University of Florida Thesis Or Dissertation

CHANNEL BED CHANGES IN THE CENTRAL ATCHAFALAYA RIVER AND PATTERNS OF SEDIMENTATION IN EAST GRAND LAKE, LOUISIANA, 1967 TO 2006

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

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.

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

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

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

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

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

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.

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.

Figure 1-1. Digital Elevation Model (DEM) with selected features of the Central Atchafalaya River.

CHAPTER 2 BACKGROUND AND STUDY AREA

The Atchafalaya River Basin is located in south-central Louisiana and extends

220 kilometers (~150 miles) south where it empties into the Gulf of Mexico (Figure 2-1).

It is a shallow interdistributary basin and the largest contiguously forested bottomland in

North America (Hupp et al., 2008; Piazza, 2014). The Atchafalaya River was created in the 16th century when the westward migration of the Mississippi River intercepted the

Red River. This event formed an intersection that serves as the beginning of the

Atchafalaya River, end of the Red River and continuation of the Mississippi River (The

Three Rivers).

The Atchafalaya River currently receives all of the Red Rivers flow, and roughly

30% of the Mississippi Rivers flow (Mossa, 2016), However, this was not always the case. The development of the Atchafalaya River has been heavily influenced by humans and natural processes. The first major alteration came in 1831 with the construction of Shreve’s cut off. Captain Henry Shreve asserted that a cutoff at a large meander loop in the Mississippi River would improve navigation and decrease silting at the mouth of the Red River (Reuss, 1998). Upon completion, Shreve’s cut off changed the junction of The Three Rivers by creating the Upper Old River and Lower Old River.

Additional human-induced and natural events promoted increased flow to the

Atchafalaya River. In 1833 and 1855 log jams were removed from the Red River and

Atchafalaya River, respectively, allowing water to pass more freely. Moreover, periods of high water and floods facilitated the diversion of more flow to the Atchafalaya River

(Mossa, 2016). This was made possible by the maintenance of the Lower Old River starting in 1878, which prevented Shreve’s cut off from developing into an oxbow lake

(Mossa, 2013). The Atchafalaya River’s shorter and steeper track to the Gulf of Mexico, compared to the Lower Mississippi, also increased flow (Mossa, 2016).

With the growing prominence of the Atchafalaya River into the early to mid 20th century, there was concern that it would capture the entirety of the Mississippi River’s flow. A 1952 report concluded that by 1975 a critical stage would be reached, and the

Mississippi would be primed to avulse (Fisk, 1952). As a result, the U.S. Congress authorized the U.S. Army Corps of Engineers to implement a plan that would maintain the current balance of flow between the Mississippi and Atchafalaya Rivers (Hale et al.,

1999). The solution came in 1963 with the completion of the Old River Control Project.

This project closed the Lower Old River and added a lock for navigation. Overbank structures, The Low Sill, and associated channels became the new primary connection between the Red, Atchafalaya and Mississippi Rivers. These structures controlled how much water could flow from the Mississippi into the Red River, which subsequently leads to the Atchafalaya (Mossa, 2016). When the Low Sill Structure was damaged by the flood of 1973 it became apparent that more structures were needed to reduce the overall stress on each structure (Roberts, 1998; Mossa, 2016). An Auxiliary Structure was built in 1987 as well as the Sidney A. Murray Jr. Hydroelectric Station in 1990 to meet this need. Combined, the structures and their channels create the Old River

Outflow channel.

One of the primary roles of the Atchafalaya River has been as a floodway for the lower Mississippi River (Piazza, 2014). The West Atchafalaya and Morganza floodways flank the upper Atchafalaya River and merge with the Atchafalaya Basin Floodway

(Figure 2-1). Guide levees define the lateral extents of the floodways, and completely

changed natural drainage in the Basin (Fisk, 1952). River levees extend to approximately river mile (RM) 55 where the Atchafalaya Basin Floodway begins. As designed by the Mississippi River and Tributaries Project, the Atchafalaya Basin is able to transport 42500 m3/s (1.5 million cfs) via this floodway system. The Morganza

Spillway and Floodway which begins near River Mile 80 on the Mississippi River was completed in 1954, the spillway was opened during the 1973 and 2011 floods. The

West Atchafalaya Floodway has not been used and appears to be unnecessary given the increased channel capacity of the river (Lopez-Llompart and Kondolf, 2016).

A defining characteristic of the Atchafalaya River Basin is its prograding deltas: the Atchafalaya and Wax Lake. The sizable sediment load of the Atchafalaya has fostered the land-building in the Gulf and presents a stark contrast to the deteriorating

Mississippi Delta (Hale et al., 1999). The Red River is a significant source of this sediment and provides a disproportionate amount compared to its contribution of total flow (Mossa, 1990). From 1984 to 1994, 5 locks and dams were constructed on the Red

River downstream of Shreveport, reducing sediment transport to the Atchafalaya

(Mossa 2016).

The Central Atchafalaya River in this study is defined as starting at RM 55 and ending at roughly RM 100 (Figure 1-1). This reach of the Atchafalaya is unbounded by levees and consists of a broad floodplain with bayous, navigation canals, oil pipelines and along-channel lakes (Hupp et al., 2008). The Atchafalaya before 1932 split into many different streams after RM 55 that drained in a deltaic pattern into Grand Lake

(Fisk, 1952). Beginning in 1932, dredging and other human influences aided in the development of a main channel for water to flow (Reuss, 1998). The main channel has

a relatively steeper gradient for ~32 km (~20 miles) after the levees (Fisk, 1952) and diverges from the relic Butte La Rose distributary at RM 55. Approximately 97.9 million m3 (128 million yd3) was dredged from the Atchafalaya Basin south of the river levees to increase carrying capacity and improve connectivity (Reuss, 1998). An additional 1.52 million m3 (2 million yd3) of maintenance dredging was conducted south of RM 55 (U.S.

Army Engineer District, New Orleans, 1974). As a result of the increased flow and capacity, the Central Atchafalaya main channel naturally deepened and widened itself, accelerating the filling of Grand Lake with sediment (Tye and Coleman, 1998b). Long, medium and short-term channel adjustments in response to changes in water and sediment discharge in alluvial rivers aids in maintaining a dynamic equilibrium

(Richards, 1982). In the case of the Central Atchafalaya River, the development of a main channel likely elicited channel adjustments that would lead to a new equilibrium.

Grand Lake once covered approximately 15% of the Atchafalaya Basin, but now only shallow remnants still exist (Hale et al., 1999). One of these remnants, named East

Grand Lake, is included in the scope of this study's analysis. In the late 1950’s the flow of the Atchafalaya River was confined to East Grand Lake, which facilitated the infilling of its southern portion and Willow Cove (Allen et al., 2010). By 1989, the outlet that connected East Grand Lake to the main channel filled with sediment, isolating it and reducing floodplain connectivity (Piazza, 2014). While land-building has significantly declined in former Grand Lake since the 1970’s, local sedimentation and low turbidity still affects areas like East Grand Lake (Allen et al., 2008; Piazza, 2014).

Figure 2-1. Selected features of the Atchafalaya River and basin. From Mossa 2016.

Figure 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.

CHAPTER 3 DATA AND METHODS

Hydrographic surveys from the United States Army Corps of Engineers (USACE) have been conducted roughly every ten years in the Atchafalaya River Basin since

1967. This study used data from the 1967, 1989, 1998 and 2006 surveys. The survey from 1998 was only used for data relating to East Grand Lake. Each survey contained individual sheets that span the length of the Atchafalaya River. Only sheets that covered the Central Atchafalaya River and East Grand Lake were used.

Every sheet contained cross-sectional sonar elevation points and channel boundaries that are crucial for determining bed elevation trends in the basin. The 1967 and 1989 surveys were in MrSID (multiresolution seamless image database) format, while 1998 and 2006 were in DGN format. Sheets in MrSID format were georeferenced to create a continuous picture of the study area. The elevation points and channel boundaries were then digitized by hand in the NAD 1983 StatePlane Louisiana South

FIPS 1702 Feet Projection, creating two respective shapefiles. Sheets in DGN format only required the selection and extraction of the points and boundaries to shapefiles. A total of 130 survey sheets and 16,223 elevation points were used, 10,016 of which were digitized by hand (Table 3-1). Figure 3-1 displays a flow chart of the methods used to process hydrographic survey data.

In order to analyze bathymetry between the cross-sectional data, an interpolation technique was needed to create a continuous raster surface. Interpolation is a mathematical process in which unknown values, such as bed elevations, can be predicted using surrounding observed values (Burrough and McDonnel, 1998). All techniques in the context of rivers systems assume that bathymetry is spatially

autocorrelated (Amante and Eakins, 2016). Although there is little consensus in the literature regarding the best interpolation technique (Chowdhury et al., 2017; Curtelli et al., 2015), Inverse distance weighting (IDW) and ordinary kriging (OK) are the most frequently used techniques when assessing 53 comparative studies (Li and Heap,

2011). IDW is a deterministic method that works under the assumption that sampled points closer to predicted points have more influence on predicted value than sample points that are further away (Wu et al., 2019). Its equation is as follows:

푛 ∑푖=1 푊푖푍푖 (3-1) Z(x) = 푛 ∑푖=1 푊푖

Kriging is a complex geostatistical method that is based on a concept of random functions. It assumes a surface is one realization of a random function with a certain spatial covariance (Mitas and Mitasova, 2005).

There are advantages to both presented techniques, and the literature reflects instances where IDW statistically performs similarly or better than kriging (Chowdhury et al., 2017; Panhalkar and Jarag, 2015). Likewise, documentation of kriging outperforming IDW also exists (Schwendel et al., 2012; Wu et al., 2019). Performance often relies on factors such as sampling density, sample spacing, and data variation

(Chaplot et al., 2006; Li and Heap, 2011), which usually favors a particular technique.

After interpolating bed elevation data using IDW and OK, analysis of root mean square error (RMSE) revealed that IDW had less error than OK. This study recognizes that kriging offers complex procedures for selecting parameters that can improve accuracy (Wu et al., 2019), however, the simplicity and availability of IDW was more appealing in this case. For these reasons, IDW was selected for this study.

Anisotropic considerations can be helpful in increasing the accuracy of interpolations, especially in rivers with high sinuosity (Merwade et al., 2006; Wu et al.,

2019). Such techniques were explored in this study; however, time constraints and the paucity of evidence of improvement excluded these considerations from this study. The need for this is not critical in the study area because the Central Atchafalaya River has a relatively straight channel.

The river boundary from 2006 was used as a common surface for interpolation. It was chosen because, in most places, it had the widest channel of all the surveyed years. This allowed the surface to capture the greatest amount of elevation points from any given survey. The use of a common surface made comparing the interpolated surfaces of different survey years possible. Elevation points from approximately RM 66 to 68 were excluded because they resided in a bend that had changed too dramatically from 1967 to 2006 for any meaningful analysis to be done. However, this area is included in the planform change assessment (Figure 3-2).

The Raster Calculator tool was used to visually show the differences between the interpolated surfaces of each year. This was simply accomplished by subtracting the earlier year’s raster from the later year’s raster. The result of this tool was a raster that displays elevation changes across the Atchafalaya’s channel and East Grand Lake.

This analysis was done for the following time periods for the Atchafalaya River: 1967-

1989, 1989-2006, and 1967-2006 and for East Grand Lake: 1967-1989, 1989-1998,

1998-2006, and 1967-2006.

The Cut Fill tool was used for volumetric changes in the study area. This tool identified and quantified areas in the channel and lake that experienced either

aggradation, degradation, or no change from one raster to the next. The tool uses the following formula:

V=(CellArea)*ΔZ (3-2)

The tool analyzes the same raster cell from two different times in order to produce its own raster that shows changes. All rasters in this study consisted of ~3m X 3 m (10ft x

10ft cells). The same time periods used for the Raster Calculator were used for the Cut

Fill tool.

In addition, a geovisualized 3D model in CityEngine Web Viewer was employed to present changes. Geovisualization is an emerging subfield that has the potential to improve geoscience education and communication through artistic and cartographic choices (Mitasova et al., 2012; Mossa et al., 2019). Its application to rivers and floodplains has been well explored and is exemplified in the high profile works of Daniel

Coe. Lidar data was merged with hydrographic survey data from river miles 95 to 100 and interpolated in order to visualize the floodplain surrounding the Atchafalaya River and East Grand Lake (Figure 3-3). The morphology of the river and lake can be seen using the comparison tool, which allows users to toggle between the models containing data from 1967, 1989, and 2006 (https://arcg.is/zTqPm0).

Daily discharge at Simmesport from 1967 to 2006 (Figure 2-1) was utilized to uncover potential drivers of aggradation and degradation in the study area. Sediment concentration and transport rates increase with discharge given that transportable sediment is available (Richards, 1982), inferring that frequency and magnitude of flows over the study period could explain changes in bed elevations and patterns of sedimentation. Because the critical point at which shear stress is (or is not) able to

transport alluvial material is not known in this system, this study generously assumes flows that exceed the 80th percentile of flows during the study period provide adequate sheer stress for sediment transport. The average number of days per year that exceeded this threshold was calculated for each time period in which bed elevations were analyzed.

Figure 3-1. Flow chart of methods used to process MrSid and DGN hydrographic survey data.

Table 3-1. Data Sources for the Central Atchafalaya River. Year Source Format Area Description Number of Number of Survey Sheets Survey Points

1967 USACE MrSID Atchafalaya Elevation 40 5223 Channel and East Grand Boundaries Lake

1989 USACE MrSID Atchafalaya Elevation 40 4793 Channel and East Grand Boundaries Lake

1998 USACE DGN East Grand Elevation 10 658 Lake and Boundaries

2006 USACE DGN Atchafalaya Elevation 40 5549 Channel and East Grand Boundaries Lake

Figure 3-2. Central Atchafalaya River channel planform, 1967-2006.

Figure 3-3. Geovisualization of the Central Atchafalaya River and East Grand Lake and floodplain features near RM 95-100.

CHAPTER 4 RESULTS

Atchafalaya River

Results from the interpolation of bed elevations reveal variation in aggradation and degradation in the Central Atchafalaya River. Figures 4-1 through 4-3, 4-4 through

4-6 and 4-7 through 4-9 display bed elevations, raster calculator and cut fill outputs, respectively, for each survey used. These figures give insight into spatial and temporal trends of aggradation and degradation in the river.

The dominant trend from 1967 to 2006 was degradation. The period saw mean bed elevation fall from -8.5 to -12.1 m (-27.9 to -39.9 feet) and the lowest surveyed elevation deepened from -31.6 to -44.5 m (-104 to -146 feet). Spatially, the deepest parts of the Atchafalaya main channel in 1967 (Figure 4-1) were in the center channel of portions of the Butte La Rose distributary and upper main channel (RM 55 to 65). Figure

4-3 shows that by 2006 the deepest parts resided all along the middle of the main channel. In terms of volume, -38.9 million cubic meters (Mm3) (-1375.1 Mft3) of sediment were gained and 189.3 Mm3 (6685.3 Mft3) were lost, resulting in a net sediment loss of

150.368 Mm3 (5310.2 Mft3) (Table 4-1). Figure 4-9 displays these changes and shows a general trend of degradation in the main channel and aggradation in the Butte La Rose distributary.

From 1967 to 1989, a significant decrease in bed elevation through the main channel can be seen in Figure 4-4. Volumetrically, -27.8 Mm3 (-983.1 Mft3) of sediment was gained and 183.1 Mm3 (6467.7 Mft3) was lost over the 22-year time period, resulting in a net sediment loss of 155.3 Mm3 (5484.6 Mft3) (Table 4-1). Approximately

81% of the sediment gained from 1967 to 1989 can be accounted for by the aggrading

Butte La Rose distributary (Table 4-1). This provides strong evidence of scouring in the main channel and infilling in the Butte La Rose distributary as the main channel increasingly captured more flow.

The time period from 1989 to 2006 conversely shows aggradation in the channel

(Figure 4-5 and 4-8). The sediment gain of -40.0 Mm3 (-1415.9 Mft3) of sediment along with a loss of 36.8 Mm3 (1300.9 Mft3), resulted in a net sediment gain of -3.2 Mm3 (-

115.0 Mft3) (Table 4-1). While the amount of sediment gain increased (45% increase) from the previous time period, the amount of sediment loss severely decreased from the previous time period (80% decrease). Tellingly, only 23% of the aggradation from 1989 to 2006 can be accounted for by the Butte La Rose distributary (Table 4-1). This is partly because the aggradation in the Butte La Rose distributary decreased by ~60% when compared to the period of 1967-1989. These results indicate that the main channel bed was slightly degrading during the time period while the Butte La Rose distributary continued aggrading at a slower rate.

Although this study does not focus on changes in channel geometry, digitized river boundaries in conjunction with elevation data show that the main channel was widening (Figure 3-2). This has significant implications for bed elevation changes in the results of this study. Since the larger channel of 2006 was used as a common surface for all analysis, elevation data from beyond the extent of the channel in some areas of the 1967 and 1989 surveys were included. This is illustrated in Figure 4-6 where high areas of change from 1967 to 2006 can be seen along the banks of the main channel.

Daily discharge data conveys interesting results. Figure 4-13 shows the average number of days per year that exceeded the high flow threshold for each time period.

Figure 4-14 displays the comparison of time periods and uses a chi squared test to determine if there are any statistical differences in days of high flow. Although 1967 to

1989 had more days of high flow than 1989 to 2006, a chi squared test revealed that there is no statistical difference between the periods. Statistical significance was found when 1989 to 1998 and 1998 to 2006 were compared to any other period. 1989 to 1998 had the most days of high flows while 1998 to 2006 had the least days of high flows.

This indicates that changes in bed elevation during these periods can be linked to the presence—or absence—of high flows.

East Grand Lake

East Grand Lake experienced aggradation over the 40-year time period of this study with average bed elevation increasing from -1.00 to -0.85 m (-3.31 to -2.82 feet) and a net sediment gain of -0.85 Mm3 (-30.3 Mft3) (Table 4-2). From 1967 to 1989, East

Grand Lake had a net sediment gain of -0.70 Mm3 (-24.7 Mft3), contrasting the scouring main channel during the same interval. Figure 4-8 shows volumetric changes from 1989 to 2006, which illustrates the -0.13 Mm3 (-4.8 Mft3) of sediment gained over the interval

(Table 4-2). Likewise, Figure 4-5 shows mostly increases in bed elevation, especially in the southernmost portion.

Further analysis using the 1998 hydrographic survey (Figures 4-10 through 4-12) gives important insight into recent patterns of sedimentation in the lake. From 1989 to

1998 East Grand Lake again gained sediment, however its gain of -1.89 Mm3 (-66.9

Mft3) was a 170% increase from the 1967 to 1989 time period (Table 4-2). Similarly, from 1998 to 2006 East Grand Lake had a proportionally larger change in volume than the ~20-year time periods. It lost 1.74 Mm3 (61.5 Mft3) of sediment, roughly matching the large amount of sediment gained from 1989 to 2006, but in the opposite direction

(Table 4-2). The smaller temporal scope that the 1998 survey provided demonstrates not only a larger magnitude of bed changes, but also a complete change in process.

Figure 4-1. Central Atchafalaya River and East Grand Lake elevation (ft) 1967.

Figure 4-2. Central Atchafalaya River and East Grand Lake elevation (ft) 1989.

Figure 4-3. Central Atchafalaya River and East Grand Lake elevation (ft) 2006.

Figure 4-4. Central Atchafalaya River and East Grand Lake elevation changes (ft) 1967- 1989.

Figure 4-5. Central Atchafalaya River and East Grand Lake elevation changes (ft) 1989- 2006.

Figure 4-6. Central Atchafalaya River and East Grand Lake elevation changes (ft) 1967- 2006.

Figure 4-7. Central Atchafalaya River and East Grand Lake volume changes (m3) 1967- 1989.

Figure 4-8. Central Atchafalaya River and East Grand Lake volume changes (m3) 1989- 2006.

Figure 4-9. Central Atchafalaya River and East Grand Lake volume changes (m3) 1967- 2006.

Table 4-1. Volume change for the Central Atchafalaya River with percentage Butte La Rose distributary contribution to total gain. Year Total Gain Loss Difference Dominant % of % Butte Volume (Mm3) (Mm3) (Mm3) Trend Dominant La Rose (Mm3) Dist. Contr. to Total Gain 1967- 210.986 -27.839 183.147 155.307 Sediment 86.8% 81.3% 1989 Loss 1989- 76.935 -40.096 36.839 -3.256 Sediment 52.1% 22.9% 2006 Gain 1967- 228.246 -38.939 189.307 150.368 Sediment 82.9% 76.8% 2006 Loss

Table 4-2. Volume change for East Grand Lake. Year Total Gain Loss Difference Dominant % of Volume (Mm3) (Mm3) (Mm3) Trend Dominant (Mm3) 1967-1989 1.34 -1.021 0.319 -.701 Sediment 76.1% Gain 1989-1998 2.012 -1.954 0.058 -1.896 Sediment 97.1% Gain 1998-2006 1.831 -0.045 1.786 1.741 Sediment 97.5% Loss 1989-2006 1.165 -0.651 0.514 -.136 Sediment 55.8% Gain 1967-2006 1.804 -1.332 0.472 -.859 Sediment 73.8% Gain

Figure 4-10. East Grand Lake elevation (ft) 1998.

Figure 4-11. East Grand Lake elevation changes (ft) 1989-1998 and 1998-2006.

Figure 4-12. East Grand Lake volume changes (m3) 1989-1998 and 1998-2006.

Figure 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).

Figure 4-14. Comparison of average days of high flows for each time period using Chi- squared tests.

CHAPTER 5 DISCUSSION

Atchafalaya River

The results of this study diverge from the existing literature on the upper and lower Atchafalaya River (Reynolds, 2019; Abdulrahman, 2018). Whereas aggradation was the dominant trend in the other reaches, except for the Wax Lake Outlet which experienced considerable degradation, degradation characterizes most of the Central

Atchafalaya riverbed over the 40 years studied (Figure 4-6 and 4-9). Changing flows over the study period is a potential driver of change in the basin and was tested for significance. The expected fluvial response to an increase in flow is increased incision and an increase in bank erosion (Schumm, 2005). However, Mossa (2016) also noted that most of the change that occurred in the past century was widening rather than deepening, possibly because this large river cannot scour much more below sea level due to base level constraints.

The large human influence in the Central Atchafalaya played a key role in its riverbed morphology. The closing of channels and dredging through the lakes and lacustrine deltas acted to funnel flow down a steeper main navigable channel. By design, the main channel eroded its banks and bed to accommodate the increasing flow, lessening the need for dredging. Channel training, the building up of natural levees, was utilized to maintain and increase flow capacity in the channel for similar reasons. This can be seen in Figures 4-4 through 4-6 where decreases in bed elevation throughout the main channel are present.

Bank and bed materials are also an essential factor when discussing degradation in the Central Atchafalaya. According to Fisk’s 1952 report, RM 46 to 67 contains a

shallow fine-grained top stratum and a sand substratum. In the flatter leveed section

(RM 46 to 55), the report documented scouring into the sand substratum, however, after the leveed section (RM 55 to 67) little penetration of the substratum was seen. Results show evidence of scouring in this section (Figure 4-6), meaning that the steeper gradient, increasing flow and natural levee height likely allowed for penetration of the easily transportable substratum.

From RM 67 to 112, Fisk’s report states that the topstratum consists of thin deltaic silt deposits and that the substratum consists of heavy clay that extends -36.5 m

(-120 feet) below sea level. Up to 1952, the flow of the Atchafalaya topped its banks because it was unable to erode the clay. Results indicate that increased flow and channel training may have aided this reach in penetrating the substratum, therefore decreasing bed elevations (Figures 4-6 and 4-9). This agrees with Fisk’s expectation that a straight, narrow and deep channel would develop following the aforementioned flow and levee changes.

Regarding the large reduction of degradation in the Central Atchafalaya’s channel from 1989 to 2006 (Figure 4-8), there are many factors to consider. Firstly, results from analyzing changes in frequency of high flow indicates that the 1989 to 2006 period was not statistically different than high flows from 1967 to 2006 or 1967 to 1989.

This demonstrates that, for the purposes of this study, discharge cannot account for the decline in degradation during this period. Although high flows were not proved to be significantly different, this study may be incorrect in its grouping and definition of high flows. The flood of 1973 is an example of historic flow (~22,500 m3/s) that likely had a large influence on the riverbed morphology of the reach. Due to this study’s

generalization of high flows, it may underestimate the significance of extreme events on the transport of sediment and scouring of the riverbed.

Additionally, lack of scouring from 1989 to 2006 in the main channel could indicate that the base level, being sea level at the Gulf of Mexico, constrains or prevents further downcutting. With most of the Central Atchafalaya River’s bed elevations being below sea level, base level must be considered a limitation to incision in the study area and basin as a whole (Mossa, 2016). Assuming that high flow during this period had no significant impact on changes in bed elevations, results of this study point to a new equilibrium being reached in the Central Atchafalaya River, likely as a response to the development of a main channel in the reach.

Despite a net gain of sediment from 1989 to 2006, there is still evidence of degradation in the main channel. Mean bed elevations decreased during the period from -11.4 to -12.1 m (-37.6 to -39.9 feet) and its minimum surveyed elevation decreased by 6 m (20 feet). Omitting the silting Butte La Rose distributary, the main channel lost some sediment over the time period, indicating a continued, but lesser, trend of degradation.

The Butte La Rose distributary exemplifies what is happening to many channels and canals in the Central Atchafalaya. Since the dredged main channel has captured the majority of flow, channels and canals directly connected to the sediment rich river have aggraded (Piazza, 2014). The Butte La Rose distributary, although larger than most affected channels, is no exception to the trend. Figures 4-6 and 4-9 clearly show aggradation and an increase in bed elevation over the years studied.

East Grand Lake

East Grand Lake was included in this study to provide a bed elevation perspective from beyond the banks of the main channel. Floodplain connectivity in the

Central Atchafalaya has garnered attention in recent years due to the presence of stagnation and hypoxic conditions in backswamp areas, especially during low flow periods (Hupp et al., 2008; Piazza, 2014; Baustian et al., 2019). With flow being confined to large channels and natural levees increasing in height, East Grand Lake offers insight into the transport of sediment to backswamp areas as well as sedimentation patterns in former Grand Lake.

Previous studies have already documented land-building in East Grand Lake using lidar and aerial photography (Allen et al., 2010; Piazza, 2014). This study utilized bed elevations to determine sedimentation patterns in an area of the lake with no documented land-building. From 1967 to 2006 the results of this study show localized aggradation, specifically in the southernmost portion, and net sediment gain in East

Grand Lake (Figures 4-6 and 4-9).

Changing the temporal scope revealed markedly different results. Figure 4-12 illustrates the gain of sediment from 1989 to 1998 and the loss of sediment from 1998 to

2006. Table 4-2 emphasizes the larger magnitude of sediment loss and gain of the ~10- year periods when compared to the ~20-year periods.

When looking at ~20-year periods, the results of this study agree with Hale et al.

1999 in that accretion in former Grand Lake has slowed and is reaching an equilibrium.

Conversely, ~10-year periods show a reversal from aggradation to degradation, a striking shift that appears to be related to discharge.

Figure 4-14 indicates that there is a significant difference in average days of high flow during the ~10-year periods when compared to every other time period, including each other. From 1989 to 1998, there were significantly more days of high flow. This corresponds with Figure 4-12 which displays pronounced aggradation in East Grand

Lake during the same period. In contrast, from 1998 to 2006 there were significantly less days of high flow. This corresponds with Figure 4-12 which displays pronounced degradation during the same period. This provides evidence that high flows transport sediment laden waters to East Grand Lake where velocity decreases and sediment is deposited. Likewise, the absence of high flows prevents river water from reaching East

Grand Lake, causing the lake to lose sediment over time.

Although the ~10-year periods display opposite processes that are likely driven by the frequency of high flows, the processes were relatively equal, affirming that an equilibrium is being reached in the study area. These results underscore the dynamic nature of fluvial systems, and how a balance can be struck despite shorter term variance in morphologic variables.

CHAPTER 6 CONCLUSION

Aggradation has been a prominent characteristic in much of the Atchafalaya

River Basin and has had negative impacts on endemic ecosystems, navigation and floodplain connectivity. Results of this study have shown that the Central Atchafalaya

River experienced degradation from 1967 to 2006. However, recent trends indicate that erosion in the main channel has been slowing.

East Grand Lake experienced aggradation over the study period and similarly experienced a slowing of its dominant process. Sub periods of 1989 to 2006 revealed a reversal, of relatively equal magnitude, from aggradation to degradation. This reversal is linked to a statistically significant decrease in frequency of high flows, which ultimately did not affect the broader time period of 1989 to 2006. Since 1967 to 1989 and 1989 to

2006 had no significant difference in frequency of high flows, the disparity in magnitude of sediment loss (Central Atchafalaya main channel) and sediment gain (East Grand

Lake) is due to a dynamic equilibrium being reached in the study area. Results also convey that the Central Atchafalaya River in times of high flow transported and deposited sediment in East Grand Lake.

Channel changes and sedimentation patterns explored in this study have significant implications for flood control, navigation, floodplain connectivity and ecosystem health in the Central Atchafalaya River. The deepening of the main channel by an average of 3.6 m indicates that dredging is continuing to be unnecessary in the reach, but that the filling Butte La Rose distributary may be an exception. The addition of .859 Mm3 of sediment to East Grand Lake suggests that floodplain habitats have

shrank due to altered hydrology, and the potential for hypoxia during low flow periods has increased.

Results of this study are limited primarily by the time scales at which bed elevations were observed. While the spatial data is extremely detailed and daily discharge aides in filling the void of up to 22 years between hydrographic surveys, it is well known that alluvial rivers modify and respond quickly to changes in morphologic variables. This study acknowledges that the selected time periods may encompass and be influenced by events that quickly shaped the study area, particularly those that occurred just prior to a hydrographic survey.

LIST OF REFERENCES

Abdulrahman, M. (2018). Channel bed and geometry changes in the upper and middle Atchafalaya River 1967-2006. Ph.D. Dissertation. Dept. of Geography, University of Florida.

Allen, Y.C., Constant, G.C., Couvillion, B.R. (2008). Preliminary classification of water areas within the Atchafalaya Basin Floodway System by using Landsat imagery. U.S. Geological Survey Open-File Report 2008-1320.

Allen, Y.C. (2010). A brief synopsis of land-water changes in the former extent of Grand Lake in the Atchafalaya Basin – early 1900s to late 2000s. Interim Report to Accompany Deliverables to the Louisiana Department of Natural Resources, Atchafalaya Basin Program, Baton Rouge, Louisiana.

Amante, C.J., Eakins, B.W. (2016). Accuracy of interpolated bathymetry in digital elevation models. Journal of Costal Research 76, 123-133.

Baustian, J.P., Piazza, B.P., Bergan, J.F. (2019). Hydrologic connectivity and backswamp water quality during a flood in the Atchafalaya Basin, USA. River Research and Applications, 1-6.

Burrough, P.A., McDonnell, R.A. (1998). Principles of geographical information systems. Oxford University Press, Oxford.

Chaplot, V., Darboux, F., Bourennane, H., Leguedois, S., Silvera, N., Phachomphon, K. (2006). Accuracy of interpolation techniques for the derivation of digital elevation models in relation to landform types and data density. Geomorphology 77, 126- 141.

Chowdhury, E., Hassan, Q., Achari, G., Gupta, A. (2017) Use of bathymetric and lidar data in generating digital elevation model over the lower Athabasca River watershed in Alberta, Canada. Water 9, 19.

Curtarelli, M., Leao, J., Ogashawara, I., Lorenzzetti, J., Stech, J. (2015). Assessment of Spatial Interpolation Methods to Map the Bathymetry of an Amazonian Hydroelectric Reservoir to Aid in Decision Making for Water Management. ISPRS International Journal of GeoInformation 4, 220-235.

Fisk, H.N. (1952). Geological Investigation of the Atchafalaya Basin and the Problem of the Mississippi River Diversion. United States Army Waterways Experiment Station. Mississippi River Commission, Vicksburg, MS.

Hale, L., Waldon, M.G., Bryan, C.F., Richards, P.A. (1999). Historic patterns of sedimentation in Grand Lake, Louisiana. In: Rozas, L.P., Nyman, J.A., Proffitt, C.E., Rabalais, N.N., Reed, D.J., Turner, R.E. (Eds.), Symposium Recent Research in Coastal Louisiana: Natural System Function and Response to Human Influence. Louisiana Sea Grant College Program, 3-12.

Hupp, C.R., Demas, C.R., Kroes, D.E., Day, R.H., Doyle, T.W. (2008). Recent sedimentation patterns within the central Atchafalaya Basin, Louisiana. Wetlands 28, 125-140.

Leopold, L.B., Miller, J.P., Wolman, M.G. (1964). Fluvial processes in geomorphology. San Francisco, CA: W.H. Freeman Company.

Li, J., Heap, A.D. (2011). A review of comparative studies of spatial interpolation methods in environmental sciences: Performance and impact factors. Ecol. Inform. 6(3-4), 228-241.

Lopez-Llompart, P., Kondolf, G.M. (2016). Encroachments in floodways of the Mississippi River and Tributaries Project. Nat Hazards 81, 513-542.

Merwade, V.M., Maidment, D.R., Goff, J.A. (2006). Anisotropic considerations while interpolating river channel bathymetry. Hydrology 331, 731-741.

Mitas, L., Mitasova, H. (2005) Spatial Interpolation. In: Longley, P.A., Goodchild, M.F., Maguire, D.J. and Rhind, D.W., Eds., Geographic Information Systems: Principles, Techniques, Management and Applications, 2nd Edition, Vol. 1, Part 2, Chapter 34

Mitsova, H., Harmon, R.S., Weaver, K.J., Lyons, N.J., Overton, M.F. (2012). Scientific visualization of landscapes and landforms. Geomorphology 137, 122-137.

Mossa, J. (1990). Discharge-suspended sediment relationships in the Mississippi- Atchafalaya River system, Louisiana. Ph.D. Dissertation. Dept. of Geography, Louisiana State University.

Mossa, J. (2013). Historical changes of a major juncture: Lower Old River, Louisiana. Phys. Geogr. 34, 315-334.

Mossa, J. (2016). The changing geomorphology of the Atchafalaya River, Louisiana: A historical perspective. Geomorphology 252, 112-127.

Mossa, J., Chen, Y., Wu, C. (2019). Geovisualization geoscience of large river floodplains, Journal of Maps 15, 75-91.

Panhalakr, S. S., and A. P. Jarag. 2016. “Assessment of Spatial Interpolation Techniques for River Bathymetry Generation of Panchganga River Basin Using Geoinformatic Techniques.” Asian Journal of Geoinformatics 15.

Piazza, B.P. (2014). The Atchafalaya River basin: History and ecology of an American wetland, River Basin: History and Ecology of an American Wetland. Texas A&M University Press, College Station, TX.

Powell, N. (1996). The Wax Lake Outlet Weir and Channel Response. 6th FISC, Federal Interagency Sedimentation Conference, Las Vegas III. Fluvial: Channel Evolution and Channel Stabilization, 46-53.

Reuss, M. (1998). Designing the Bayous: The Control of Water in the Atchafalaya Basin 1800-1995. Office of History, United States Army Corps of Engineers, Alexandria, VA.

Reynolds, J. (2019). Channel bed changes in the lower Atchafalaya River and Wax Lake Outlet, LA, 1967-2006. Master’s Thesis. Dept. of Geography, University of Florida.

Richards, K. (1982). Rivers, form and process in alluvial channels. London. Methuen. 359.

Roberts, H.H. (1998). Delta Switching: Early Responses to the Atchafalaya River Diversion. J. Coast. Res 14, 882-899.

Schumm, S.A. (2005). River variability and complexity. Cambridge, UK. Cambridge University Press, 220.

Schwendel, A.C., Fuller, I.C., Death, R.G. (2012). Assessing DEM interpolation methods for effective representation of upland stream morphology for rapid appraisal of bed stability. River Res. Appl. 28, 567–584.

Shaw, J.B., Mohrig, D., Whitman, S.K. (2013). The morphology and evolution of channels on the Wax Lake Delta, Louisiana, USA. J. Geophys. Res. Earth Surf. 188, 1562-1584.

Tye, R.S., Coleman, J.M. (1989b). Evolution of Atchafalaya lacustrine deltas, south- central Louisiana, Sediment. Geol. 65, 95-112.

Wu, C., Mossa, J., Mao, L., Almulla, M. (2019). Comparison of different spatial interpolation methods for historical hydrographic data of the lowermost Mississippi River. Annals of GIS 25, 133-151.

BIOGRAPHICAL SKETCH

John Newman earned a Bachelor of Science in geography from the University of

Florida in 2019. He continued his education at the University in pursuit of a Master of

Science in geography. After graduation, he will commission as a Second Lieutenant in the United States Marine Corps and attend The Basic School in Quantico, Virginia.