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Home , Coastal Plain, Land loss

Trends and Causes of Historical Loss in Coastal Louisiana

Project Summary Wetland losses in the northern region of the United States are so extensive that they represent critical con- cerns to government environmental agencies and natural resource managers. In Louisiana, almost 3,000 square kilometers (km2) of low-lying converted to open water between 1956 and 2004 (fig. 1), and billions of dollars in State and Federal funding have been allocated for coastal restoration projects intended to compensate for some of those wetland losses. Recent research at the St. Petersburg Coastal and Marine Science Center (SPCMSC) focused on understanding the physical processes and human activities that contributed to historical wetland loss in coastal Louisiana and the spatial and temporal trends of that loss. The physical processes (land-surface and sediment ) responsible for historical wetland loss were quantified by comparing marsh-surface elevations, water depths, and vertical displacements of stratigraphic contacts at 10 study areas in the Mississippi plain and 6 sites at Sabine National Wildlife Refuge (SNWR) in the western che- nier plain (fig. 1). The timing and extent of land loss at the study areas was determined by comparing historical maps, aerial photographs, and satellite imagery; the temporal and spatial trends of those losses were compared with historical subsidence rates and hydrocarbon production trends.

Chenier Plain

Delta Plain

MS AL GA LA TX FL 0 50 KILOMETERS

Gulf of Mexico 0 50 MILES

Figure 1. Satellite image of coastal Louisiana showing extent of historical (1956 to 2004) wetland loss (in red). White boxes encompass study-area locations in the western chenier and delta plains. Landsat 5 Thematic Mapper image acquired December 13, 2006.

settings, including (1) an upper delta- Geologic Setting plain levee flank of the , Historical Wetland Loss The plain (2) a preexisting upper delta-plain interior In the upper delta plain, extensive was constructed from fluvial sediments , (3) four upper delta-plain areas of formerly emergent marsh that were deposited in overlapping delta interdistributary areas, (4) three lower converted to open water before 1978, lobes. Thick peat deposits that accumu- delta-plain interdistributary areas, and (5) with little significant land-area change lated in interdistributary areas underlie a lower delta-plain -ridge margin since (fig. 2). Analysis of historical aerial the modern delta-plain wetlands. In com- area. The six western chenier-plain study photography showed that the most rapid parison, the chenier plain was constructed areas were located in Sabine National wetland loss at many of these study areas by the alongshore progradation of broad Wildlife Refuge (SNWR), which occu- occurred during the late 1960s and 1970s. capped by thin peat deposits and pies a broad, -parallel, low-lying In comparison, at most of the lower delta- wetland vegetation with intervening nar- area that formed between the higher plain study areas, some areas of wetland row, sandy beach ridges (cheniers). elevation beach ridges along the gulf loss were apparent by the middle to late The 10 delta-plain study areas encom- shoreline to the south and upland areas to 1970s, but the majority of historical land passed several different physiographic the north. loss occurred between 1978 and 1990.

U.S. Department of the Interior Fact Sheet 2013-3017 Printed on recycled paper U.S. Geological Survey March 2013 At some sites, areas of formerly continu- 1958 1974 ous marsh were alternately emergent or partially submerged on historical aerial photographs acquired prior to 1978, depending on whether the images were acquired under low or high water condi- tions, respectively. Between 1978 and 1990, most of these “wet marsh” areas became permanently submerged. In the western chenier plain, most of the histori- cal wetland loss occurred prior to 1978; 1989 2004 however, some expanses of wet marsh in the western part of SNWR persisted into the 1980s. Wetland Subsidence and Erosion The two primary physical processes EXPLANATION responsible for historical wetland loss in 0 10 KILOMETERS coastal Louisiana are land-surface subsid- Core location 1978 - 1990 land loss 1956 - 1978 land loss 1990 - 2004 land loss ence and erosion. These processes create 0 10 MILES accommodation space, which is described by the difference between the emergent Figure 2. Pre- and post-land-loss aerial photographs overlaid with historical wetland-loss marsh-surface elevation and the existing trends at selected upper delta-plain interdistributary study areas. Image sources are: Edgar water depth at a core site (fig. 3). The Tobin Aerial Surveys controlled aerial mosaics (1958), National Aeronautics and Space predominance of subsidence or erosion at Administration color-infrared aerial photography (1974), and U.S. Geological Survey Digital Orthophoto Quarter Quadrangles (1989 and 2004). Data sources from which the land-loss the study areas varied by physiographic trends were derived are: National Wetlands Inventory habitat data (1956 and 1978) and and geologic setting. Compared with the Landsat 5 Thematic Mapper satellite imagery (1990 and 2004). upper delta-plain interdistributary study areas, where subsidence greatly exceeded erosion, erosion was about equal to or and then slow since the early 1990s. Sub- ence. If accelerated historical subsidence greater than subsidence at most of the sidence rates for the slow periods were rates and wetland loss were induced by lower delta-plain study areas. At all of comparable to rates that are attributed to hydrocarbon production, then the recent the delta-plain study areas, organic-rich natural processes such as shallow sedi- reductions in subsidence rates likely marsh deposits (peats) were preserved ment . reflect a balancing of subsurface stresses where formerly emergent wetlands were Some areas of wetland loss in the after production decreased. converted to open water. In contrast, ero- Gulf Coast region coincide with some of sion generally exceeded subsidence at the the Nation’s largest oil-and-gas fields. western chenier plain study areas, and the Several lines of evidence support the Marsh Open-water Marsh thinner peat deposits were mostly eroded hypothesis that long-term, large-volume core core core h surface at the open-water sites. hydrocarbon production resulted in land- Reference emergent-mars surface subsidence and wetland loss in Elevation datum coastal Louisiana. There is close temporal Induced Subsidence A and spatial correlation between rates of T Related to Hydrocarbon R E historic wetland loss, subsidence, and Sed iment - water interface hydrocarbon production on the Louisiana Refe Production rence contact coastal plain. Peak annual production TW Historical delta-plain subsidence rates from onshore Louisiana oil-and-gas fields S were assessed by analyzing rates of rela- EXPLANATION coincided with the period of greatest wet- Organics Mud and tive sea-level rise derived from tide-gage land loss (fig. 4). In addition, historical records, elevation changes at established subsidence rates measured at benchmarks Figure 3. Conceptual diagram showing the benchmarks, and Global Positioning Sys- were generally higher over oil-and-gas stratigraphic relations used to estimate tem vertical velocities at Continuously fields (fig. 5). Numerical modeling by Operating Reference Stations. Integra- subsidence and erosion at an open-water SPCMSC collaborators at Stanford core site. Abbreviations: A, accommoda- tion of these data sources indicated that University confirmed that reservoir tion; E, erosion; S, subsidence; TR, reference subsidence rates were slow (≤ 5 millime- depletion and pore-pressure reductions stratigraphic thickness; TW, stratigraphic ters per year, mm/yr) from 1947 until the at large oil-and-gas fields in the south- thickness in the open-water core. mid-1960s, relatively fast (≥ 10 mm/yr) ern Mississippi River delta plain were from the mid-1960s until the early 1990s, capable of inducing land-surface subsid- 6

6 0 250 20 25

EXPLANATION 18 Oil GOLDEN LEEVILLE MEADOW 200 Gas 16 20 Water -5 Wetland loss

14 /yr) 2 150 12 15

10 -10

100 8 10

6 Land loss (km Subsidence rate (mm/yr) -15 50 4 5 1952-1965: average -7.8 mm/yr 2 1965-1982: average -7.9 mm/yr CUT OFF - LAROSE 1982-1993: average -11.1 mm/yr BULLY CAMP BULLY 0 RACELAND VALENTINE Annual production, natural gas (mcf) x 10 0 0 -20 1930 1940 1950 1960 1970 1980 1990 2000 2010 Annual production, oil and water (bbls) x 10 0 25 50 75 100 Year Distance (km) Figure 4. Composite annual fluid production (oil, gas, and formation Figure 5. Historical subsidence rates calculated by the National water) from delta-plain oil-and-gas fields with wetland-loss rates Geodetic Survey from repeat leveling surveys at benchmarks along measured at five representative delta-plain wetland-loss hotspots. Bayou Lafource in the delta-plain study area. Shaded areas delineate Abbreviations: mcf, millions of cubic feet; bbls, barrels; km2/yr, square approximate subsurface extents of nearby oil-and-gas fields. Abbre- kilometers per year. viations: mm/yr, millimeters per year; km, kilometers.

less than formed at the delta plain study totally contained within the peat section Historical areas (about 108 × 106 m3). In part, this and did not penetrate into the underlying Accommodation reflects the much greater spatial extent mud and sand, even at sites where erosion of wetland loss and total area in the delta exceeded subsidence and extant water Formation plain (fig. 1); however, normalized mag- depths were greater than the thicknesses Accommodation is the space available nitudes of accommodation that formed of the organic-rich marsh sediments. It is for sediment accumulation as a result of in the western chenier plain are generally physically impossible to erode to those sea-level rise and (or) land-surface sub- less than formed in the delta plain and depths and still preserve some of the sidence. Historical conversion of coastal- are significantly less than at some upper marsh deposits. In addition, the expanses plain wetlands of the northern Gulf of delta-plain study areas. These differences of wet marsh that were identified on the Mexico to open water is an example are attributed to greater subsidence and, historical aerial photography also indicate of accommodation that formed at the consequently, deeper water bodies and that subsidence was the process that initi- decadal scale. Bathymetric data acquired greater accommodation in the delta plain ated historical wetland loss. Wet marsh is at the study areas (fig. 6) were integrated (figs. 6 and 7). an intermediate stage in the progression with the areal extent of historical wetland Despite differences in geologic setting, from emergent wetlands to open water loss between 1956 and 2004 to estimate similarities in temporal and spatial trends and represents nearly uniform drowning the total volume of accommodation space of wetland loss indicate that historical of large sections of marsh. In the western that formed historically on the western accommodation formation was likely chenier and lower delta plains, initial chenier and delta plains. initiated by similar processes in both the subsidence likely lowered the emergent Magnitudes of historical accommoda- delta and western chenier plains. The marshes to a position where they were tion that formed locally at the western importance of land-surface subsidence more susceptible to erosion. chenier plain study areas (about 20 × to initiating delta-plain wetland loss is 106 cubic meters, m3) were significantly demonstrated by the fact that erosion was

EXPLANATION Elevation (cm) Core location 0 to 15 -60 to -40 Analysis extent -20 to 0 -80 to -60 -40 to -20 < -80

Figure 6. Example bathymetric grids with superimposed core locations from the lower delta plain (left) and western chenier plain (right). Image sources are U.S. Geological Survey Digital Orthophoto Quarter Quadran- 0 1,000 METERS 0 500 METERS gles (2004). Abbreviation: cm, centimeters. Implications and Future 350 Considerations 250 150 Figure 7. Comparison of one-dimensional This study provides the first compre- 50 hensive analyses of the three-dimensional (vertical) accom- -50 alue (cm) modation parameters aspects of historical wetland loss in V coastal Louisiana and characterizes the -150 and measurements in the western likely causes of those losses. Calculated -250 EXPLANATION Average and range, western chenier-plain study areas chenier and delta accommodation volumes provide esti- -350 Average and range, delta-plain study areas plains. Abbreviation: mates of the new sediment that would be -450 cm, centimeters. needed just at the study areas to restore the coastal-plain wetlands to their pre- Erosion Subsidence Water depth 1956 areal extent and elevations. Incorpo- Peat thickness Marsh elevation Accommodation rating these results into project plans may improve wetland restoration strategies. Both episodic events, such as storms, For example, the volume of sediment Selected References and gradual sea-level changes leave a used to create vegetated earthen terraces • Barras, J.A., Bernier, J.C., and Morton, signature within the sediment record that around Greens Lake in SNWR (about 0.7 R.A., 2008, Land area change in coastal can be used to reconstruct environmental × 106 m3) is an order of magnitude less Louisiana — A multidecadal perspective conditions through time (fig. 8). Storm than the total accommodation that formed (from 1956 to 2006): U.S. Geological deposits and magnitudes of sediment in the same area between 1956 and 2004. Survey Scientific Investigations Map erosion and compaction in near-surface Similarly, at sites where wetland loss was 3019, scale 1:250,000, 14 p. pamphlet. marsh sediments can be measured driven primarily by subsidence, structures • Bernier, J.C., Morton, R.A., and Kelso, directly in sediment cores. Temporal designed to mitigate shoreline erosion K.W., 2011, Trends and causes of histori- elevation measurements can provide rates may not be effective. cal wetland Loss, Sabine National Wild- of change. Together these data provide Considering recent and historical life Refuge, Southwest Louisiana: U.S. a comprehensive characterization of Geological Survey Open-File Report land-loss trends, and given that histori- the environment over several centuries 2011–1169, 36 p., plus app. cal subsidence was not driven entirely by and can be used to improve predictive • Morton, R.A. and Bernier, J.C., 2010, natural processes that would be expected capability in modeling future change. In Recent subsidence-rate reductions in to persist in the future, rates of future addition, coastal managers are recogniz- the Mississippi Delta and their - accommodation formation in coastal ing the need to incorporate subsidence logical implications: Journal of Coastal Louisiana will likely be low except for rates into management practices and Research, v. 26, no. 3, p. 555-561. losses associated with episodic storm project designs. These methodologies can • Morton, R.A., Bernier, J.C., Kelso, K.K., impacts. For example, about 850 km2 of be applied to regions across the Gulf of and Barras, J.A., 2010, Quantifying new water areas formed in coastal Loui- Mexico to develop standardized proto- large-scale historical formation of accom- siana since 2004 following the passage modation in the Mississippi Delta: Earth cols for monitoring and measurement of of Hurricanes Katrina and Rita in 2005 Surface Processes and , v. 35, coastal and wetland change. and Hurricane Ike in 2008; however, not no. 14, p. 1625-1641. enough time has passed to assess which • Morton, R.A., Bernier, J.C., and Kelso, of these changes are transitory and (or) K.W., 2009, Recent subsidence and which changes will persist as areas of erosion at diverse wetland sites in the permanent land loss and new accommo- southeastern Mississippi delta plain: U.S. dation space. Geological Survey Open-File Report Increased storm intensities and sea- 2009–1158, 39 p., plus app. (p. 41-221). level rise due to climate change will • Morton, R.A., Bernier, J.C., Barras, J.A., and Ferina, N.F., 2005, Rapid subsidence likely have a significant effect on the and historical wetland loss in the Missis- resilience of the coastal wetlands along sippi delta plain: Likely causes and future the northern . Under- implications: U.S. Geological Survey standing the processes driving landscape Open-File Report 2005–1216, 116 p. change, as well as measurements of rates of change, is necessary to characterize the resiliency and fate of these ecosys- Contact Information tems and provide scientific support for Julie Bernier mitigation and restoration activities. Data Figure 8. Top: Example of a push core col- U.S. Geological Survey from this study characterize historical lected at SNWR. Top of the core is to the 600 4th Street South processes that contributed to wetland right. Photograph by Bob Morton (2008). St. Petersburg, FL 33701 change and establish baseline conditions Bottom: Shoreline-protection structure [email protected] for comparing future change. under construction in the upper delta plain. http://coastal.er.usgs.gov/gc-subsidence/ Photograph by Julie Bernier (2007).