Erosion and Sediments Yields in the Changing Environment (Proceedings of a symposium held at the Institute of Mountain Hazards and Environment, CAS-Chengdu, China, 11–15 October 2012) (IAHS Publ. 356, 2012). Copyright  2012 IAHS Press 104 Are riverine sediment discharges sufficient to offset the sinking coast of Louisiana? Y. JUN XU & TIMOTHY ROSEN School of Renewable Natural Resources, Louisiana State University Agricultural Center, Baton Rouge, LA 70803, USA yjxu@lsu.edu Abstract The Mississippi River and four other major rivers along Louisiana’s coast (USA) discharge a combined total of 620 km 3 of water annually into the Gulf of Mexico. In addition to the vast quantity of freshwater, these river systems carry substantial sediments that affect physical, biological and human domains in the northern Gulf of Mexico. In the past century, river engineering and land use practices in the river basins have changed dramatically. A large number of locks and dams were built along the major tributary rivers including the Upper Mississippi River, Illinois River, Missouri River, Ohio River, Tennessee River, Arkansas River, and Red River, which has greatly contributed to the reduction in sediment yield to the continental shelf of the northern Gulf of Mexico. Concurrently, Louisiana’s coast has experienced the highest rate of relative sea-level rise of any region in the USA. In the past 50 years land loss rates along Louisiana’s coast have exceeded over 60 km 2 year -1 , and in the 1990s the rate has been estimated to be between 40 and 56 km 2 year -1 . This change represents 80% of the coastal wetland loss annually in the entire continental USA. The highest relative sea-level rise is 17.7 mm year -1 at Calumet, Louisiana, compared to 6.3 mm year -1 at Galveston, Texas, 1.5 mm year -1 at Biloxi, Mississippi, and 2.3 mm year -1 at Pensacola, Florida. Riverine sediments are precious resources to coastal Louisiana, and their effective management is of long-term strategic importance. This paper reports combined sediment yields from two major distributaries of the Mississippi River and four major coastal rivers in Louisiana for the most recent three decades, and discusses the actual availability of sediment and a new diversion approach – controlled overbank flow – that mimics the natural process of sediment replenishment over large areas. Key words riverine sediment; TSS yield; Mississippi-Atchafalaya River; coastal Louisiana INTRODUCTION The global average sea level has risen from 1961 to the present time at an average rate of 1.8 mm per year, ranging from 1.3 to 2.3 mm (IPCC, 2007). The rise rate was found to be faster for the recent decade, averaging about 3.1 mm per year with a range between 2.4 and 3.8 mm. The Louisiana coast of the northern Gulf of Mexico has experienced one of the highest sea-level rises over the past century (Dixon et al., 2006; Ivins et al., 2007). Within the last 50 years land loss rates have exceeded 60 km 2 per year, and in the 1990s the rate has been estimated as between 40 and 56 km 2 each year. This loss represents 80% of the coastal wetland loss annually in the entire continental United States. The highest relative sea-level rise (RSLR) is 17.7 mm per year at Calumet station in St. Mary’s Parish of Louisiana, according to the US Army Corps of Engineers tide gauge stations (Penland & Ramsey, 1990) compared to 10.4 mm per year at Grand Isle, Louisiana, 6.3 mm per year at Galveston, Texas, 2.3 mm per year at Pensacola, Florida, 2.2 mm per year at Key West, Florida, 1.7 mm per year at Cedar Key, Florida, 1.5 mm per year at Biloxi, Mississippi, and 3.1 mm per year eustatic sea-level rise. Based on their assessment using long-term historical aerial photos and satellite images, Morton et al. (2010) found that much of the land loss in coastal Louisiana is caused by land subsidence, rather than erosion. While the land has been sinking and the sea level has been rising, sediment yields from the Mississippi River and four other major coastal rivers in southern Louisiana have been declining (Horowitz, 2010; Rosen & Xu, 2011). River engineering has confined sediment distribution to only the continental shelf of the northern Gulf of Mexico. Consequently, the Louisiana Gulf coast has been subject to the highest rate of relative sea-level rise of any region in the USA. Couvillion et al. (2011) found that approximately 4900 km 2 of low- lying coastal land on Louisiana’s delta plain have become submerged since 1932. Previous studies (Britsch & Dunbar, 1993; Barras et al., 2003) reported peak delta-plain land losses of 60 to 75 km 2 each year from the 1960s to 1980s.