high-magnitude flood event (or events) that drove large geomorphic change around 750 CE (1200 BP), with implications for settlement patterns and chronology. Based on topographical analysis of the eastern-distal sector of the Usumacinta–Grijalva delta, we propose a new model for the evolution of this area with implications for the human occupation during the Mesoamerican Terminal Classic and Early Postclassic on the delta. As one of the main conclusions, we propose that the Pom–Atasta water bodies predate much of the Usumacinta–Grijalva delta and the most recent phase of delta building overlays the original lagoon barriers, resulting in a geomorphic setting more attractive to local human occupation after the Terminal Classic period. According to one of the geochronological models of the delta, this dates to ca. 900 CE, preceding the establishment of nearby settlements such as Atasta.
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1007/s11625-016-0374-4
