Causes for the decline of suspended‐sediment discharge in the Mississippi River system, 1940–2007

TLDR

The Missouri–Mississippi River system transported ~400 Mt yr⁻¹ of sediment before 1900, but this dropped to ~145 Mt yr⁻¹ in 1987–2006, a decline largely attributed to dams built on the Missouri River in the 1950s. Analysis of over 60 years of data shows dams account for only half the sediment loss, as the system shifted from transport‑limited to supply‑limited and additional engineering works and soil‑erosion controls have further trapped or removed sediment. Published 2009 by John Wiley & Sons, Ltd.

Abstract

Abstract Before 1900, the Missouri–Mississippi River system transported an estimated 400 million metric tons per year of sediment from the interior of the United States to coastal Louisiana. During the last two decades (1987–2006), this transport has averaged 145 million metric tons per year. The cause for this substantial decrease in sediment has been attributed to the trapping characteristics of dams constructed on the muddy part of the Missouri River during the 1950s. However, reexamination of more than 60 years of water‐ and sediment‐discharge data indicates that the dams alone are not the sole cause. These dams trap about 100–150 million metric tons per year, which represent about half the decrease in sediment discharge near the mouth of the Mississippi. Changes in relations between water discharge and suspended‐sediment concentration suggest that the Missouri–Mississippi has been transformed from a transport‐limited to a supply‐limited system. Thus, other engineering activities such as meander cutoffs, river‐training structures, and bank revetments as well as soil erosion controls have trapped sediment, eliminated sediment sources, or protected sediment that was once available for transport episodically throughout the year. Removing major engineering structures such as dams probably would not restore sediment discharges to pre‐1900 state, mainly because of the numerous smaller engineering structures and other soil‐retention works throughout the Missouri–Mississippi system. Published in 2009 by John Wiley & Sons, Ltd.