LAND LOSS IN THE MISSISSIPPI RIVER DELTAIC PLAIN

Sherwood M. Gagliano1, Klaus J. Meyer-Arendt1, and Karen M. Wicker1

ABSTRACT

Systematic measurements and comparisons.of maps, black-and-white aerial photographs, and color infrared imagery taken at five periods within the interval from 1890-1978 have been used to document land loss and habitat change within the Mississippi River Deltaic Plain. The studies show that the long-term trend of net progradation, which persisted through most of the past 5000 years, was reversed during the late nineteenth century, and that during the twentieth century land-loss rates have accelerated geometrically. Within the 11,500 mi2 study area, land-loss rates have progressed from approximately 6.7 mi2/year in 1913 to a projected 39 .4 mi2/year in 1980. The greatest loss has occurred in the wetlands, but barrier islands and natural-levee ridges are also disappearing at a very high rate. The data can be used not only to document past change, but also to project future conditions. The findings have great significance to fish and wildlife resources, flood-p:rotection planning, and land ownership. Apparent causes of the high rates of land loss include the harnessing of the Mississippi River by levees and control structures which reduce tendencies toward natural diversion and funnel valuable sediments to deep, offshore waters. Additional factors include canal dredging and accelerated subsidence related to mineral extraction, both of which are often associated with saltwater intrusion. The net effect is a rapidly accelerating man-induced transgression of a major coastal system.

INTRODUCTION The vast deltaic plain of coastal Louisiana is a product of 5000 years of Mississippi River delta building. Land was built as a result of sediment deposition in the vicinity of active outlets of the Mississippi River during an interval of sea-level stability. Through periodic upstream diversions the river has been able to construct a complex deltaic plain. Each diversion has initi­ ated a cyclic episode of delta growth and coastal environmental change, including an interval of growth followed by an interval of breakup and decline with waxing and waning of river out­ flow and sediment deposition. The net result has been a gra­ dual progradation of the deltaic plain.

These processes have resulted in an extensive coastal low­ land characterized by a skeleton of alluvial natural-levee ridges LEGEND along active and abandoned distributaries, an outer fringe of m PLEISTOCENE OUTCROP c:J tNTERD1STRt9UTA~Y IAS,NS sandy barrier islands and Gulf beaches, and vast areas of inter­ IT:) NATUfO,U LEVEE­ (8ETWEIEN LEVEES> distributaty swamps, marshes, lakes and bays (Fig. 1). CHANNEL COMPLEXES ~8AIIIIIEII ISLANDS

The extent and character of the deltaic coast, as well .as the Figure 1. Major physiographic · features of the Mississippi River cyclic habit of delta building and the environmental succession Deltaic Plain. which it drives, have resulted in a bounty of renewable re­ sources. The magnitude and importance of these values, re­ lated primarily to fish, wildlife, and recreational resources, thought of as outbuilding, the latter as upbuilding or mainte­ have been well documented (Chabreck, 1973; Day et al., 1979; nance. Upbuilding is necessary to offset deterioration and ero­ Fruge and Ruelle, 1980; Lindall et al, 1972; Perret et al., 1971; sion related t.o marine processes and subsidence. U.S. Fish and Wildlife Service [FWS] 1981), and need not be Wetlands in the deltaic plain are truly "living surfaces." That repeated here, though it is noteworthy that they do represent a is, the marsh and swamp plant communities usually become resource of national importance. established initially on a substrate of elastic sediment derived from the river (subaqueous natural levees, bars, mudflats, Two aspects of deltaic processes are vital to maintenance of natural-levee backslopes, etc.). Early colonizers hold newly the system and its high level of productivity. The first is its deposited sediment together and trap additional sediment. ability to build new land tluough sediment deposition in the Because of high, natural-subsidence rates (typically 0.7 ft/cen­ vicinity of active distributaty outlets. The second is its ability to tury, but up to 4 ft/century), the initial substrate may gradually continue to build up the land mass through overbank sedi­ sink to a level well below that of the sea. The root zone of mentation and through accumulation of organic sediments wetland plants maintains its position near sea level by gradual­ such as peat and shell deposits. The former processes can be ly building up organic litter (peats) and trapping clays at a rate equal to, or faster than, that of subsidence. Radiocarbon dates from the base of thick sections of peat indicate that in many 1Coastal Environments, Inc. (CEI), 1260 Main Street, Baton Rouge, LA instances these living surfaces have persisted for 3000 years or 70802 more. In other instances, especially in fresher interior marshes,

295 , .96 TRANSACTIONS-----GULF COAST ASSOCIATION OF GEOLOGICAL SOCIETIES Volume XXXI, 1981 the rate of sinking may exceed that of sediment accumulation, In modem years, hydrnlogic conditions in the deltaic plain but the living surface is maintained through development of have changed greatly, and in most areas there has been acceler­ floating vegetation mats (flotant). If the floating mat is frag­ ated saltwater intrusion via an extensive, deep, Gulf-to­ mented or fails to develop and subsidence exceeds the rate of interior-canal network. This has caused major changes in the sediment accumulation, the vegetated wetland reverts to open distribution of wetland vegetation. In general,-there has been a water. landward shift in marsh zones, i.e., saline marshes displacing The distribution of wetlands vegetation in the deltaic plain is brackish marshes and brackish marshes displacing intermedi­ controlled by such factors as elevation, drainage, soil type, ate marshes. However, even slight salinity increases will kill hydroperiod, and salinity. However, it is well recognized that freshwater swamps and marshes, and in many places these salinity is a critical factor. Marsh plants, for example, are map­ habitat types have reverted to barren mudflats or open water. ped as vegetative types whose zonation is based primarily on average salinity conditions (Fig. 2) . In general, there is a broad LAND LOSS MEASUREMENTS landward-to-seaward zonation from fresh through intermedi­ While patterns of marsh deterioration and land loss along the ate and brackish to saline marshes. The main exception is an lower Mississippi River were documented as early as the 1930' s "island" of fresh and intermediate marsh in the active birdfoot (Russell, 1936), initial attempts at quantifying areal extent of delta area where conditions are predominately fresh through­ water bodies within the coastal zone did not begin until the late out the year. 1960's. Methodologies consisted of planimetering water­ This zonation of marshes reflects a gradual dilution of inland surface features from U.S. Geological Survey (USGS) topo­ freshwater runoff by saline gulf tidal inflow. Freshwater is graphic quadrangles (Barrett, 1970) and multiplying percentage presently derived largely from local precipitation and runoff, frequency of water-occupied sampling stations by the area of but, under natural conditions, was supplemented by overbank coverage (Chabreck, 1971). The first systematic analysis of land­ floodwaters from the Mississippi River. Freshwater moved sea­ water ratio changes over three successive periods of topo­ ward through an intricate network of tidally influenced, back­ graphic-map coverage employed a grid-sampling technique swamp, drainage streams. Marine water was introduced to (Gagliano and van Beek, 1970). Center points of each grid, marsh systems through relatively shallow-tidal streams and spaced at 0.50-mi intervals, were differentiated as to land or inlets between barrier islands. water. Point estimates of the distribution were then used to

i INLAND LIMIT OF COASTAL MARSHES I ~ FRESH

INTERMEDIATE 25

0 30km

/

G {./ l

0 F M E X I C 0

Figure 2. Zonation of marsh types in the Mississippi River Deltaic Plain in 1978. (From Chabreck and Linscombe, 1978.) GAGLIANO, MEYER-ARENDT, WICKER 297

evaluate changes in land-water ratios. For a study area covering P2-1930-1950. USGS planimetric and topographic maps. Ae­ 20,480 mi2, a land loss rate of 16.5 mi2/yr was derived (Gagliano rial photo controlled with field verification. 15 and 7-Y2 and van Beek, 1970). minute quadrangle format. Point count measurement Aerial black-and-white and color infrared imagery was first technique (Gagliano and van Beek, 1970). used in land-loss analyses of Barataria Basin during the late P3-1942-1967. USGS topographic maps. Aerial photo con­ 1970's (Adams et al., 1976; Craig et al., 1979). Surface coverage trolled with field verification. 15 and 7-Y2 minute quad­ of land and water was electronically digitized for the periods of rangle format. Point count measurement technique (Gag­ study. A more comprehensive examination of habitat change in liano and van Beek, 1970). the Mississippi River Deltaic Plain, conducted for the FWS,, P4-1955-1956. AMMANN Aerial Surveys and Tobin Aerial employed large-scale, black-and-white, 1955-56 air photos and Surveys controlled, black-and-white, aerial photo 1978 NASA color infrared photos (Wicker, 1980). Habitat types mosaics. 7- 1/2 minute quadrangle format. Digitizer were interpreted within a 1:24,000 uses topographic quad­ measurements of interpretative habitat overlays (Wicker rangle framework for both periods (Wicker et al., 1980a). Resul­ et al., 1980a). tant overlay maps were manually planimetered with an electro­ nic digitizer, and the data subsequently transferred to compu­ P5-0ctober, 1978. NASA color infrared imagery. Reformat­ ter tape for analysis (Wicker, 1980: App. 9). ted and rectified to 7- 1/2 minute quadrangle format. Digi­ tizer measurements of interpretative habitat overlays Comparative measurements from five mapping intervals are (Wicker et al., 1980a). now available from an 11,500 mi2 study area of southeastern and south central Louisiana (Fig. 3). Characteristics of these data sets and the primary references describing the measure­ ment techniques are as follows: RESULTS OF THE STUDY

P1-1890-1914. USGS topographic maps. Based on field sur­ Since the reversal of the long-term trend of land-building in veys before the advent of aerial photography. 15 minute the late nineteenth century, land loss has increased steadily and quadrangle format. Point count measurement technique. geometrically. Data from the 1970 study show a land loss rate of Maps available for only 7300 mi2 of total study area (Gag­ 6.7 mi2/yr from 1913 and a rate of 15.8 mi2/yr for 1946 (Gagliano liano and van Beek, 1970). and van Beek, 1970}2. The 1955 to 1978 period (P4-P5) showed a

legend MISSISSIPPI DEL TAIC PLAIN D 1930's to present (11,500 mi2) MAP AND PHOTO COVERAGE 1890's to present (7,300 ml2 )

-MISSISSIPPI RIVER

·- 30'

29' 0 50 ml

92" 91" 90° 89"

Figure 3. Land loss study area in coastal Louisiana. 1298 TRANSACTIONS----GULF COAST ASSOCIATION OF GEOLOGICAL SOCIETIES Volume XXXI, 1981

loss of 413,000 ac of land, from 3,646,000 to 3,233,000 ac within The above figures may be regarded as slightly rnisleading,as the study area, representing a land Joss rate of 28.1 mi2/yr for the average land loss rates were used. It should be emphasized 1967, the chronological mid-point. By regression analysis and th.at while most of the acreage lost has been in wetland areas, graphic display, a geometric curve becomes apparent (Fig. 4). the natural-levee ridge lands along the Mississippi River and its Based on the analysis, the projected rate for 1980 is approx­ former distributaries (which constitute most of the habitable imately 39.4 mi2/yr. lands of coastal Louisiana) are sinking very rapidly (Fig. 6). Resistant, coarser-grained barrier islands and beaches are also •o losing acreage as a result of subsidence and marine forces. A MISSISSIPPI DEL TAIC PLAIN study recently completed by CEI for the Terrebonne Parish •• LAND CHANGE RA TES Police Jury indicates that between 1955 and 1978 the Terrebon­ STUDY AREA t 1,500 mi> I ne barrier islands decreased in size by 44 percent (Wicker et al., 39.4 Ml2/YR l 1980b) (Fig. 7). At this rate of erosion, the life expectancy of 40 1980 I these islands is less than 30 years. I .. I RECOMMENDATIONS a: . >- 28.1 Ml2 / YR -.. .' ;; 30 ~ As long as the Mississippi River is confined to its present i •P• 1967 Ps• ;:; channel by means of protection levees and control structures,

legend MISSISSIPPI DEL T AIC PLAIN / 5 °'- ACRES/YEAR LAND CHANGE RATES•1955-1978 400-500

500-600 ACRES PER YEAR PER 7112 MIN. QUADRANGLE I >600 (7v2' quadrangle:approximately 41,420 acres)

COASTAL ENVIRONMENTS, INC.

30'

I N 29' 0 50 ml

g2• g1• go• 89'

Figure 5. Variation in land loss rates within the Mississippi River Deltak Plain.

LIFE EXPECTANCY PLAQUEMINES PARISH, LA. ISLE DERNIERES TERREBONNE PARISH, LA. I' lllllil 195 5 D 191a

Gulf of

Figure 7. Changes in position and size of the Isle Dernieres, Terrebonne Parish, Louisiana, 1955-1978

Figure 6. Map showing projected loss of land along the lower reaches of the Mississippi River in Plaquemines Parish, Louisiana. Values based on present land area of each 7-% minute quadrangle divided by IIIAJIAH& Of 1955-1978 loss rate. Contours plotted on values at quadrangle center points. 300 TRANSACTIONS-GULF COAST ASSOCIATION OF GEOLOGICAL SOCIETIES Volume XXXJ, 1981

ACKNOWLEDGMENTS Lindall, W. J. Jr., Hall, J. R., Sykes, J.E., and Arnold, E. L. Jr., 1972, Louisiana coastal zone: analyses of resources and resource de­ Comparative habitat mapping studies which provided an velopment needs in connection with estuarine ecology: Prepared invaluable source of data upon which this paper is based were by National Marine Fisheries Service Biological Laboratory, St. supported by the U.S. Fish and Wildlife Service, Office of Petersburg Beach, Florida, for U.S. Army Corps of Engineers, New Orleans District, Contract No. 14-17-002-430. Biological Services. Additional data were drawn from studies conducted for the Police Juries of St. Bernard Parish and Terre­ Perrett, W. S., Barrett, B., Latapie, W., Pollard, J., Mock, W., Adkins, bonne Parish, Louisiana. B., Guidry, W., White, C., and Gillespie, M., 1971, Cooperative Gulf of Mexico estuarine inventory and study, Louisiana: Louisiana Wildlife and Fishery Commission, New Orleans, 2 vols. Russell, R. J., 1936, Physiography of the Lower Mississippi River Delta: Louisiana Geological Survey, Lower Mississippi Delta Geological SELECTED REFERENCES Bulletin, v. 8, p. 3-199. U.S. Fish and Wildlife Service, 1981, Mississippi Deltaic Plain Region Adams, R. D., Barrett, B. B., Blackmon, J. H., Gane, B. W., and ecological characterization: an introduction to the literature: Office McIntire, W. G., 1976, Barataria Basin: Geological processes and of Biological Services, in press. framework: Louisiana State University, Center for Wetland Re­ sources, Sea Grant Publication, LSU-T-76~06. van Beek, J. L. and Meyer-Arendt, K. J., 1981, Sediment - asset or liability?: in Proc., Symposium on Freshwater Inflow into Barrett, B., 1970, Water measurements of Coastal Louisiana: Louisiana Estuaries, San Antonio, 1980: U.S. Fish and Wildlife Service, Wildlile and Fisheries Commission, Division of Oysters, Water Office of Biological Services, Slidell, Louisiana, in press. Bottoms and Seafood, 196 p. Wicker, K. M ., 1980, Mississippi Deltaic Plain Region ecological charac­ Chabreck, R. H., 1971, Ponds and lakes of the Louisiana coastal mar­ terization: a habitat mapping study, a user's guide to the habitat shes and their value to fish and wildlife: Presented at 25th Annual maps: U.S. Fish and Wildlife Service, Office of Biological Services, Conference, Southeast. Assoc. Game and Fish Comm., 19 p. FWSIOBS-79-07. Chabreck, R. H., (ed.), 1973, Proceedings of the coastal marsh and Wicker, K. M., et al., 1980a, The Mississippi Deltaic Plain Region estuary management symposium: Louisiana State University, Di­ habitat mapping study: U.S. Fish and Wildlife Service, Office of vision of Continuing Education, Baton Rouge. Biological Services, FWS/OBS-79~7, 464 maps. "!=habreck, R. H. and Linscombe, G., 1978, Vegetative type map of the Wicker, K. M.,DeRouen, M., O'Connor, 0., Roberts, E., and Watson, Louisiana coastal marshes: Louisiana Wildlife and Fisheries Com­ J., 1980b, Environmental characterization of Terrebonne Parish: mission, Baton Rouge. 1955-1978: Coastal Environments, Inc., Baton Rouge, 29 p. Craig, N. N., Turner, R. E., and Day, J. W. Jr., 1979, Land loss in Coastal Louisiana: in Proc., Third Coastal Marsh and Estuary Management Symposium: Louisiana State University, Division of Continuing Education, p. 227-254. Day, J. W. Jr., Culley, D. R. Jr., Turner, R. E., and Mumphrey, A. J. Jr. (eds), 1979, Proc. of the Third Coastal Marsh and Estuary Manage­ ment Symposium: Louisiana State University, Division of Con­ tinuing Education, Baton Rouge. Fruge,D. W. and Ruelle, R., 1980, A planning aid report on the Missis­ sippi and Louisiana estuarine areas study: U.S. Fish and Wildlife Service, Lafayette, Louisiana. Gagliano, S. M., Kwon, H.J., and van Beek, J. L., 1970, Deterioration and restoration of coastal wetlands: Paper presented at the 12th International Conference on Coastal Engineering, Washington, D.C., September 13-18. Gagliano, S. M., Light, P. , and Becker, R. E., 1973, Controlled diver­ sions in the Mississippi Delta system: An approach to environ­ mental management: Louisiana State University, Center for Wet­ land Resources, Hydrologic and Geologic Studies of Coastal Louisiana, Report 8. Gagliano, S. M., and van Beek, J. L., 1970, Geologic and geomorphic aspects of deltaic processes, Mississippi Delta system: Louisiana State University, Center for Wetland Resources, Hydrologic and Geologic Studies of Coastal Louisiana, Report 1. Gagliano, S. M., 1973, Environmental management in the Mississippi Delta system: Gulf Coast Assoc. Geo. Socs. Trans., 23rd Annual Convention, Houston, October 24-26. Gagliano, S. M., 1975, An approach to multiuse management in the Mississippi Delta system, in M. L. Broussard, (ed.). Deltas, mod­ els for exploration, Houston Geological Society, reprint. Gagliano, S. M., 1976, Mississippi River sediment as a resource: in R.S. Saxena, (ed.). Modem Mississippi Delta - depositional environ· ments and processes, A guide book for the AAPG/SEPM field trip, May 23-26, reprint. Kazmann, R. G. and Johnson, D. B., 1980, If the Old River Control Structure fails?: Louisiana State University, Louisiana Water Re­ sources Research Institute Bulletin 12. Kolb, C. R., 1980, Should we permit Mississippi-Atchafalaya diver­ sion?: Gulf Coast Assoc. Geol. Socs. Trans., v. 30, p. 145-150.