Watertown Water Is Very Hard — Here's What It Costs You Each Year

Why Is Watertown Water So Hard?

Watertown pulls its drinking water from the Big Sioux River, and that river does the heavy lifting when it comes to hardness. The Big Sioux doesn’t flow through granite or volcanic rock. It drains a region of South Dakota and western Minnesota where the geology is dominated by glacial till and ancient limestone and dolomite deposits. As rain and snowmelt work their way through the soil and into the river, the water picks up carbon dioxide and turns mildly acidic. That weak carbonic acid then dissolves the calcium and magnesium carbonate in the underlying rock. The result is water that arrives at the tap carrying a heavy load of dissolved minerals.

At 13.4 GPG (228.9 ppm), Watertown’s water sits firmly in the “very hard” category. For context, the USGS classifies anything above 10.5 GPG as very hard, and the national average hovers around 7.3 GPG. So Watertown is running roughly double the national norm. The Big Sioux is a relatively shallow, fast-moving river, which means it has plenty of contact with the mineral-rich soil and rock along its course. That contact time is exactly what you need to produce hard water. The river isn’t just carrying suspended sediment; it’s carrying dissolved calcium and magnesium that won’t settle out no matter how long you let it sit.

There’s another layer to this. The glacial history of eastern South Dakota left behind thick deposits of ground-up limestone and shale. Those deposits act like a slow-release mineral bank. Every time the river rises and cuts into new banks, or every time groundwater seeps into the riverbed, more dissolved solids enter the flow. You’re not imagining the film on your glassware or the scale on your faucets. That’s the Big Sioux doing what it has always done, carrying the dissolved remains of the region’s bedrock straight to your pipes.

How Watertown Compares to Nearby Cities

Watertown isn’t alone in dealing with hard water, and that’s no accident. Cities along the Big Sioux River and its tributaries draw from the same mineral-rich surface water, so they face similar challenges. The specific hardness numbers for neighboring communities aren’t published in the same way, but the geological story is consistent across the region. If you’re filling a glass in Castlewood or Estelline, you’re drinking water that has traveled through the same glacial till and limestone formations that make Watertown’s water so hard.

The bigger contrast comes when you look west. Cities that pull from the Missouri River or from deeper wells in the western part of the state often see different hardness profiles, sometimes lower, sometimes higher, depending on exactly which aquifer or river stretch they tap. But for the immediate Watertown area, the Big Sioux watershed is the common thread. The river’s hardness doesn’t drop dramatically from one town to the next because the geology doesn’t change at city limits. The mineral content is a function of the landscape, not the municipal boundary.

One thing that does separate Watertown from some neighbors is the treatment approach. The city’s water is treated to meet EPA safety standards, and hardness itself is not regulated as a health hazard. The EPA classifies hardness as an aesthetic issue only, covered under Secondary Maximum Contaminant Levels. That means the minerals stay in the water unless the utility actively softens it, and most utilities in this region don’t bother because hardness isn’t a safety concern. So the water that comes out of your tap in Watertown is essentially as hard as the river delivers it, minus whatever treatment the city applies for other contaminants like arsenic and disinfection byproducts.

According to the Water Quality Association, hard water carries a real cost in energy, appliance wear, and cleaning products. Battelle Memorial Institute research found that gas water heaters lose roughly 4% efficiency for every 5 GPG of hardness, and the DOE notes that just a quarter inch of scale can cut water heater efficiency by up to 40%. At 13.4 GPG, Watertown homes are on the steep end of that curve. The WQA’s industry estimate suggests the annual cost scales with your actual hardness, so plug your numbers into the calculator on this page to see what it means for your household. The short version is that Watertown’s water is hard, it’s hard for a reason, and that reason is written into the region’s geology.

How does Watertown stack up against the rest of South Dakota? It's very hard at 13.4 GPG — Sioux Falls sits nearby. See the full hardest-water ranking for context.

See where Watertown sits on the South Dakota water hardness map →

Want the broader explainer on what hardness actually is and how it behaves? Read hard water vs. soft water, explained.