The effect of commercial and recreational traffic on the resuspension of sediment in Navigation Pool 9 of the Upper Mississippi River

This study was conducted to determine some effects of commercial and recreational traffic on the resuspension of sediment in Navigation Pool No. 9 of the Upper Mississippi River. Fifty commercial vessel passages were examined at five different main channel locations and at side channels that were adjacent to each of the main channel locations. Sixteen recreational vessel passages were examined at one main channel location and its adjacent side channel and at a channel located in the backwaters. The backwater channel was not directly influenced by navigation in the main channel. Changes in total non-filterable residue (TNFR) and average particle size of suspended silts were used to assess some effects of navigation. Seventy-eight percent of the commercial vessel passages resulted in significant increases of TNFR and/or average particle size in the water column. In the main channel, TNFR increased from 3.4% to 15% above ambient levels; in the side channels, increases ranged from 2.5% to 21.7%. The average diameter of the resuspended silts increased by 0.21 to 2.34 m. In the main channel, 50% of the recreational vessel passages caused increased TNFR and all passages increased average particle size. In the backwater channel, all of the recreational passages caused increases in TNFR and average particle size. Total resuspended sediment transported downstream ranged from an estimated 0.82 to 1015.7 mTons/passage in the main channel for commercial vessels, 0.39 to 0.64 mTons/passage in the main channel for recreational vessels, 0.22 to 28.12 mTons/passage in side channels due to commercial vessels in the main channel, and 0.54 to 2.08 mTons/ passage in the backwater channel for recreational vessels. Bed-sediment composition, location of the vessels in the channel, channel geometry, the number of successive passages, and vessel speed were identified as factors that affected the magnitude of the resuspension.