How are human activities like levee building and damming changing the way the Mississippi River carries water and sediment, and what does that mean for Louisiana’s coast?

To protect cities and communities along the Mississippi River, large levees were built along the lower reaches following the historic 1927 flood. These levees prevent overbank flooding but also cut off freshwater and sediment delivery to adjacent wetlands and floodplains, and instead funnel water and sediment directly into the Gulf of Mexico (Kesel, 1988). Historically, most of the sediment reaching the mouth of the Mississippi River was sourced from the Missouri Watershed and tributaries draining from the Rocky Mountains (Meade & Moody, 2010). However, beginning in the 1950s, extensive dam construction on the Missouri River and its major tributaries have resulted in large volumes of sediment being trapped, reducing the total sediment load reaching Louisiana’s delta plain from ~390 Mt y-1 (pre-dam conditions) to ~100 Mt y-1 (Kemp et al., 2016).

The effects of substantially reduced sediment loads and restrictions to the exchange of freshwater and sediment between the river and deltaic wetlands have critically impacted Louisiana’s coast. Without the periodic input of river sediment to balance natural subsidence and sea-level rise, wetlands and barrier islands are rapidly eroding. Since the 1930s, nearly 5,000 km² of land has been lost, contributing to ecosystem degradation and greater vulnerability to hurricanes and storm surge (Couvillion et al., 2017).