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CHANNEL BED CHANGES IN THE LOWER ATCHAFALAYA RIVER AND WAX LAKE OUTLET, LOUISIANA, 1967-2006 By JEREMY REYNOLDS A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2019 © 2019 Jeremy Reynolds To my parents, my brothers, and all of my friends who have helped support me. ACKNOWLEDGMENTS I would like to thank the members of my committee, Dr. Joann Mossa, Dr. Peter Waylen, and Dr. Liang Mao, for their support and encouragement. I would like to specifically thank Dr. Joann Mossa for her guidance and advice that kept me well on track during this complex process. I would like to thank my peers, specifically Mohammad Abdulrahman and Chia-Yu Wu, for their support and advice on how to tackle this study. 4 TABLE OF CONTENTS page ACKNOWLEDGMENTS ........................................................................................................... 4 LIST OF TABLES ...................................................................................................................... 6 LIST OF FIGURES .................................................................................................................... 7 ABSTRACT ............................................................................................................................... 8 CHAPTER 1 LOWER ATCHAFALAYA CHANNEL BED CHANGE .................................................... 9 Background .......................................................................................................................... 9 Data and Methods .............................................................................................................. 12 2 RESULTS .......................................................................................................................... 15 Elevation Models ............................................................................................................... 15 Changes in Bed Elevation .................................................................................................. 15 Atchafalaya River Elevation Changes .......................................................................... 16 Wax Lake Outlet Elevation Changes ........................................................................... 16 3 DISCUSSION AND CONCLUSION ................................................................................. 17 LIST OF REFERENCES .......................................................................................................... 32 BIOGRAPHICAL SKETCH ..................................................................................................... 34 5 LIST OF TABLES Table page 1-1 Sediment contributions to Atchafalaya Bay in the Gulf of Mexico (Roberts et al., 1997) ............................................................................................................................. 21 1-2 Data Sources for the Lower Atchafalaya River System. ................................................. 21 1-3 Total volume channel bed change for the Lower Atchafalaya River and Wax Lake Outlet combined. ........................................................................................................... 24 1-4 Total volume channel bed change for the Lower Atchafalaya channel. .......................... 25 1-5 Total volume channel bed change for the Wax Lake Outlet channel. .............................. 25 6 LIST OF FIGURES Figure page 1-1 United States Geological Survey streamflow field measurements from April 11, 1973 to May 22, 2019. Measurements begin before the 1973 flood. ....................................... 20 1-2 Wax Lake Outlet and Atchafalaya channel bed elevation (m) 1967. ............................... 22 1-3 Wax Lake Outlet and Atchafalaya channel bed elevation (m) 1989. ............................... 23 1-4 Wax Lake Outlet and Atchafalaya channel bed elevation (m) 2006. ............................... 24 1-5 Lower Atchafalaya elevation changes (m) and channel bed volume changes (m3) 1967-1989. .................................................................................................................... 26 1-6 Lower Atchafalaya elevation changes (m) and channel bed volume changes (m3) 1989-2006. .................................................................................................................... 27 1-7 Lower Atchafalaya elevation changes (m) and channel bed volume changes (m3) 1967-2006. .................................................................................................................... 28 1-8 Wax Lake Outlet elevation changes (m) and channel bed volume changes (m3) 1967- 1989. ............................................................................................................................. 29 1-9 Wax Lake Outlet elevation changes (m) and channel bed volume changes (m3) 1989- 2006. ............................................................................................................................. 30 1-10 Wax Lake Outlet elevation changes (m) and channel bed volume changes (m3) 1967- 2006. ............................................................................................................................. 31 7 Abstract of Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science CHANNEL BED CHANGES IN THE LOWER ATCHAFALAYA RIVER AND WAX LAKE OUTLET, LOUISIANA, 1967-2006 By Jeremy Reynolds August 2019 Chair: Joann Mossa Major: Geography The Lower Atchafalaya River system is experiencing major changes in channel bed sediment elevations such as aggradation and degradation in the past four decades. These changes are in part associated with construction of the Wax Lake Outlet in 1941 which was intended to divert floodwaters from the Lower Atchafalaya, and completion of the Old River Control Project in 1963 which was built to regulate flow into the Atchafalaya River and keep the Mississippi River in its course. Cross sectional elevation data were used to interpolate channel bed elevations from 1967-2006, and to identify areas in both the Wax Lake Outlet and Lower Atchafalaya that experienced changes in channel bed elevation. Due to human intervention both upstream and in the Lower Atchafalaya River system, the Wax Lake Outlet is degrading possibly due to its shorter route while the Atchafalaya channel has been severely aggrading across the entire time period. 8 CHAPTER 1 LOWER ATCHAFALAYA CHANNEL BED CHANGE Alluvial rivers can change quickly and are very dynamic systems that require an understanding of the characteristics and causes of underlying processes. Water flow is the most important factor in determination of a river channel’s dimensions whereas the suspended sediment load contributes most to the shape of the river and its channel (Schumm, 1985; Mueller, 2013). Almost all of the rivers in the United States have been altered in some way by human interactions (Pringle, 2003). Human interference in the form of locks, dams, levees, and flow regulation systems can lead to large changes in suspended sediment and significant downstream channel bed changes (Hupp et al., 2009). These changes to the river’s natural processes can lead to large scale changes in the river network that quickly become visible and have a negative impact on the ecosystems contained in the drainage and floodplain (Gregory, 2006). Background The Atchafalaya River begins at the confluence of the Red and Mississippi rivers just north of Simmesport, LA (Piazza, 2014). The river spans 200 kilometers (146 river miles) and at the southern end is divided into the Lower Atchafalaya River and the Wax Lake Outlet, a river control structure designed for flood management (Mossa, 2016). The Atchafalaya River has acted as a distributary river for the Mississippi River for as long as records have kept for in Louisiana, predating the mid-1500’s, but also received all of the flow of the Red River (Fisk, 1952). In its role as a distributary river, the Atchafalaya currently carries all of the discharge from the Red River as well as ~30% of the discharge and suspended sediment from the Mississippi River (Mossa, 2016). 9 At first, the Atchafalaya River began to increase in prominence due to the construction of Shreve’s cutoff. Completed in 1831, Shreve’s cutoff reduced the length of a bend in the Mississippi channel in order to improve navigation into Simmesport (Fisk, 1952; Mossa, 2016). Following completion of Shreve’s cutoff and settlement in the area, a 32 kilometer log jam formed that blocked the Atchafalaya River (Fisk, 1952). Removal of the log jam began in 1838 and was completed in 1861 at the behest of Henry Shreve who believed this course of action to be the only way to prevent silting in the Red River (Reuss, 1998; Mossa, 2016). Shreve was correct in that the Atchafalaya River took on all of the discharge and sediment from the Red River. However, flow from the Mississippi River into the Atchafalaya increased steadily as the Atchafalaya was poised to capture all of the Mississippi’s discharge leading to a major delta switch (Roberts, 1998). This effect was compounded by the Atchafalaya’s shorter distance from the convergence of the two rivers to the Gulf of Mexico as well as its gradient advantage over the Mississippi (Mossa, 2016). The Old River control project, which includes structures to regulate the amount of flow contributed to the Atchafalaya from