Study finds Clear Fork Reservoir dependable source of drinking water for next 50 years

MANSFIELD — The 77-year-old Clear Fork Reservoir should remain a dependable source of drinking water in Mansfield for at least the next 50 years, according to a recent sediment survey done by Stantec Consulting Services.

City officials believe that’s a good thing, especially considering the staggeringly high costs of dredging, the painfully slow permitting process needed and a dearth of grant funding available to help pay for it all.

That was the gist of the conversation Tuesday with Mansfield City Council, which met with Kyle Blakley, senior geotechnical engineer for Stantec, and city of Mansfield engineer Bob Bianchi, during a public utilities committee meeting.

The process behind the proactive survey began in 2025 when the city signed a $149,799 contract with Stantec to do a bathymetric survey of the 4.4-billion gallon body of water that supplies 40 percent of the city’s water needs each day. The remaining 60 percent come from a well system.

The bathymetric survey, sometimes referred to as a fathometric survey, is a type of hydrographic (water-based) survey that maps the depths and shapes of underwater terrain to illustrate the land that lies below. It covered the 997 surface acres of the lake.

Bathymetric surveyors worked to obtain an exact representative image of the bottom and identify specific elevations, anomalies, and the presence of any scour around submerged structures. They also analyzed and defineed the surrounding shorelines, tides, currents, and waves to create a better model of the area features overall.

Blakely said he saw no need to sound an alarm.

“Sedimentation is normal. It can’t really be avoided. It’s something that happens. It’s happening all across the country in these types of reservoirs and at the age of these types of reservoirs, it’s going to happen, … but unmanaged, it can reduce the volume of water available,” he said.

He told lawmakers large dredging projects can cost tens of millions of dollars and require five to 10 years of planning. He said the recently completed survey can help in that process. His recommendations touched on less intrusive, cheaper alternatives, none of which he recommended at the moment.

He said it appeared that about 5.5 million cubic yards of sediment have accumulated in the reservoir since it opened in 1949.

“That sounds like a big number, but it comes down to about 26 percent of the capacity over that 70-plus year period.

“This study showed that the reservoir still reliably meets the water demand today, so no immediate corrective action came out as a recommendation from the study,” Blakley said.

“The reservoir currently supplies about 4.5 million gallons per day of the city’s water supply. That’s about 40 percent of the supply. The other 60 percent comes from a well network.”

He said the survey found that based on current rates of sedimentation and water usage that the reservoir could easily function another 50 years, as long as intake structures remain operational, the water supply is reliable and the usage rate of 4.5 million gallons a day continues.

“Continued monitoring and planning around sedimentation is a good idea just as it continues, but no major sediment removal is urgently required at this time based on the results of our study,” Blakley said.

“If the demand were to increase, we also try to take a look at that too, knowing that as communities grow and developments happen and things like that, we wanted to look at that.”

(Below is a PDF of the presentation made to City Council on Tuesday evening.)

Stantec was the only company to submit “qualifications” for the reservoir projects, according to Bianchi.

“We have worked with Stantec before, as well. They helped us on the dam seepage project in 2023,” he said at the time.

Bianchi said the four goals of the study were to determine how much silt has collected, the composition of the silt, whether it makes sense to remove it, and the best method to use in removing it.

No harmful chemicals or materials were found in the silt during the survey.

Bianchi has said the silt enters the low-flow reservoir from streams and tributaries within a 35-mile drainage area.

“There is a lot of energy in the streams to carry that silt, but there is not as much energy in the reservoir, so the silt will drop to the bottom,” Bianchi has said.

Blakley said the reservoir “reliably refills annually.”

“So enough rainfall, snow melt, everything … to replenish based on the withdrawals and that evaporation that we found and currently the sedimentation level doesn’t impact the operation of the intake structure, sending the water to the plant,” Blakley said.

Continued economic development would lead to additional water usage, he said. Currently, the city consumes about 9.5 million gallons of water each day from the reservoir and wells system.

“One big key takeaway from that study for me was that the water demand has a greater influence on the supply reliability than does the sedimentation. So looking ahead future-planning wise, having a good understanding and an idea of what the water need is into the future will help drive some decision making on any kind of projects or things to engage moving forward there,” Blakley said.

“So while it’s important to focus and continue to focus on the sediment that accumulates, it’s more important to look at our demand. As economic development improves and the capacity is needed at the plant, that’s when we start to look at the reservoir again in terms of capacity and sediment,” he said.

Public Works Director Louis Andres said the city took a “proactive approach” with the study.

“This is something that needed to be done and this survey gives us a good baseline of where we’re at currently with our sedimentation. With our consultants and things, we’re going to have a better idea of where we’re at and where we can go in the future.

“If we had not done this, we would know none of this information. So this is a good baseline for us and as you can tell, we’re good for right now, but we need to be proactive and start planning because at some point we’re gonna have to get rid of the sediment to keep the capacity of the reservoir,” Andres said.

After the presentation during the public utilities committee meeting, environmental citizen groups led by Mansfield attorney Eric Miller asked lawmakers to consider the potential for Harmful Algae Blooms that they said could threaten the reservoir.

Such HABs have impacted the western basin of Lake Erie in Maumee Bay and have been a recurring problem at Grand Lake St. Marys in Auglaize/Mercer counties, impacting the water supply in Celina.

Miller helped lead the successful election effort behind the Water Main Initiative now replacing aging water mains around the city.

“We recognized the Water Main Initiative as a precursor to being able to address the reservoir. We’re very grateful that the administration jumped right on this and commissioned this study. It was certainly very thorough, 123-page study, and I got the opportunity to read it when it became available to the city,” Miller said.

The citizen group concerns are about the quality of the water, as much as they are the quantity.

“HABs is a term for a variety single-cell organisms that can poison a reservoir if they have water and nutrient conditions that are ideal for explosive growth,” Miller said. “Such events have already impacted other water systems in Ohio. They can escalate rapidly and resist treatment.”

The citizen groups asked the utilities committee to schedule a hearing to take testimony from experts regarding HABs to hear from citizens expressing views on the need for additional alternative measures to protect the city’s long-term water supply.