My research focuses on hydrologic and biogeochemical processes in both natural and managed ecosystems in a wide range from uplands to wetlands. In hydrology, I am mostly concerned with watershed scale modeling, GIS applications in surface hydrology, and water quality. In biogeochemistry, I have been working on carbon/nitrogen cycles and riverine sediment and nutrient transport. I am also interested in climate change effects on hydrologic and biogeochemical cycles. Phone: 225-578-4168
This study analyzed bathymetric changes of the 77-km Yangtze River Estuary in China over the past... more This study analyzed bathymetric changes of the 77-km Yangtze River Estuary in China over the past ten years in order to understand the impacts of recent human activities on the estuary of a large alluvial river. Morphological changes were assessed by analyzing digitized bathymetric data of the estuarine channels from 2002 to 2013. Additionally, multi-beam bathymetric measurements made in 2012, 2014 and 2015 were utilized to investigate microtophographic bedforms of the lower reach of the estuary. Our results showed that the middle and upper reaches of the Yangtze River Estuary experienced substantial channel bed erosion in the past 10 years, and that the recent human activities have contributed to the change. These included the construction of a 70 km 2 reservoir along the Yangtze River Estuary, the Qingcaosha Reservoir, for drinking water supply for the City of Shanghai, which has caused progressive bed erosion in the North Channel. The net volume of channel erosion in the Hengsha Passage from 2002 to 2013 was 0.86 Â 10 8 m 3. A large amount of the eroded sediment was trapped downstream, causing overall accretion in the upper reach of the North Passage. The middle and upper reaches of the South Passage also experienced intense erosion (0.45 Â 10 8 m 3) in the past ten years, while high accretion occurred in the lower reach because of the Deepening Waterway Project. The channel dredging left a large range of dredging marks and hollows in the North Passage. The increasing saltwater intrusion found in the Yangtze River Estuary may have been a consequence of either dredging or erosion, or both combined.
A recent study reported considerable sediment trapping by three large channel bars downstream 18–... more A recent study reported considerable sediment trapping by three large channel bars downstream 18–28 km of the Mississippi–Atchafalaya River diversion (commonly known as the Old River Control Structure, ORCS) during the 2011 Mississippi River flood. In this study, we analyzed 3-decadal morphological changes of the 10-km river channel and the three bars to elucidate the long-term effects of river engineering including diversion, revetment and dike constructions. Satellite images captured between 1985 and 2015 in approximate 5-year intervals were selected to estimate the change of channel morphology and bar surface area. The images were chosen based on river stage heights at the time when they were captured to exclude the temporal water height effect on channel and bar morphology. Using a set of the satellite images captured during the period of 1984–1986 and of 2013–2014, we developed rating curves of emerged bar surface area with the corresponding river stage height for determining the change in bar volume from 1985 to 2013. Two of the three bars have grown substantially in the past 30 years, while one bar has become braided and its surface area has shrunken. As a whole, there were a net gain of 4,107,000 m 2 in surface area and a net gain of 30,271,000 m 3 in volume, an equivalent of approximately 36 million metric tons of sediment assuming a bulk density of 1.2 t/m 3. Sediment trapping on the bars was prevalent during the spring floods, especially during the period of 1990–1995 and of 2007–2011 when large floods occurred. The results suggest that although revetments and dikes have largely changed the morphology of the channel and the bars, they seem to have a limited impact on the overwhelming trend of sediment deposition caused by the river diversion.
The formation of channel bars has been recognized as the most significant sediment response to th... more The formation of channel bars has been recognized as the most significant sediment response to the highly trained Mississippi River (MR). However, no quantitative study exists on the dynamics of emerged channel bars and associated sediment accumulation in the last 500-kilometer reach of the MR from the Gulf of Mexico outlet, also known as the lowermost Mississippi River. Such knowledge is especially critical for riverine sediment management to impede coastal land loss in the Mississippi River Delta. In this study, we utilized a series of satellite images taken from August 2010 to January 2012 to assess the changes in surface area and volume of three large emerged channel bars in the lowermost MR following an unprecedented spring flood in 2011. River stage data were collected to develop a rating curve of surface areas detected by satellite images with flow conditions for each of the three bars. A uniform geometry associated with the areal change was assumed to estimate the bar volume changes. Our study reveals that the 2011 spring flood increased the surface area of the bars by 3.5% to 11.1%, resulting in a total surface increase of 7.3%, or 424,000 m 2. Based on the surface area change, we estimated a total bar volume increase of 4.4%, or 1,219,900 m 3. This volume increase would be equivalent to a sediment trapping of approximately 1.0 million metric tons, assuming a sediment bulk density of 1.2 metric tons per cubic meter. This large quantity of sediment is likely an underestimation because of the neglect of subaqueous bar area change and the assumption of a uniform geometry in volume estimation. Nonetheless, the results imply that channel bars in the lowermost MR are capable of capturing a substantial amount of sediment during floods, and that a thorough assessment of their long-term change can OPEN ACCESS Water 2015, 7 6080 provide important insights into sediment trapping in the lowermost MR as well as the feasibility of proposed river sediment diversions.
Strontium and barium to calcium ratios are often used as proxies for tracking animal movement acr... more Strontium and barium to calcium ratios are often used as proxies for tracking animal movement across salinity gradients. Many estuarine rivers face saltwater intrusion due to sea level rise, potentially causing changes in mobility and distribution of these metals upstream. From May 2013 to August 2015, monthly water samples were collected and in-situ measurements were performed at six sites along an estuary strongly affected by saltwater intrusion, the Calcasieu River, with salinity ranging from 0.02 to 29.50 Parts Per Thousand (ppt). Results showed that the total Sr concentration and the Sr/Ca ratio both increased significantly with increasing salinity. The average Sr concentration at the site closest to the Gulf of Mexico (Site 6) was 46.21 µmol/L, about 130 times higher than that of the site furthest upstream (Site 1, 0.35). The average Sr/Ca ratio at Site 6 (8.41 mmol/mol) was about three times the average Sr/Ca ratio at Site 1 (2.89). However, the spatial variation in total Ba concentration was marginal, varying from 0.36 to 0.47 µmol/L. The average Ba/Ca ratio at Site 1 (4.82 mmol/mol) was about 54 times the ratio at Site 6 (0.09), showing a negative relation between the Ba/Ca ratio and salinity. All elemental concentrations and ratios had considerable seasonal variations, with significant differences among sampling months for the Sr and Ba concentrations and the Ba/Ca ratio (p < 0.01). The results suggest that for low-gradient estuarine rivers such as the Calcasieu River, water chemistry upstream would experience substantial Sr and Ca enrichment, potentially affecting aquatic environments and biological communities.
TMDL 2010: Watershed Management to Improve Water Quality Proceedings, 14-17 November 2010 Hyatt Regency Baltimore on the Inner Harbor, Baltimore, Maryland USA, 2010
Timber harvest can increase streamwater temperature, surface runoff, organic input, and subsurfac... more Timber harvest can increase streamwater temperature, surface runoff, organic input, and subsurface leaching of nutrients, which can reduce dissolved oxygen (DO) concentrations in adjacent water bodies. To limit water quality degradation, total maximum daily load (TMDL) guidelines ...
This study investigated long-term (1980-2009) yields and variability of total organic carbon (TOC... more This study investigated long-term (1980-2009) yields and variability of total organic carbon (TOC) from four major coastal rivers in Louisiana entering the Northern Gulf of Mexico where a large-area summer hypoxic zone has been occurring since the middle 1980s. Two of these rivers drain agriculture-intensive (> 40%) watersheds, while the other two rivers drain forest-pasture dominated (> 50%) watersheds. The study found that these rivers discharged a total of 13.0×104 t TOC annually, fluctuating from 5.9×104 to 22.8×104 t. Seasonally, the rivers showed high TOC yield during the winter and early spring months, corresponding to the seasonal trend of river discharge. While river hydrology controlled TOC yields, land use has played an important role in fluxes, seasonal variations, and characteristics of TOC. The findings fill in a critical information gap of quantity and quality of organic carbon transport from coastal watersheds to one of the world’s largest summer hypoxic zones.
The Mississippi River Delta Plain has undergone substantial land loss caused by subsidence, relat... more The Mississippi River Delta Plain has undergone substantial land loss caused by subsidence, relative sea-level rise, and loss of connectivity to the Mississippi River. Many restoration projects rely on diversions from the Mississippi River, but uncertainty exists about the timing and the amount of actually available sediment. This study examined long-term (1980–2010) suspended sediment yield as affected by different hydrologic regimes to determine actual suspended sediment availability and how this may affect diversion management. A stage hydrograph-based approach was employed to quantify total suspended sediment load (SSL) of the lower Mississippi River at Tarbert Landing during three river flow conditions: Peak Flow Stage (stage = 16.8 m, discharge >32,000 m3 s−1), High Flow Stage (stage = 14.6 m, discharge = 25,000–32,000 m3 s−1), and Intermediate Flow Stage (Stage = 12.1 m, discharge = 18,000–25,000 m3 s−1). Suspended sediment concentration (SSC) and SSL were maximized during High Flow and Intermediate Flow Stages, accounting for approximately 50% of the total annual sediment yield, even though duration of the stages accounted for only one-third of a year. Peak Flow Stage had the highest discharge, but significantly lower SSC (p < 0.05), indicating that diversion of the river at this stage would be less effective for sediment capture. The lower Mississippi River showed significantly higher SSC (p < 0.0001) and SSL (p < 0.0001) during the rising than the receding limb. When the flood pulse was rising, Intermediate Flow and High Flow Stages showed greater SSC and SSL than Peak Flow Stage. Together, Intermediate Flow and High Flow Stages on the rising limb annually discharged 28 megatonnes over approximately 42 days, identifying this to be the best period for sediment capture and diversion.
This study analyzed bathymetric changes of the 77-km Yangtze River Estuary in China over the past... more This study analyzed bathymetric changes of the 77-km Yangtze River Estuary in China over the past ten years in order to understand the impacts of recent human activities on the estuary of a large alluvial river. Morphological changes were assessed by analyzing digitized bathymetric data of the estuarine channels from 2002 to 2013. Additionally, multi-beam bathymetric measurements made in 2012, 2014 and 2015 were utilized to investigate microtophographic bedforms of the lower reach of the estuary. Our results showed that the middle and upper reaches of the Yangtze River Estuary experienced substantial channel bed erosion in the past 10 years, and that the recent human activities have contributed to the change. These included the construction of a 70 km 2 reservoir along the Yangtze River Estuary, the Qingcaosha Reservoir, for drinking water supply for the City of Shanghai, which has caused progressive bed erosion in the North Channel. The net volume of channel erosion in the Hengsha Passage from 2002 to 2013 was 0.86 Â 10 8 m 3. A large amount of the eroded sediment was trapped downstream, causing overall accretion in the upper reach of the North Passage. The middle and upper reaches of the South Passage also experienced intense erosion (0.45 Â 10 8 m 3) in the past ten years, while high accretion occurred in the lower reach because of the Deepening Waterway Project. The channel dredging left a large range of dredging marks and hollows in the North Passage. The increasing saltwater intrusion found in the Yangtze River Estuary may have been a consequence of either dredging or erosion, or both combined.
A recent study reported considerable sediment trapping by three large channel bars downstream 18–... more A recent study reported considerable sediment trapping by three large channel bars downstream 18–28 km of the Mississippi–Atchafalaya River diversion (commonly known as the Old River Control Structure, ORCS) during the 2011 Mississippi River flood. In this study, we analyzed 3-decadal morphological changes of the 10-km river channel and the three bars to elucidate the long-term effects of river engineering including diversion, revetment and dike constructions. Satellite images captured between 1985 and 2015 in approximate 5-year intervals were selected to estimate the change of channel morphology and bar surface area. The images were chosen based on river stage heights at the time when they were captured to exclude the temporal water height effect on channel and bar morphology. Using a set of the satellite images captured during the period of 1984–1986 and of 2013–2014, we developed rating curves of emerged bar surface area with the corresponding river stage height for determining the change in bar volume from 1985 to 2013. Two of the three bars have grown substantially in the past 30 years, while one bar has become braided and its surface area has shrunken. As a whole, there were a net gain of 4,107,000 m 2 in surface area and a net gain of 30,271,000 m 3 in volume, an equivalent of approximately 36 million metric tons of sediment assuming a bulk density of 1.2 t/m 3. Sediment trapping on the bars was prevalent during the spring floods, especially during the period of 1990–1995 and of 2007–2011 when large floods occurred. The results suggest that although revetments and dikes have largely changed the morphology of the channel and the bars, they seem to have a limited impact on the overwhelming trend of sediment deposition caused by the river diversion.
The formation of channel bars has been recognized as the most significant sediment response to th... more The formation of channel bars has been recognized as the most significant sediment response to the highly trained Mississippi River (MR). However, no quantitative study exists on the dynamics of emerged channel bars and associated sediment accumulation in the last 500-kilometer reach of the MR from the Gulf of Mexico outlet, also known as the lowermost Mississippi River. Such knowledge is especially critical for riverine sediment management to impede coastal land loss in the Mississippi River Delta. In this study, we utilized a series of satellite images taken from August 2010 to January 2012 to assess the changes in surface area and volume of three large emerged channel bars in the lowermost MR following an unprecedented spring flood in 2011. River stage data were collected to develop a rating curve of surface areas detected by satellite images with flow conditions for each of the three bars. A uniform geometry associated with the areal change was assumed to estimate the bar volume changes. Our study reveals that the 2011 spring flood increased the surface area of the bars by 3.5% to 11.1%, resulting in a total surface increase of 7.3%, or 424,000 m 2. Based on the surface area change, we estimated a total bar volume increase of 4.4%, or 1,219,900 m 3. This volume increase would be equivalent to a sediment trapping of approximately 1.0 million metric tons, assuming a sediment bulk density of 1.2 metric tons per cubic meter. This large quantity of sediment is likely an underestimation because of the neglect of subaqueous bar area change and the assumption of a uniform geometry in volume estimation. Nonetheless, the results imply that channel bars in the lowermost MR are capable of capturing a substantial amount of sediment during floods, and that a thorough assessment of their long-term change can OPEN ACCESS Water 2015, 7 6080 provide important insights into sediment trapping in the lowermost MR as well as the feasibility of proposed river sediment diversions.
Strontium and barium to calcium ratios are often used as proxies for tracking animal movement acr... more Strontium and barium to calcium ratios are often used as proxies for tracking animal movement across salinity gradients. Many estuarine rivers face saltwater intrusion due to sea level rise, potentially causing changes in mobility and distribution of these metals upstream. From May 2013 to August 2015, monthly water samples were collected and in-situ measurements were performed at six sites along an estuary strongly affected by saltwater intrusion, the Calcasieu River, with salinity ranging from 0.02 to 29.50 Parts Per Thousand (ppt). Results showed that the total Sr concentration and the Sr/Ca ratio both increased significantly with increasing salinity. The average Sr concentration at the site closest to the Gulf of Mexico (Site 6) was 46.21 µmol/L, about 130 times higher than that of the site furthest upstream (Site 1, 0.35). The average Sr/Ca ratio at Site 6 (8.41 mmol/mol) was about three times the average Sr/Ca ratio at Site 1 (2.89). However, the spatial variation in total Ba concentration was marginal, varying from 0.36 to 0.47 µmol/L. The average Ba/Ca ratio at Site 1 (4.82 mmol/mol) was about 54 times the ratio at Site 6 (0.09), showing a negative relation between the Ba/Ca ratio and salinity. All elemental concentrations and ratios had considerable seasonal variations, with significant differences among sampling months for the Sr and Ba concentrations and the Ba/Ca ratio (p < 0.01). The results suggest that for low-gradient estuarine rivers such as the Calcasieu River, water chemistry upstream would experience substantial Sr and Ca enrichment, potentially affecting aquatic environments and biological communities.
TMDL 2010: Watershed Management to Improve Water Quality Proceedings, 14-17 November 2010 Hyatt Regency Baltimore on the Inner Harbor, Baltimore, Maryland USA, 2010
Timber harvest can increase streamwater temperature, surface runoff, organic input, and subsurfac... more Timber harvest can increase streamwater temperature, surface runoff, organic input, and subsurface leaching of nutrients, which can reduce dissolved oxygen (DO) concentrations in adjacent water bodies. To limit water quality degradation, total maximum daily load (TMDL) guidelines ...
This study investigated long-term (1980-2009) yields and variability of total organic carbon (TOC... more This study investigated long-term (1980-2009) yields and variability of total organic carbon (TOC) from four major coastal rivers in Louisiana entering the Northern Gulf of Mexico where a large-area summer hypoxic zone has been occurring since the middle 1980s. Two of these rivers drain agriculture-intensive (> 40%) watersheds, while the other two rivers drain forest-pasture dominated (> 50%) watersheds. The study found that these rivers discharged a total of 13.0×104 t TOC annually, fluctuating from 5.9×104 to 22.8×104 t. Seasonally, the rivers showed high TOC yield during the winter and early spring months, corresponding to the seasonal trend of river discharge. While river hydrology controlled TOC yields, land use has played an important role in fluxes, seasonal variations, and characteristics of TOC. The findings fill in a critical information gap of quantity and quality of organic carbon transport from coastal watersheds to one of the world’s largest summer hypoxic zones.
The Mississippi River Delta Plain has undergone substantial land loss caused by subsidence, relat... more The Mississippi River Delta Plain has undergone substantial land loss caused by subsidence, relative sea-level rise, and loss of connectivity to the Mississippi River. Many restoration projects rely on diversions from the Mississippi River, but uncertainty exists about the timing and the amount of actually available sediment. This study examined long-term (1980–2010) suspended sediment yield as affected by different hydrologic regimes to determine actual suspended sediment availability and how this may affect diversion management. A stage hydrograph-based approach was employed to quantify total suspended sediment load (SSL) of the lower Mississippi River at Tarbert Landing during three river flow conditions: Peak Flow Stage (stage = 16.8 m, discharge >32,000 m3 s−1), High Flow Stage (stage = 14.6 m, discharge = 25,000–32,000 m3 s−1), and Intermediate Flow Stage (Stage = 12.1 m, discharge = 18,000–25,000 m3 s−1). Suspended sediment concentration (SSC) and SSL were maximized during High Flow and Intermediate Flow Stages, accounting for approximately 50% of the total annual sediment yield, even though duration of the stages accounted for only one-third of a year. Peak Flow Stage had the highest discharge, but significantly lower SSC (p < 0.05), indicating that diversion of the river at this stage would be less effective for sediment capture. The lower Mississippi River showed significantly higher SSC (p < 0.0001) and SSL (p < 0.0001) during the rising than the receding limb. When the flood pulse was rising, Intermediate Flow and High Flow Stages showed greater SSC and SSL than Peak Flow Stage. Together, Intermediate Flow and High Flow Stages on the rising limb annually discharged 28 megatonnes over approximately 42 days, identifying this to be the best period for sediment capture and diversion.
Delta growth at the Atchafalaya River Delta Complex is dictated by large floods.Tropical systems ... more Delta growth at the Atchafalaya River Delta Complex is dictated by large floods.Tropical systems have major impact on delta growth when there is no large flood.Without large floods vegetation does not buffer against higher energy storms.Consistent floods and sediment supply are most important for continued delta growth.The Mississippi River Delta Plain has experienced substantial wetland loss from subsidence, erosion, and sea level rise, threatening coastal communities and the ecosystems that support them. The Atchafalaya River, the largest distributary of the Mississippi River, has one of the few prograding delta features along the ~ 200-km deltaic coastline. Understanding changes in the Atchafalaya River Delta complex (ARDC) development has critical implications for future prediction and management strategy for the Mississippi River Delta Plain. This study was organized to answer two major questions: (1) how did development of the ARDC respond to fluctuation in riverine sediment supply over the period 1989–2010, and (2) has vegetation succession helped stabilize subaerial land? The study quantified annual total suspended sediment yields to the two ARDC subdeltas—Atchafalaya River subdelta (ARSD) and Wax Lake outlet subdelta (WLSD)—classified delta land cover using satellite imagery over ~ 5-year intervals into three classes: barren land, vegetation, and open water and investigated the relationship of delta land change with sediment yield and vegetation succession. Over the entire 21-year study period, we found a net land gain of 59 km2, with the ARSD accounting for 58% of this gain and WLSD 42%. Sediment yield to the subdeltas decreased from an average annual of 38 megatonnes (MT) for ARSD and 18 MT for WLSD during 1989–1995 to an average annual of 24 MT for ARSD and 17 MT for WLSD during 2004–2010, corresponding to the decrease in riverine suspended sediment concentration. Concurrently, total land growth rate decreased from 2.4 km2 y− 1 to 1.6 km2 y− 1 for ARSD and 3.2 km2 y− 1 to 0.6 km2 y− 1 for WLSD. However, the ARDC had a net land loss of 2.1 km2 during 1999–2004 because of tropical system effects in conjunction with the lack of large river floods (defined as discharge > 13,800 m3 s− 1). On average, more than 60% of newly vegetated land remained vegetated in subsequent years, and when compared with barren areas, vegetated land was less likely (7.3% vs. 32%) to be converted to water, indicating vegetative stabilization effect. However, during the period without a major flood, vegetation buffering against tropical system erosion was limited. This indicates that over the period 1989 to 2010 land growth of the ARDC was dictated by large flood events.
Uploads
Books by Y. Jun Xu
Papers by Y. Jun Xu