Mississippi River Delta: an Overview

James Coleman, Harry Roberts, Gregory Stone
Plat e 1. The Mississippi River delta covers an a rea of -30,000 km 2 a nd its coasta l wetla nds compris e 41% of those in th e United St ates. Over geologic ti me major relocat ions of t he Mississip pi's course have resulted in five Holocene delt a complexes and a sixth one is in a n ea rly stage of developmen t as a product of th e lat est Atchafalaya River diver sion. ABSTRACT _ COLE MAN, J .M.; ROBE RTS, H .H., a nd ST ONE , G.W., 1998. Mississippi River Delt a : a n Overv iew. Journa l
more » ... Coasta l Research, 14(3 ), 698-716. Royal Palm Bea ch (Florida), ISSN 0749 -0208. Over t he last cen tury, the riv er -domin ated Mississippi delt a has received increasing attention from geoscient ists, biologist s, enginee rs, a nd env iron mental pla n ner s becau se of t he importance of th e riv er and it s deltaic environ men ts to t he economic well-b ein g of t he st ate of Loui sia na and t he nat ion. Popula t ion gr owt h, subs urface re source extraction , and increased land-wat er use have plac ed demands on the delt a's natural geologic, biologic, and che mical systems, t herefore modifying the time and spa t ial scales of natural processes within the delta a nd its lower a lluvial va lley . As a result , the combined effects of natural an d human-induced pro cesse s, such as subs idence, eu static se a level ri se, sa lt water intrusion, and wetl and loss, have produ ced a dyna micall y cha ngi ng landscap e and socioeconomic fra mework for th is complex delt a . Under natu ral conditions, the fu ndament a l cha nges th at result in land-building a nd land loss in the Holocen e Mississipp i River delt a pla in a re rooted in t he sys temat ic diversion of water and sediment associa te d wit h majo r shifts in th e river 's cours e-the process of delt a swi tchi ng. Research over the la st half cent ury has shown that major relocations of th e Mississipp i's course have res ulted in five Hol ocen e del ta compl exes a nd a sixt h one in an ea rly stage of development as a produ ct of the latest Atcha fal aya River diversion . Collect ively, t hese Holocene delt as hav e produ ced a delt a plain t ha t cover s an area of -30,000 km-and accou nts for 41% of the coast a l wetl ands in the Un ite d States. After a river diver sion t akes pla ce, t he r esul ting delta evolves th rough a systemati c a nd se mipredictable set of stages gene ra lly cha racte rized by: (a) ra pid progr ad a t ion wit h in creasing-to-st abl e disch a rge, (b) rela tive stability durin g initial stages of wa ning discharge, (c) aba ndon men t by t he ri ver in favor of a higher gradie nt course to the receiving ba sin , and (d) ma rine reworki ng of a se dime n t-starved delt a as it un der goes pr ogr essive su bmergence by the combine d processes of subs idence. Delt a switch ing has t aken place every 1000 to 2000 ye ars duri ng Holocene times, and resulting deltas have a n average th ickne ss of approxi mately 35 m. Within a single delt a t here a re subdelt as , bayfills, a nd cre vasse-splays that have hi gher frequ ency delt a cycles ra ngi ng from several hundred year s to a few decad es. Th ese deposit ional featur es a re usu all y less than 10 m thick, a nd some ha ve produ ced ma rshla nd areas of over 300 krn -. The net result of these delta-building events is a low-lyin g la ndscap e wit h com pone nts that are cha nging (building and deteriorating ) at differ ent rates. Geologica lly, the se depositi on al cycles produ ce a th ick accu mulation of coarsening, up wa rd delt aic depo sit s th at have va r ious thi cknesses in re sponse t o developm en t on a variety of tempora l a nd spa t ial sca les. In th is river-domina te d delt a system, distributaries can pro gr ad e se a ward at rates of over 100 m/year . The cum ulat ive effect of t he Holocene depository has been to depress the u nd er lying Pl eist ocene surface . In a local setting, e.g., the modern Bali ze Lobe, differ ential load ing causes t he vertica l displ acem ent of u nd erl yin g clay-rich facies (shale dia pir s-mu dlumps ). The delt a front of th is lobe, which has progr ad ed into deep wa t er of the outer continental she lf, is cha ra cteri zed by ra pid deposition of silt -a nd clay-rich sedime nts a nd slope inst ab ility, wh ich resul ts in sea wa r d disp lacem ent of sedi me nts by a variety of mass-m ovemen t processes. Sup erimpose d on t he natural processes a nd forms of the Mississip pi delt a ic plai n a nd its associated es tua rine env ironme nts , are huma n im pacts, most of which have been imp osed in this century. Th e mos t signi fica nt impacts have resu lt ed from a decr ease in se dime nt inp ut to the river from its t rib utaries a nd the a lte ration of t he river 's natural se diment disp er sal process es through t he construction of levees. Measu res a re now being taken to rei nstate some of t he delt a's natural processes, th ereby mit igating landloss so that declin e in animal a nd plan t producti vity can be mitigat ed. ADDITIONAL INDEX WORDS: Mississippi Ri ver delta , delta cycle, progradation, subsidence, shoreline change.