the formation of a new delta is oc- 

 curring on the Louisiana coast ap- 

 proximately 77 miles west of the 

 modern bird's-foot delta of the Mis- 

 sissippi . 



INTRODUCTION 



All runoff from the large drain- 

 age basin of the Mississippi conver- 

 ges in south Louisiana and exits into 

 the Gulf of Mexico (Figure 1). Over 

 the past 6,000 to 8,000 years the Mis- 

 sissippi River has occupied several 

 positions (Figure 2) resulting in 

 progradation of the shoreline and 

 development of a large low-relief 

 deltaic plain in south Louisiana 

 (Fisk 1952). The Atchafalaya Basin, 

 a large lowland bounded by the high 

 natural levees of the Mississippi and 

 Bayou Lafourche on the east and Bayou 

 Teche on the west, is the most promi- 

 nent feature in the Lower Valley 

 (Figure 3). Within the Atchafalaya 

 Basin, the Atchafalaya Floodway and 

 Atchafalaya Bay are currently experi- 

 encing morphological changes on a 

 grand scale (Figure 4) . 



Prior to the 1840' s, the Missis- 

 sippi was the primary route for de- 

 livery of water and sediments to the 

 gulf. Discharges up to bankfull re- 

 mained in the channel and exited into 

 the gulf with the exception of minor 

 percentages that diverted through 

 distributaries. Discharges exceeding 

 bankfull flowed generally southward 

 through the Atchafalaya Basin low- 

 lands and into the gulf through nu- 

 merous bayous and outlets. Distribu- 

 tion of flow and sediment is signifi- 

 cantly different today. The Atcha- 

 falaya River, a dynamic distributary 

 of the Mississippi, has a distinct 

 gradient advantage and is currently 

 controlled to carry about 25 percent 

 of the flow and sediment load of the 



Mississippi River. Flood flows now 

 enter the Atchafalaya Basin in a con- 

 trolled fashion, rather than through 

 levee crevasses. Major natural dis- 

 tributaries, Bayou Manchac, Bayou 

 Plaquemine, and Bayou Lafourche have 

 been closed by flood protection 

 levees. This paper describes the 

 natural regime of the river, reviews 

 alterations of the Mississippi River 

 drainage system, and discusses their 

 effect on the flow regime and sedi- 

 ment loads entering estuaries. 



NATURAL DRAINAGE PATTERN OF 

 SOUTHERN LOUISIANA 



Estuaries of south Louisiana 

 (Figure 5) can be divided into three 

 zones: zone 1 lies east of the 

 modern Mississippi River and its 

 bird's-foot delta and includes Lakes 

 Maurepas, Ponchartrain, and Breton 

 Sound; zone 2 is bounded on the east 

 by the Mississippi River and its 

 delta and on west by the high natural 

 ridges of the Lafourche system; and 

 zone 3 is the broad Atchafalaya Basin 

 bounded on the east by the Lafourche 

 system and on the west by the natural 

 higher levees of the former Teche 

 delta system (Figure 2). In its nat- 

 ural state, distribution of water 

 and sediment to estuaries in zone 1 

 was via Manchac Bayou, a distributary 

 located about 15 miles downstream of 

 Baton Rouge, Louisiana, to Lakes 

 Maurepas, Ponchartrain, and Borgne, 

 and Pass A Loutre at the mouth of 

 the Mississippi River; in zone 2 

 via Bayou Lafourche, a distributary 

 located at Donaldsonville, Louisiana, 

 and southwest pass at the mouth of 

 the Mississippi River; and in zone 

 3 via the Atchafalaya River and 

 Bayou Plaquemine, a distributary lo- 

 cated at Baton Rouge, Louisiana. The 



335 



