Posted on March 4, 2019
Ryan Hermens, Journal Staff
After an attempted dredging of Rapid City’s Canyon Lake was thwarted several years ago, another attempt will be made this spring.
This time, the South Dakota Department of Game, Fish and Parks will apply a newly purchased floating dredge to the task.
Officials had hoped to dredge Canyon Lake while it was drained in late 2014 and early 2015, when the spillway was being reconstructed. That plan included driving heavy equipment onto the lake bed to scrape out the built-up sediment.
But higher-than-expected inflows overwhelmed a diversion pipe, and the lake bed remained too wet for the dredging work.
The GF&P has since purchased a floating dredge, which will make its maiden voyage on Canyon Lake. John Carreiro, of the GF&P’s Rapid City office, hopes to begin the dredging in April and said it could last several months.
The total cost to acquire the dredge and its associated equipment and supplies was $273,740. Of that amount, $100,000 was supplied by a grant from the state Department of Environment and Natural Resources, $150,000 was contributed by the West Dakota Water Development District, $13,740 came from the city of Rapid City, and $10,000 was provided by Black Hills Fly Fishers.
The dredge looks like a small barge. It’s a DinoSix model, made by GeoForm International of Olathe, Kansas. The DinoSix is 21 feet long, about 6 feet wide, about 5 feet tall, weighs 3,800 pounds and floats on stainless steel pontoons. The spartan deck consists of guardrails, controls, a seat and a diesel engine.
A 5.5-foot-wide cutting head on the front of the dredge looks like a giant garden tiller. The cutting head can be lowered to a depth of 13 feet to churn up sludge and sediment, while a pump sucks the sediment up a metal tube and sends it through a length of portable pipe that runs back to the shore.
On the shore, the sediment is discharged into permeable tubes measuring roughly 100 feet by 6 feet. The tubes trap the sediment but allow water to filter and drain back into the lake. Carreiro said the tubes at Canyon Lake will be placed near the boat ramp.
When the tubes have been sufficiently de-watered, the sediment in the bags can be hauled away and dumped. Carreiro said those arrangements are still being made and the destination of the Canyon Lake sediment is not yet known.
Carreiro said the floating dredge is predicted to shave anywhere from a foot to several feet of sediment from various areas of the lake bottom, and the anticipated total harvest of sediment is 15,000 cubic yards. That’s the equivalent of up to 1,500 dump-truck loads, but it’s less than the more than 33,000 cubic yards that was expected to be dredged out of Canyon Lake while it was drained during the winter of 2014-2015.
Using a floating dredge instead of dredging a drained lake is a trade-off, Carreiro said. While a floating dredge will remove less sediment, it won’t require the removal or mass killing of fish, and public access to parts of the lake will be retained while the floating dredge is in use.
After this summer, the GF&P hopes to use the floating dredge on other small lakes throughout western South Dakota.
Source: rapidcityjournal.com
