CHANNEL BED CHANGES IN THE CENTRAL ATCHAFALAYA RIVER AND PATTERNS OF SEDIMENTATION IN EAST GRAND LAKE, LOUISIANA, 1967 TO 2006 By JOHN NEWMAN 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 2020 © 2020 John Newman To my parents ACKNOWLEDGMENTS I would like to thank the members of my committee, Dr. Joann Mossa, Dr. Peter Waylen, and Dr. Yujie Hu for their guidance. I would also like to thank Dr. Yin-Hsuen Chen for her technical help, patience, and encouragement. Finally, I would like to thank everyone who has inspired and challenged me to learn along my journey through life; there is no greater gift. 4 TABLE OF CONTENTS page ACKNOWLEDGMENTS ...................................................................................................... 4 LIST OF TABLES ................................................................................................................ 6 LIST OF FIGURES .............................................................................................................. 7 CHAPTER 1 INTRODUCTION ........................................................................................................ 10 2 BACKGROUND AND STUDY AREA ......................................................................... 12 3 DATA AND METHODS ............................................................................................... 18 4 RESULTS .................................................................................................................... 25 Atchafalaya River ........................................................................................................ 25 East Grand Lake ......................................................................................................... 26 5 DISCUSSION .............................................................................................................. 38 Atchafalaya River ........................................................................................................ 38 East Grand Lake ......................................................................................................... 41 6 CONCLUSION ............................................................................................................ 43 LIST OF REFERENCES ................................................................................................... 45 BIOGRAPHICAL SKETCH ................................................................................................ 48 5 LIST OF TABLES Table page 3-1 Data Sources for the Central Atchafalaya River.................................................... 23 4-1 Volume change for the Central Atchafalaya River with percentage Butte La Rose distributary contribution to total gain. ........................................................... 33 4-2 Volume change for East Grand Lake..................................................................... 34 6 LIST OF FIGURES Figure page 1-1 Digital Elevation Model (DEM) with selected features of the Central Atchafalaya River. .................................................................................................. 11 2-1 Selected features of the Atchafalaya River and basin. From Mossa 2016. ......... 16 2-2 Daily discharge of the Atchafalaya River at Simmesport, 1930-2015. Increasing minimum flows can be seen until roughly 1963. The flood of 1973 is also prominent. From Mossa 2016..................................................................... 17 3-1 Flow chart of methods used to process MrSid and DGN hydrographic survey data. ........................................................................................................................ 23 3-2 Central Atchafalaya River channel planform, 1967-2006. .................................... 24 3-3 Geovisualization of the Central Atchafalaya River and East Grand Lake and floodplain features near RM 95-100. ..................................................................... 24 4-1 Central Atchafalaya River and East Grand Lake elevation (ft) 1967. ................... 29 4-2 Central Atchafalaya River and East Grand Lake elevation (ft) 1989. ................... 29 4-3 Central Atchafalaya River and East Grand Lake elevation (ft) 2006. ................... 30 4-4 Central Atchafalaya River and East Grand Lake elevation changes (ft) 1967- 1989. ....................................................................................................................... 30 4-5 Central Atchafalaya River and East Grand Lake elevation changes (ft) 1989- 2006. ....................................................................................................................... 31 4-6 Central Atchafalaya River and East Grand Lake elevation changes (ft) 1967- 2006. ....................................................................................................................... 31 4-7 Central Atchafalaya River and East Grand Lake volume changes (m3) 1967- 1989. ....................................................................................................................... 32 4-8 Central Atchafalaya River and East Grand Lake volume changes (m3) 1989- 2006. ....................................................................................................................... 32 4-9 Central Atchafalaya River and East Grand Lake volume changes (m3) 1967- 2006. ....................................................................................................................... 33 4-10 East Grand Lake elevation (ft) 1967. ..................................................................... 34 4-11 East Grand Lake elevation changes (ft) 1989-1998 and 1998-2006. ................... 35 7 4-12 East Grand Lake volume changes (m3) 1989-1998 and 1998-2006. ................... 35 4-13 Plotting of average day per year each time period exceeded high flow percentile threshold (upper). Correspondence of daily discharge and selected high flow percentiles (lower)................................................................................... 36 4-14 Comparison of average days of high flows for each time period using Chi- squared tests. ......................................................................................................... 37 8 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 CENTRAL ATCHAFALAYA RIVER AND PATTERNS OF SEDIMENTATION IN EAST GRAND LAKE, LOUISIANA, 1967 TO 2006 By John Newman May 2020 Chair: Joann Mossa Major: Geography The Atchafalaya River Basin in south-central Louisiana underscores the complicated and delicate relationship between alluvial river systems and human engineering. The Central Atchafalaya River has rapidly changed over the past century due to natural and human-induced processes which have severely altered its natural hydrology. The present study has analyzed bed elevation data from 1967 to 2006 (40 years) in the Central Atchafalaya River and East Grand Lake in order to identify channel bed erosion and sedimentation patterns in both water bodies. Results indicate that the main channel has experienced pronounced sediment loss (150.3Mm3) and that East Grand Lake has gained sediment (-.859Mm3). Recent trends suggest that sediment loss has slowed in the main channel and East Grand Lake’s rates of accretion have reduced and appear to be reaching an equilibrium. Changes and patterns explored in this study have significant implications for flood control, navigation, floodplain connectivity and ecosystem health in the Central Atchafalaya River. 9 CHAPTER 1 INTRODUCTION The Atchafalaya River Basin is the largest distributary basin in North America and has undergone prominent morphological changes related to natural processes and human intervention over the past century (Piazza, 2014). Most notably, the construction of levees, dredging, pipelines associated with the petroleum industry, and flow control structures have altered hydrology and geomorphology of the basin. Human intervention often changes the magnitude of certain morphologic variables, such as discharge and sediment load, usually causing an acceleration or deceleration of natural processes (Leopold et al., 1964). The Central Atchafalaya River, river mile (RM) 55 to 100 (Figure 1-1), is of particular interest in that it has been transformed from a lacustrine deltaic system to a main channel and surrounding backswamp in a relatively short time (Fisk, 1952). Fittingly, floodplain interaction is the subject of much research in this reach of the river (Hupp et al., 2008; Baustian et al., 2019), however, there is an absence of work regarding bed elevations in the main channel and some along-channel lakes. Bed elevations are useful in determining areas of aggradation and degradation in fluvial and lacustrine systems over a given time; therefore, they are instrumental in examining how the development of a main channel has affected the reach, and patterns of sedimentation in East Grand Lake. Analysis of bed elevations in the Upper and Lower Atchafalaya River have shown trends of aggradation - a process that can be detrimental to ecosystems and navigation. This study seeks to identify channel bed changes in the Central Atchafalaya River and sedimentation patterns in East Grand Lake by using hydrographic surveys with cross sectional bed elevation data from 1967 to 2006. 10