The navigation channel of the Mississippi River apparently rose during last year’s record flood and is keeping the U.S. Army Corps of Engineers busier than usual as the river keeps sinking to near-record depths.

The Corps’ Vicksburg District has operated four dredges to keep the channel open, its Dredge Jadwin and three other contracted dredges, a release from the district said.

“This unusually high amount of dredging is necessary due to the large quantity of silt that was deposited on the harbor and riverbeds during the historic flood of 2011 and has subsequently affected channel depths,” read part of the release.