COASTAL LAND LOSS AND WKTLANB RESTORATION tSI R. E. Turner estuaryare causallyrelated to the landlosses this sealevel ri se,climate change~, soil type,geomorphic century." I then comparethe strengthof this frameworkand age, subsidence or tnanagement. hypothesisto someof theother hypothesized causes of land loss on this coast, There are laboratoryand Four Hypotheses small-scale field trials that support various hypotheses,It seemsto me thatthe mostreliable Four hypothesesabout the causes of indirect interpretationsare basedon what happensin the wetlandlosses in BaratariaBay will be addressed field, andnot on the resultsof computermodels, here adapted from Turner 1997!: laboratorystudies or conceptualdiagrams. H l. i ct n ences of The test results discussed herein are derived t !tin oil banks v d solelyfrom data derived at a landscapescale. The 'ori of 1 loss sin h data set is restricted to a discussion of the Barataria watershed. This watershed is a significant H2. componentof theLouisiana coastal zorie 14,000 lv ha!and there are a varietyof habitatdata available i tl on it. Its easternboundary is the MississippiRiver from whichoccasional overflowing waters are v n.vi hypothesizedto deliver enoughsediinents and on 1 v tno I freshwaterto significantlyinfluence the balanceof rit f i land lossor gain in the receivingwatershed, and whosere-introduction would restore the estuary's wetlands. Improvingour understandingof the H4. w rin si ecologicalprocesses operating in this watershed h ' ' of mightassist in the managementof others. The effect of geologicalsubsidence and sea DIrect and Indirect Causes of Wetland Loss level rise are not included in this list because both factorshave remained relatively stablethis century Wetlandloss is essentiallythe same as land loss when the land-loss rates rose and fell, Local on thiscoast Baurnann and Turner 1990!. We can subsidencecaused by oil andgas fluid withdrawal discriminatebetween wetland loss that is a direct in Louisianahas been estitnatedto be relatively consequenceof humanactivities, and the lossesthat insignificant comparedto soil sttbsidencerates arean indirect consequence of various other factors. Martin and Serdengecti,1984; Suhayda 1987!. The initial habitat conversions from humati Thereare clearlyIong-term variations in wind, and activities,or "direct impacts", are about 12% of the therefore sea level. However, the trend in water- total land lossesin the Louisianacoastal zone from level rise for the last 80 yearsis essentiallylinear, the1930s to l 990 Britsch and Dunbar 1993!. These and there is no acceleration in relative sea level rise directimpacts are almost exclusively the resultof at any tide gagesite with a long-termrecord up to dtedgingfor oil and gas exploration and recovery, the 1990s Turner 1991!. as well as navigationchannels, Additional direct itnpactsarose from failed agricultural impound- The four hypothesesidentified above were ments.The 'indirect losses' make up the retnaining examinedusing data on a varietyof habitatchanges 88% of all landlosses. Some of thecauses of these obtainedfrom photo-interpretationsof both7 I/2' indirectlosses, or impacts,include reductions in and 15' quadranglemaps that cover the Barataria sedimentsupply, dredging, from subsurface fluid watershed. These analysesinclude documentation withdrawal,or hydrologicalterations. The ratio of of the numberof new pondsforming nearcanals, direct:indirectimpacts resulting frotn human the numberof pondsfilling in near canals.,and the activitiesmay vary under influences such as global amountof landloss and canal density over various BaratariaEstuary Wetiand t-oss 185 time periods. The major question asked is: "Docs land lossresult from the hydrologic changesarising from dredged canals and the consequentialspoil 80 bankparallel to the canal?" 60 Spatial RelationshipsBetween Land i~ and HydrologicChange; 7 l/2' Quadrangle Maps A newlydredged canal is typically >20 m wide and is 5 rn deepand has a spoil hank built from the 'IG dredgedmaterials that is several meters wide and many times higher than a natural levee. If 0 hydrologicchanges cause wetland loss,then land- 0 3 lossrates should be higher ncarcr,rather than farther, Distance nl from a dredgedcanal and spoil bank. Wetland fragmentationinto ponds is presuinedto bethe first 800;. stage of wetland loss. This hypothesis was Y = 0.<6x+ 14.1 e I examinedusing data on thc spatial distributionof c 6001 k- -0.58 different sized ponds found in 1955/56 and 1978 c fromsixty-three pairs of USFWS 7 1/2'quadrangle e 400' habitatmaps Turner and Rao 1990!. Pondsthat hadmerged or enlargedto becamepart of a larger zimI open-waterbody during the intervalwere identified 0 0 00 400 600 800 lOGO 1 00 and not included in the analysis. The total land loss l978 Canal Area ha1 examinedrepresented 38% of thetotal landloss for the coastalzone in the sameperiod. Someponds Fig. 1. Top:The re!aiionshipbetween the percentof foundin 1955/56were not presentin 1978,and the pondsthat arenew, persisting.and ephemeral for the vast tnajorityof pondswere new. The pondsin interval 1955/6 to 78 ! and distance to the nearesi Baratariawatershed and elsewhere! between I and canal. Bottom: The re}arionshipbetween the areaof 50 ha were the most numerous n = 1104! and tnostly new ponds<60 ha formed between1955/6-78 and canalsurface area ha! in Baraiariaestuary adapted formed after 1955/56 n=935!, from Turner and Rao 1990!. The appearanceof 'new ponds and the persistenceof existingponds waspositively related to the distancefrom the canal Fig. 1, top panel!- regressionof'the two variables pond area and canal Morethan half of all new andpersisting ponds, but area!gave an intercept statistically indistinguishable lessthan 10 % ofthe ephemeral ponds, were within from zero p = 0.03!. Thesepatterns were also 1 km of a canal. If canals had no effect on new documentedin the neighboringSt. Bernardand pondformation, then the distributionpattern of new, Terrebonnewatershecls Turner and Rao 1990!, The persistingand ephemeral ponds should overlap. The hypothesisthat canals and spoil banks caused new frequencydistribution of these pondsdo overlap pondformation is notrejected. withinthree kin of thecanal. The greatestdisparity betweenthe distributionof ephemeraland new TemporalRelationships Between Land Loss pondswas within 1 km of the canal. andHydrologic Change: Salt Marshes The area of new pondsbetween 0 and 60 ha Thetemporal relationships between canal area that formed between 1955/56 and 1978 in each and landloss for the St. Bernard,Barataria, and quadranglemap was positivelyrelated to the area Terrebonnewatersheds were investigated by Bass «canals in 1978 Fig, 1, bottompanel!. A linear and Turner 997! using aerial photographs. BaratanaEstuary Wetland Loss lIT /pat}a} and Temporal ~ 1930s to 195ps;R,- = 0,aq geh}tionsh}psBetween Land 01930s to 1974 R2 0 95 Lossand HydrologicChange: 01930s to 1983; R- =0.93 15' Quadrangle Maps 12000 v1930a to 1990 R2 0 91 Data from 15' quadrangle mapsare available Britsch and 8000 Dunbar 1993! to test for a spatial and temporal relationship betweenlandloss and hydrologic ~g 4MN changes.Britsch and Dunbar's land inventories for coastal 0 Louisiana from the 1930s to 1990 0 500 1000 1500 MOO 500 3000 were derived from colored Direct ha! overlays on 15 base tnaps approximately63 X 10' ha!. They usedthese 0.3 overlaysto map the open-waterhabitat changes betweenmapping intervals. Theseauthors used a consistent photo-interpretationmethod that is slightly differentfrom methodsothers have used, The data set is based on grossland-loss rates, rather thannet land-loss rates, and represents the only data setof its kind for Louisiana! that goes back to the 1930s and that covers the whole coast in a consistent manner.The }nappingdates were from the 1930s 0 0. 0,01 0.02 0,03 0.04 0.05 range 1931 to 1949; p,=1934!, the mid-1950s range Direct Loaa 1951 to 1958; p.=1957!, 1974, 1983 and 1990, Land Britschand Dunbar 993! classified Man-made loss Fig. 4. The relationshipbetween direct land lossand as land that became open water as a direct indirectland loss primarilycanal density!in the consequenceof hurzummodification. Natural loss 8 aratanawatershed for eight 15' quadranglemaps was all other land loss. The 'Man-made loss' in the analyzedby Britsch and Dunbar 993!. No data was Britschand Dunbar 993! analysisis the sameas excluded. Four different mapping intervals are what1 consideredto bedirect land loss in this paper. coinpared: 1930sto 1950s,1930s to 1950s,1930s ro The 15' quadranglemaps included all of the 1974, and 1930sto 1990. Top: Area of direct land loss vs. area of indirect land loss ha!. A linear regression Barataria watershed. of the data is showntogether with the Coefficient of Determination R'!. p < 0.01 in all cases!. Bottom: T~er 997! usedall of thesedata except those The percentindirect land loss vs. the percent indirect niapswith >85% openwater, or <10' ha land within landloss. A polynomial fit of the data is shown a 15'quadrangle map. Only oneof nine mapsthat togetherwith the R'-for eachdata sei p c 0.01 in ail cases!. included the Barataria watershed was excluded in thatanalysis. This one map includedthe Delta Farmsregion, where an agricultu}alimpoundment failedin the1960s and the area became open water. Thisdramatic conversion toopen water represented the Barataria watershed and the indirect land loss Aevast majority of landloss from the 1930sto 1990 Fig. 4; p < 0.01 in all cases!.The interceptv as fortllat quadrangle map. zero,or less,indicating that no significant