Mississippi Valley Division

Engineer Research and Development Center

Geomorphic Assessment of Pass a Loutre and 

South Pass, Mississippi River Delta

Mississippi River Geomorphology and Potamology Report No. 3

A geomorphic assessment was conducted for Pass a Loutre and South Pass, both 

distributaries located in the bird’s-foot delta of the lower Mississippi River. The 

geomorphic assessment includes analysis of historical changes in channel geometry and 

flow distribution at the passes, as well as investigation of the effects of natural and maninduced activities. Historical trends in the long-term morphology of the passes are 

described.

Background

The dominant morphological processes that shape the lower Mississippi River channel and 

deltaic channels such as Pass a Loutre and South Pass can operate over a very large range 

of spatial and temporal scales. There are many factors, both natural and man-induced, that 

can contribute to these processes. The effects of large flood events and tropical storms, 

changing sediment loads and characteristics, channel maintenance activities, dredging 

practices, diversions (natural and man-made), subsidence, and relative sea level rise are 

just a few such factors. Formulating the most accurate assessment of river morphology 

over these large scales requires evaluation of long-term trends in geometry adjustment, 

flow distribution, and sediment loading based on observed data. These morphological 

trends can be determined by means of a geomorphic assessment. 

A geomorphic assessment was conducted for the Pass a Loutre and South Pass deltaic 

channels of the Mississippi River bird’s-foot delta. The purpose of the study was to 

document long-term morphological trends in the channels through the analysis of historical 

stream gage data, hydrographic surveys, dredge data, and other information on maninduced activity in the study area. The physical limits of the study were from Head of 

Passes (HOP) to the Gulf of Mexico outlet for both passes, and the time period was 

focused from 1960 to the present. Specifically, the geomorphic assessment tasks consisted 

of data compilation, geometric data analysis, analysis of stage and discharge data, and 

assessment of historical river engineering activity in the study area. The results of all 

analyses were integrated to develop an understanding of the morphologic trends in the 

study area. Information derived from the geomorphic assessment assists in evaluating 

potential activities in the area such as river diversions for freshwater marsh restoration.

Assessment Summary

Both Pass a Loutre and South Pass have demonstrated depositional trends throughout the 

study period. Average depths in Pass a Loutre have decreased from 4 to 20 feet (ft) 

throughout the pass due to sediment deposition, with decreases of 80 to 100 ft observed 

near the head of the pass. Average depth decreases due to sediment deposition observed in 

South Pass range from 5 to 10 ft. The flow distribution in Pass a Loutre as a percentage of 

Mississippi River discharge decreased from approximately 30% to 10% during the study 

period, with only a slight decrease from 15% to 12% observed for South Pass.
The observed trends of sediment deposition and flow distribution for Pass a Loutre and South Pass are influenced by 

many factors, both natural and anthropogenic. River floods and tropical storms, changing river sediment regimes, 

navigation channel maintenance, and dredge material disposal practices have contributed to the geometric and flow 

distribution changes observed in the passes. Identification of the causative factors with increased certainty will require 

additional investigation with detailed hydrodynamic numerical models.

Additional Information

To access the full version of Geomorphic Assessment of Pass a Loutre and South Pass, Mississippi River Delta report, 

click the following link: http://acwc.sdp.sirsi.net/client/search/asset/1036385

Additional MRG&P reports, historic reports, and MRG&P information can be accessed from the technology transfer 

section link at the MVD Historic Studies website: http://www.mvd.usace.army.mil/mrgp_pubs.

Points of Contact

Charles D. Little, Jr., P.E.

Author, ERDC-CHL

Charles.D.Little@usace.army.mil

Barb Kleiss, Ph.D.

MRG&P Technical Director

USACE MVD

Barbara.A.Kleiss@usace.army.mil

Freddie Pinkard, P.E.

MRG&P Sr. Project Manager

USACE MVK

Freddie.Pinkard@usace.army.mil

Summary of Significant Findings

 Mississippi River deltaic channels are 

part of a dynamic system with 

morphologic processes operating over 

large time scales.

 Pass a Loutre and South Pass have 

experienced depositional trends over 

the past 5 decades.

 Decrease in flow distribution as a 

percentage of Mississippi River 

discharge has been more significant in 

Pass a Loutre than South Pass.

 Both natural and anthropogenic factors 

influence the observed morphology in 

Pass a Loutre and South Pass.