Elsevier

Geomorphology

Volume 194, 15 July 2013, Pages 108-120
Geomorphology

Recent decadal growth of the Atchafalaya River Delta complex: Effects of variable riverine sediment input and vegetation succession

https://doi.org/10.1016/j.geomorph.2013.04.020Get rights and content

Highlights

  • Delta growth at the Atchafalaya River Delta Complex is dictated by large floods.

  • Tropical systems have major impact on delta growth when there is no large flood.

  • Without large floods vegetation does not buffer against higher energy storms.

  • Consistent floods and sediment supply are most important for continued delta growth.

Abstract

The Mississippi River Delta Plain has experienced substantial wetland loss from subsidence, erosion, and sea level rise, threatening coastal communities and the ecosystems that support them. The Atchafalaya River, the largest distributary of the Mississippi River, has one of the few prograding delta features along the ~ 200-km deltaic coastline. Understanding changes in the Atchafalaya River Delta complex (ARDC) development has critical implications for future prediction and management strategy for the Mississippi River Delta Plain. This study was organized to answer two major questions: (1) how did development of the ARDC respond to fluctuation in riverine sediment supply over the period 1989–2010, and (2) has vegetation succession helped stabilize subaerial land? The study quantified annual total suspended sediment yields to the two ARDC subdeltas—Atchafalaya River subdelta (ARSD) and Wax Lake outlet subdelta (WLSD)—classified delta land cover using satellite imagery over ~ 5-year intervals into three classes: barren land, vegetation, and open water and investigated the relationship of delta land change with sediment yield and vegetation succession. Over the entire 21-year study period, we found a net land gain of 59 km2, with the ARSD accounting for 58% of this gain and WLSD 42%. Sediment yield to the subdeltas decreased from an average annual of 38 megatonnes (MT) for ARSD and 18 MT for WLSD during 1989–1995 to an average annual of 24 MT for ARSD and 17 MT for WLSD during 2004–2010, corresponding to the decrease in riverine suspended sediment concentration. Concurrently, total land growth rate decreased from 2.4 km2 y 1 to 1.6 km2 y 1 for ARSD and 3.2 km2 y 1 to 0.6 km2 y 1 for WLSD. However, the ARDC had a net land loss of 2.1 km2 during 1999–2004 because of tropical system effects in conjunction with the lack of large river floods (defined as discharge > 13,800 m3 s 1). On average, more than 60% of newly vegetated land remained vegetated in subsequent years, and when compared with barren areas, vegetated land was less likely (7.3% vs. 32%) to be converted to water, indicating vegetative stabilization effect. However, during the period without a major flood, vegetation buffering against tropical system erosion was limited. This indicates that over the period 1989 to 2010 land growth of the ARDC was dictated by large flood events.

Introduction

Among coastal margins, deltas may be the most economically important regions, serving as transportation hubs and commercial centers while providing abundant natural resources that support large populations of people and wildlife. However, many of the world's deltas today are under unprecedented pressure from land loss because of reduced riverine sediment supply (e.g., Walling and Fang, 2003), coastal land erosion (e.g., Smith and Abdel-Kader, 1988, Chen and Zong, 1998), subsidence (e.g., Day et al., 1995, Syvitski et al., 2009), and sea level rise (e.g., Day et al., 1995, Gornitz, 1995). As with other delta regions, the Mississippi River Delta Plain has not been immune from land loss and has had one of the most significant conversions of land to open water, with over 4877 km2 submerged since 1932, endangering large coastal communities (Couvillion et al., 2011). This loss has been attributed to rapid subsidence of Holocene strata (Törnqvist et al., 1996, Törnqvist et al., 2008), exacerbated by the reduction in riverine sediment supply (Kesel, 2003), hydrocarbon extraction (Morton and Bernier, 2010), local faults, and glacial isostatic adjustment (Yuill et al., 2009). With much of the delta plain under stress, research and management interests have been building in the Atchafalaya River Delta complex (ARDC), the only noticeable prograding delta feature along the Mississippi River Delta coastline.

The Mississippi River enters the Gulf of Mexico through two distributaries: the Mississippi River main channel southeast of New Orleans, Louisiana, and the Atchafalaya River located to the west on the Louisiana central coast. The Atchafalaya River is ~ 190 km long, flowing southward through a levee-confined floodplain area of 4921 km2, much of which is riparian forested swamps (Ford and Nyman, 2011). Since the early twentieth century, because of human alterations to the Atchafalaya River and a more favorable gradient than the Mississippi River, a well-defined channel began to form, which increased the flow volume going down the Atchafalaya River (Fisk, 1952, Roberts et al., 1980). The increased discharge caused many of the open water areas in the Atchafalaya River basin to accumulate sediments, losing much of the open water and swamp habitat to lacustrine delta formations (Tye and Coleman, 1989). With the open water areas sediment filled, formation of subaqueous deltas at the two outlets of the Atchafalaya River, Morgan City main channel (ARMC) and Wax Lake outlet (WLO), became noticeable in the 1950s (Shlemon, 1975). Subaerial land started forming in 1972 and was accelerated by large floods that occurred from 1973 to 1975, forming the Atchafalaya River subdelta (ARSD) and Wax Lake Outlet subdelta (WLSD) (Roberts et al., 1980). With subaerial delta formation, vegetation succession was able to begin with emergent plants colonizing once the delta islands reached intertidal elevations. By 1979, the Atchafalaya River Delta had over 16 km2 vegetated (Johnson et al., 1985).

Several factors may have affected deltaic growth of the ARDC: sediment load, tropical systems, cold fronts, and vegetation colonization. Rouse et al. (1978) observed high growth rate of the ARSD following the 1973, 1974, and 1975 river floods. Barras (2007) found that hurricane Rita in 2005 removed much of the submerged aquatic vegetation and floating vegetation in the ARDC, while also enlarging existing ponds. Hurricane Andrew in 1992 was documented adding, on average, 16 cm of accumulated sediment to marshes surrounding the Atchafalaya Bay (Guntenspergen et al., 1995), while Walker (2001) found that hurricanes can also cause sediment-laden water to be transported off the coast, away from the ARDC. Cold frontal passage has also been documented transporting sediment out of Atchafalaya Bay, with an estimate of 400,000 t per front, which equates to ~ 10.6 megatonnes (MT) during a year (Walker and Hammack, 2000, Roberts et al., 2005). This erosion reduces the rate of subaerial delta development and leaves the delta features sandy (van Heerden and Roberts, 1980). Johnson et al. (1985) noted that with the colonization of plant species (most notably Salix nigra) physical processes were aided by biotic controls to enhance sedimentation and stabilization of the ARDC, although the long-term effect of vegetation on delta land growth in the ARDC is unclear. The above studies identify separate factors that have affected delta growth, but no systematic longer-term look into how the ARDC subdeltas have responded to varying sediment input and environmental stressors has been undertaken.

With the decline of total suspended sediment throughout the Mississippi River (Horowitz, 2010, Heimann et al., 2011) it has become imperative to track changes in total suspended sediment discharge to coastal Louisiana and the impacts on coastal processes. Sediment supply is one the most important components that shape delta growth (Orton and Reading, 1993). However, uncertainty exists regarding how much sediment is discharged from ARMC and WLO to the subdeltas. Previous studies have documented sediment discharge to the ARDC from sediment data collected from the upper Atchafalaya River (Rouse et al., 1978), but Xu (2010) reported that there is a 9% sediment load reduction in the Atchafalaya River basin before reaching the outlets, fluctuating with the river hydrological conditions. With new management plans that have suggested diversions from the Atchafalaya River to other coastal areas in Louisiana (CPRA, 2012), there is an urgent need to have a comprehensive assessment on riverine sediment discharge and how the quantity and fluctuation of the sediment discharge and vegetation succession may have influenced land growth in the ARDC. This assessment is critical since different delta progradation at the two river outlets have been reported (e.g., Barras, 2007, Xu, 2010, Couvillion et al., 2011).

Using this background information, we conducted this study to take a longer-term (1989–2010) look at total suspended sediment discharge to the Atchafalaya River subdeltas below Morgan City and Wax Lake outlet to assess how this may have affected delta land change over four ~ 5-year periods (i.e., 1989 to 1995, 1995 to 1999, 1999 to 2004, and 2004 to 2010). The primary purposes of the study were to determine the relation of ARDC development to total suspended sediment discharge from the river's two outlets and to assess the role that vegetation succession may have played in stabilizing newly created land. The results are discussed in light of the influencing factors introduced earlier.

Section snippets

The Atchafalaya River Delta complex

The Atchafalaya River Delta complex is composed of two subdeltas, Atchafalaya River subdelta (ARSD) and Wax Lake outlet subdelta (WLSD), extending approximately from 29°23′ N. to 29°32′ N. in latitude and from 91°15′ W. to 91°30′ W. in longitude (Fig. 1). Both of these features are building into the shallow, low energy (mean wave height ~ 0.5 m), microtidal (mean tidal range 0.35 m) Atchafalaya Bay and display typical lobate delta growth of a river-dominated delta, based on Galloway's delta

Discharge, total suspended sediment, and tropical systems

Daily mean discharge data were downloaded from the U.S. Geological Survey (USGS) website (http://waterdata.usgs.gov/la) for lower Atchafalaya River at Morgan City, Louisiana (ARMC, 29°41′33.4″ N., 91°12′42.6″ W.; Fig. 1), and Wax Lake outlet at Calumet, Louisiana (WLO, 29°41′52″ N., 91°22′22″ W.; Fig. 1). Discharge measurement did not begin at ARMC until October 1995. Missing discharge data for ARMC were estimated back to 1989 based off of a relationship from Xu and Wang (in review) using data

River discharge and total suspended sediment yield

Over the 21-year period, the Atchafalaya River had an average annual total flow volume of 200.0 km3, varying between 122.7 km3 (2000) and 252.8 km3 (2009). About 118.7 km3 of the average annual total flow volume (i.e., nearly 60%) passed through the ARMC outlet, with a low of 74.0 km3 (2000) and a high of 164.1 km3 (1993) (Fig. 2). The river's other outlet, WLO, had an average annual flow volume of 81.3 km3, varying between 48.8 km3 (2000) and 115.2 km3 (2009) (Fig. 2). At the beginning of the study

Discussion

The Atchafalaya River Delta complex is the only notable prograding delta feature along the Mississippi River Delta coastline in recent decades. For the past 21 years, our study found an average growth rate of the entire delta complex of 2.8 km2 y 1, which was slightly lower than two other recent estimates: 3.1 km2 y 1 (1984–2004) by Xu (2010) and 3.2 km2 y 1 (1985–2010) by Couvillion et al. (2011). The two subdeltas of the ARDC exhibited slightly different growth rates: 1.6 km2 y 1 for ARSD and 1.2 km2 y 1

Conclusion

This study shows that the land growth of the Atchafalaya River Delta complex in the past two decades was dictated by large flood events, while severe tropical systems temporarily impacted delta size to a large degree. Large floods were able to transport substantial quantities of sediment needed to create rapid subaerial land formation, which was followed by vegetation colonization. When compared to barren land, vegetation succession provided stabilization. However, during the period with no

Acknowledgments

The authors thank the United States Geological Survey and United States Army Corps of Engineers for making long-term data on river discharge and suspended sediment concentration available. Timothy Rosen received financial support from the Louisiana Sea Grant College Program during this study. The manuscript greatly benefited from the helpful comments and suggestions by two anonymous reviewers and from a very thorough editorial review by Dr. Richard A. Marston.

References (65)

  • J.A. Barras

    Changes to Cote Blanche Hydrologic Restoration (TV-04) Area After Hurricane Lili

  • J.A. Barras

    Land area changes in coastal Louisiana after the 2005 hurricanes: A series of three maps

  • J.A. Barras

    Land area changes in coastal Louisiana after Hurricanes Katrina and Rita

  • Z.X. Chu et al.

    Changing pattern of accretion/erosion of the modern Yellow River (Huanghe) subaerial delta, China: based on remote sensing images

    Marine Geology

    (2005)
  • Coastal Protection and Restoration Authority of Louisiana (CPRA)

    Louisiana's Comprehensive Master Plan for a Sustainable Coast

    (2012)
  • Couvillion, B.R., Barras, J.A., Steyer, G.D., Sleavin, W., Fischer, M., Beck, H., Trahan, N., Griffin, B., Heckman, D.,...
  • J.W. Day et al.

    Impacts of sea-level rise on deltas in the Gulf of Mexico and the Mediterranean — the importance of pulsing events to sustainability

    Estuaries

    (1995)
  • N. Duan

    Smearing estimate: a nonparametric retransformation method

    Journal of the American Statistical Association

    (1983)
  • H.N. Fisk

    Geological Investigation of the Atchafalaya Basin and the Problem of Mississippi River Diversion

    (1952)
  • K.M. Flynn et al.

    Recovery of freshwater marsh vegetation after a saltwater intrusion event

    Oecologia

    (1995)
  • M. Ford et al.

    Preface: an overview of the Atchafalaya River

    Hydrobiologia

    (2011)
  • W.E. Galloway

    Process framework for describing the morphologic and stratigraphic evolution of deltaic depositional systems

  • G.D. Glysson

    Sediment-transport curves

  • V. Gornitz

    Sea-level rise — a review of recent past and near-future trends

    Earth Surface Processes and Landforms

    (1995)
  • G.R. Guntenspergen et al.

    Disturbance and recovery of the Louisiana coastal marsh landscape from the impacts of Hurricane Andrew

  • J.W. Hardy et al.

    Cold front variability in the southern United States and the influence of atmospheric teleconnection patterns

    Physical Geography

    (2003)
  • D.C. Heimann et al.

    Trends in Suspend-Sediment Loads and Concentrations in the Mississippi River Basin, 1950–2009

  • D.R. Helsel et al.

    Statistical methods in water resources

  • D.R. Helsel et al.

    Computer Program for the Kendall Family of Trend Tests

  • A.J. Horowitz

    An evaluation of sediment rating curves for estimating suspended sediment concentrations for subsequent flux calculations

    Hydrological Processes

    (2003)
  • A.J. Horowitz

    A quarter century of declining suspended sediment fluxes in the Mississippi River and the effect of the 1993 flood

    Hydrological Processes

    (2010)
  • N.C. Howes et al.

    Hurricane-induced failure of low salinity wetlands

    PNAS

    (2010)
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