Dams Causing Dramatic Drop in Mississippi River Sediment

Folks who want to beat their feet in the Mississippi mud will find less of it available than in the past, a new government study shows.

Sediment discharged by the mighty river has been reduced by half in the last 35 years, and this may be a factor in receding shorelines in the Mississippi Delta, the U.S. Geological Survey reports.

The study said the dramatic drop in sediment carried by the river occurred following construction of several large dams on the Missouri River in the 1950s and 1960s. The Missouri has been the major supplier of sediment to the river, while the Upper Mississippi and Ohio river systems contribute large volumes of water but far less solid matter.

“In cases like the Mississippi, dams trap sediment in the reservoirs they impound,” said Philip Cohen, the Survey’s chief hydrologist. “The water that flows past the dams carries less sediment, so that the river has a higher capacity to erode downstream. At the river delta, instead of the river depositing sediment, the ocean waves and tide wash away the smaller sediment load.

“Just because certain rivers are carrying less sediment does not mean, however, that we can relax efforts to control soil erosion,” Cohen said. Only 10% of eroded soil is delivered to the oceans, with the rest winding up in stream valleys, flood plains and other areas.

Although the Mississippi currently discharges only about half the sediment it did in the past, that still amounts to 230 million tons annually, more than any other river, Cohen said.

Dams have had similar effects on the sediment carried by other rivers.

For example, the amount of sediment discharged into the Gulf of Mexico by the Rio Grande, once as much as 20 million tons per year, now averages less than 1 million tons due to the effect of dams and reservoirs.