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US Fish & Wildlife Service FieldNotes

U.S. Fish and Wildlife Service and U.S. Army Corps of Engineers come together with a bang!

Region 3, January 3, 2013
Brad Rogers and Donovan Henry of the Carterville FWCO collecting DIDSON footage.
Brad Rogers and Donovan Henry of the Carterville FWCO collecting DIDSON footage. - Photo Credit: n/a
DIDSON footage of a rock pinnacle
DIDSON footage of a rock pinnacle - Photo Credit: n/a
MDC setting nets downstream of blasting barge. Note the deployed buoy.
MDC setting nets downstream of blasting barge. Note the deployed buoy. - Photo Credit: n/a

The U.S. Army Corps of Engineers recently conducted a project in the Middle Mississippi River that is of regional and national concern, and has been a common news topic across the nation. The title of the project is, “Explosive Removal of Rock Pinnacles and Outcroppings Considered to be Navigation Obstructions During Low-Flow Periods on the Middle Mississippi River.” Oddly enough, the title of the project is not nearly half as long as the backstory.


USACE is authorized to operate and maintain a safe and dependable navigation channel in the Middle Mississippi River, with the authorized channel dimensions set at a minimum of 9 feet deep and 300 feet wide, with additional width in bends as required. In 1988, an extremely low-water year, there were a number of rock pinnacles and rock shelves that were deemed a potential hazard to commercial navigation traffic on the river. These rock hazards were largely eliminated from 1988 through1999 using explosives to remove or lower the elevation of the rock pinnacles and outcroppings.

After over a decade of blasting, validation of the removal of the potential hazards to navigation traffic was conducted using an I-beam attached to two cables. The I-beam was used to sweep the removal areas after the rock pinnacles were lowered or demolished by blasting. These validation methods would be considered primitive by today’s standards.

With the potential for another extremely low-water period looming in late 2006, new state-of-the-art hydrographic multi-beam surveys were conducted and a number of rock pinnacles and rock outcroppings, which were missed by the rock removal efforts in 1988 though 1999, were detected. USACE felt these remnants that were missed during the previous blasting effort posed a potential safety hazard to commercial boat traffic, could result in a negative economic impact if navigation had to be reduced, and a threat to the environment if there were a towboat grounding that could result in a hazardous spill, during these low water periods.

In December of 2012, rock removal was again resumed in the vicinity of Thebes, Illinois. This area of the Mississippi River known as Thebes Gap contains unique geologic formations of rock, boulder, and bedrock, and is recognized as important habitat for several fishes. To minimize potential impacts to the fish community of Thebes Gap, and to the federally endangered pallid sturgeon, USACE coordinated rock removal techniques, timing, and monitoring with the Service's Ecological Services (ES) in the Marion, Illinois Sub-Office. Removal was to be conducted using a combination of blasting, grinding, and excavation techniques, and limited to time periods that would likely not affect migration or spawning of pallid sturgeon. Much of the rock was removed with mechanical techniques, using long reaching excavators, rock hammers, and rock grinders. Blasting was typically utilized on large rock formations or in some cases rock features too hard for the mechanical methods to budge.

To determine the actual impact to the fish community in the blasting areas, a final monitoring plan was developed in the field with USACE, Carterville Fish and Wildlife Conservation Office, Marion Ecological Service, and the Missouri Department of Conservation.  Ultimately, it was decided that Carterville FWCO would employ pre- and post-blast surveys in and around the rock pinnacles and outcroppings identified for removal with a Dual Frequency Identification Sonar (DIDSON) unit. In addition, DIDSON surveys of adjacent off-channel habitats were conducted for direct comparison of fish use and abundance during this mid-winter period. MDC deployed conical catch nets downstream of the blast sites, with nets being suspended at the surface and mid-water depths, as well as on the river bottom. In addition, Carterville FWCO, MDC, and USACE all provided spotter boats to search for moribund or stunned fishes floating downstream of the rock removal sites immediately after blasting was employed. In addition to our physical monitoring efforts, USACE worked to determine potential impacts of blast pressures on fish in the vicinity of blast sites using pressure transducers. Underwater pressures of multiple blasts were measured and used to calculate potential fish mortality radii using existing models.

The near-video-quality imaging provided by the DIDSON in the turbulent and turbid Mississippi River waters was valuable in identifying the rock formations and fish abundance, as well as relating some fish behavior to the rock features. Upon preliminary review, fish use of the rock features that were removed was very minimal. The removal areas were largely in or on the edge of the navigation channel where velocities were high. The combination of swift flows and the cold winter water temperatures likely made these areas less appealing to the fish community. This was further illustrated by the DIDSON footage taken in the adjacent off-channel habitats. Fishes were teaming in many of these slack water areas, with large schools orienting to the same type of rock features as were observed in the main channel, where fishes were absent.

This was good news for the fish, because once Carterville FWCO finished collecting footage at the blast sites, the drilling/blasting barge was moved over the rock pinnacles and preparation began to remove or lower the formation. Bore holes were drilled and the drill holes loaded with explosives.

While the blast site was being prepared, MDC hurriedly deployed the conical nets in an array downstream from the site. The Coast Guard boats set up a perimeter around the blast site, and the spotter boats moved into position downstream of the barge. A lone buoy would be dropped off the side of the barge indicating that the blast charge was in place, and a five minute warning was given over the marine band radio, along with three blasts of the tow boat’s horn. Then we waited. What seemed like an eternity later, a 90 second warning was given, and everyone eagerly watched the blast site. From our position outside the Coast Guard perimeter, you could hear an audible “crack,” then a “ping” would echo through the aluminum boat hull, and the water spout would appear. Depending on the depth of the blasting, the water spouts varied in size from a small splash to 10 to 20 feet across, and several feet high.

Spotter boats began scanning the area downstream of the blast. Sometimes the spotters worked in concentric circles when few boats were available, or they would maintain a line across the river when there were numerous boats filled with eager biologists. After a long thorough search, the conical nets were retrieved.

On two particular days, we monitored seven blasts with various combinations of spotter boats, DIDSON, and conical nets. In all, three fish were captured - one moribund silver carp captured by spotter boats, which ppeared to be injured by a propeller strike, not a blast, one dead gizzard shad captured by conical net, and one lively channel cat captured by conical net.

Overall, the monitoring crew felt the impacts were minimized by conducting the blasting during midwinter months. In addition, the noisy methods required to prepare the blast sites would likely also contribute to moving fish out of the blasting area.

Once USACE has completed the removal of the remnant rock pinnacles and rock outcroppings, the navigation channel should be able to remain open, even during drought years when river levels are approaching record lows. Hydrographic multi-beam surveys are being conducted in the removal areas after blasting and excavation to ensure that minimum navigation channel dimensions will be met at low flows. Hopefully, the blasting barges will never have to return to the Middle Mississippi River, and the only explosions the fish will hear are a few river rats celebrating the 4th of July.

Donovan Henry

Contact Info: Jeff Stewart, 618-997-6869 ex11, jeff_stewart@fws.gov