Restoration of Delta Streams with the Help of Conceptual Models: an Example from the Lower Mississippi River

Delta streams form in the floodplains of large river systems and are widespread throughout the United States; Delta streams are particularly prevalent in the lower Mississippi River Basin. The alluvial flood-plain deposits are typically rich in organic material. Consequently, most delta streams have been altered through intense agricultural activities and flood control measures. Low water, excessive sedimentation in smaller delta streams, and the accumulation of pesticides such as DDT are some of the consequences of these anthropogenic disturbances resulting in dominance of tolerant fish species. Attempts have been made to restore habitat conditions as part of ecosystem restoration or mitigation of flood control projects.

The Mississippi Embayment
Upper Steele Bayou System (USBS) in west central Mississippi encompasses 282 square miles in the Yazoo Delta of the lower Mississippi River basin. All of the streams in the USBS lie within a predominately agricultural landscape and were highly degraded from low water and sedimentation. As the result of a flood control project, between 1995 and 2001, seven weirs were constructed; 66 drop pipes were placed in surrounding agricultural lands next to the stream bank to reduce erosion; and approximately 63 miles of channel enlargement, cleanout, and selective snagging were completed to remove soft, unconsolidated substrates and improve flow.

In 2009, Hurricane Rita caused major flooding in the Yazoo Delta. Contaminant levels were elevated along with a increase in biological oxygen demand caused by influx of organic debris, which resulted in widespread hypoxia throughout the delta.

The USBS has been sampled throughout the 15-year period described above, and these data were studied to describe long-term trends of the fish assemblage associated with ecosystem restoration in delta streams.

Findings
Study of the USBS indicated a shift in species composition from simple to more complex communities and from tolerant to more intolerant fish assemblages. Three principal stressors on fish communities were apparent: extreme low water during late summer and autumn, accretion of soft unconsolidated sediment, and lack of instream structure. The first two stressors were at least partially remedied during the USBS project, leading to increased species richness and greater numbers of intolerant species. A conceptual model was developed to help illustrate the expected benefits of management practices on specific groups of fishes in delta streams that were as a result of the data collected from the system (Figure 1).

Figure 1. Conceptual model of the expected outcome of restoration measures in delta streams




The USBS is an example of a restoration project on a watershed scale with documented environ-mental benefits (Wohl et al. 2005). Total cost of the project was approximately $35 million, but included a return on investment for flood damage reduction (i.e., cost:benefit = 1.4). The apparent long-term benefits to the aquatic ecosystem further justify these expenditures. It is unrealistic to assume that delta streams can be restored back to pre-settlement conditions. Agricultural activities are too widespread, usually encompassing over 80 percent of the land use. However, this study indicates that focusing on the three primary stressors as part of water resource projects in these systems will have beneficial results to the fish community in delta streams.

By including a conceptual model to represent the system stressors and expected benefits from various management practices, the management team can more easily communicate with others the purposes for the management actions as well has have a better understanding of the linkages between the system components.

Conceptual Models on Your Restoration Project
The model building and development processes consist of determining system parts, identifying relationships that link those parts, specifying the mechanisms of system interaction, and exploring model behavior. Click here to view the 7 Steps to Build a Conceptual Model for your restoration project.

Additional Resources

• Restoration of Delta Streams: A Case History and Conceptual Model. Killgore, K. J., Hoover, J. J., Murphy, C. E., Parrish, K. D.; Johnson, D. R., and Myers, K. F. (2008)

EBA Resources

• Conceptual Models