
Brad Johnson
A proposed $28 million flood control project for Watertown that provides no protection for Lake Kampeska property is flawed and opens the lake to environmental damage, a new engineering analysis said.
Commissioned by the Lake Kampeska Water Project District (LKWPD), RESPEC, an engineering firm which has expertise in flood modeling, delivered a 10 page report to the U.S. Army Corps of Engineers (USACE) and the City of Watertown last week.
The Corps’ Tentatively Selected Plan (TSP) includes “a combination of seasonally lowering Lake Kampeska, constructing levees, and widening the Big Sioux River channel,” through Watertown, RESPEC noted.
The plan requires Lake Kampeska to drop about 9.6 inches in the fall from its full elevation. The plan is to remove a one foot by 12 feet section of stop logs in a downstream weir. That weir sets the lake’s full elevation, as required by state law. The corps said this was a “no-cost” plan component.
RESPEC said the Corps plan “as proposed is impracticable.” It further said, “no serious analysis of the potential for adverse water quality impacts to Lake Kampeska has been performed.”
RESPEC also said, “considerable costs are associated with this plan that have not been accounted for.”
Finally, it said the “assumptions used for hydrologic and hydraulic modeling appear to be flawed.”
RESPEC noted that the corps dismissed the Mahoney Creek Dam project, which for nearly 30 years was identified as the only plan that would solve all flooding for the lake and city. RESPEC said the corps was “placing particular emphasis on Tribal and related USACE Environmental Justice policies for justification” to ignore the dam project.
The Sisseton Wahpeton Oyate Tribal Council adopted a resolution adopting only the no action alternative which was sent to the corps on Nov. 8, 2023.
RESPEC wondered why manipulating Lake Kampeska is “not subject to similar policies regarding apparent Tribal opposition.”
According to RESPEC’s analysis, the Corps plan to lower the lake “has a success probability of only 30 to 40 percent.”
RESPEC’s analysis “suggest that the seasonal lowering of Lake Kampeska is impracticable as proposed (simply removing stop logs) and unable to reliably provide 3,300 acre-feet of additional flood storage from year to year as compared to existing conditions.”
If RESPEC’s report is correct, the basic foundation of the Corps’ plan is flawed.
RESPEC also said the Corps’ plan “provided little discussion on the potential adverse impacts that may result at Lake Kampeska” if the lake was artificially lowered.
It noted that the Corps said there would be “negligible effects to water quality,” although it has done no analysis to prove that statement.
RESPEC said that if the lake could be lowered, “any flow event of the Big Sioux River that overtops the sediment diversion weir has the potential to contribute greater pollutant load than without the lake lowered.”
The main pollutants threatening the lake include sediment, phosphorous, nitrogen and E. Coli.
At a minimum, the Corps should study the potential adverse water quality impacts and consider a mitigation plan. Costs also are associated with pollutant removal.
RESPEC further challenged the Corps “no cost” statement.
“Costs associated with amending or applying for a new flood control permit include administrative costs, additional engineering and attorney fees.” RESEPC also noted that the LKWPD “currently intends to contest this process for the TSP as proposed.”
The lake district’s has said that if the Corps is not going to reduce lake flooding, do not harm it by using it opening it to pollution.
Finally, RESPEC challenged the Corps’ modeling premise of creating simultaneous record flooding on the Big Sioux and Mud Creek resulting in a flood at Lake Kampeska that is nearly twice the size of the 1997 flood, which was the largest in the last 143 years.
RESPEC noted that “Mud Creek peak flow regularly occurs one day before the Big Sioux River peak flow.” It said the corps should consider history “to determine the appropriateness of modeling the Big Sioux River and Mud Creek with coincident 100-year peak flow events.” An offset in the timing of water flows “could reduce the required extent of flood mitigation measures.”
In other words, the Corps’ whole plan may be unreliable.
Brad Johnson is a Watertown real estate appraiser and long-time journalist. He is vice president of the Lake Kampeska Water Project District, president of the South Dakota Wildlife Federation and past president of the South Dakota Lakes and Streams Association.