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All images and underlined text are links to more information.

1. Backhoe dredger of the kind that has been used on the lower Minnesota River.

2. This grab dredger employs clamshell buckets of the kind that are sometimes used in river dredging. In fact, the contractor may use whatever equipment it prefers; the COE's only concern is that it dredge safely and meet the minimum production rate.

3. Another view of a clamshell bucket rig, this from St. Paul harbor dredging.

4. This 26-foot COE launch takes soundings to measure river depth.

5. What are some beneficial uses of dredged materials? (They don't just dump it, do they?)

6. Which Lower Minnesota River cuts require dredging?

the corps of engineers perspective:
what is dredging and what is dredging material?

From his office in Fountain City,Wisconsin, a few miles north of Winona on the Mississippi’s Wisconsin side, Steve Tapp, Channel Maintenance Coordinator for the Channels and Harbors Project Office (see photo below), watches over dredging operations for his district, which reaches from the head of navigation on the Mississippi in Minneapolis to Lock 10 at Gutenberg ,Iowa, a 250-mile stretch. And if you ask, he’ll tell you just about everything you wanted to know about river dredging.

What is dredging?

First of all, what exactly is dredging, and how does it work?

The COE, explains Tapp, hires private dredging contractors, who mount mechanical backhoes on barges. The hoes scoop out buckets of sand and related materials from the river channel and deposit them in the barges. Once full, the barges move to the dredge materials deposit site, and other backhoes, on shore, unload the material. On-shore bulldozers then distribute the material over the sites.

What is dredging material?

A second question — what is “dredging material” and why does it need to be dredged — yields the following answers.

Tapp explains that on the Mississippi the material is “coarse sand in some places, gravel in others, but mostly it’s a medium grade sand.” On the Minnesota, the material tends to be “a finer sand with some silt mixed in.”

If the average dredging operation yields, say, 20,000 cubic yards of material … and a dump truck holds roughly 10 cubic yards, then an average dredging equals 2,000 truckloads of sand. And, if an average barge holds 200 cubic yards, that’s 100 barges’ worth of material that gets delivered per operation.

By any measure, a whole lot of material needs to be disposed of, eventually. For it can’t keep piling up indefinitely in a finite space.

The problem with the materials is their sheer volume, not their quality or usability. Once the material is dredged, “our goal is to get it on land,” Tapp says, “where there’s a good beneficial use so people can come in and take it out.” The local COE district has been very successful in finding beneficial uses for the material, people who want to come in and take it.       

How often must dredging be done?

A third question involves how often dredging is done, and where it’s done.

From year to year, Tapp acknowledges, only a handful of areas ever need to be dredged. Most of the channel stays open on its own. But certain “cuts,” as they are called, do historically collect sediments, “and then we have to dredge, typically every couple of years on the Minnesota.” 

How does the Corps know when it needs to dredge? Every year it surveys the last 14.7 miles of the lower Minn from Savage to the confluence. Soundings are taken, and when the depth in any area falls to less than 10.5 feet in the channel, then dredging operations are scheduled. 

The 2000 plan

The COE has been working with the District for many years, says Tapp, and there seems to be a struggle to find a suitable site for all this material. For whatever reasons, sites that look promising at first turn up not to work out.

As its October 2000 draft of a Dredged Material Management Plan spells out, the Corps needs to find sites close to its historical dredging locations, because barging the materials more than four miles from a cut means extra charges by the contractor. Until the District and the COE can find a location within four miles of the confluence of the Minnesota with the Mississippi , material from that stretch will be barged down to the nearest site on the Mississippi River, a distance of approximately eight miles.