Updated November 1, 2026

Denver's water starts as snow on both sides of the Divide

Denver Water has been running the city's supply since 1918 and now serves about 1.5 million people in Denver and many surrounding suburbs. Almost all of it is mountain runoff, and that shapes everything from how soft it is to how the utility handles drought years.

Rivers, reservoirs and the trip down

Denver Water lists its sources as the upper South Platte River, the upper Blue River, Bear Creek, South Boulder Creek, Ralston Creek, tributaries of the Fraser River and the upper Williams Fork River. Several of those lie west of the Continental Divide. Water is stored in five mountain reservoirs: Antero, Eleven Mile Canyon, Cheesman, Dillon and Gross. It then moves through a network of streams, canals and pipes to three terminal reservoirs, Strontia Springs, Marston and Ralston, before treatment. The state's source water assessment for those terminal reservoirs lists the usual mountain and Front Range risks, including old mine sites, oil and gas wells, septic systems, roads, quarries and forest land. The assessment is a screening tool, not a finding that contamination has occurred.

Storage is growing. Denver Water is raising Gross Dam by 131 feet, which will roughly triple the reservoir's capacity.

Four plants, one usually idle

Treated water enters the system from the Foothills, Marston, Moffat and Northwater treatment plants. Moffat is the backup, used mainly in dry years, and that detail is behind the utility's only 2025 violation. The state requires synthetic organic compound sampling at Moffat in two quarters over a three-year period. Because the plant was offline for much of that time, only one set was collected. Denver Water reports that no synthetic organics have been detected at Moffat, and none at Northwater, which draws the same source water. It has since changed its scheduling to account for seasonal plant operation and planned outages.

The lead program and the numbers behind it

Denver Water has sampled homes with lead plumbing or service lines since 1992 under the federal Lead and Copper Rule. Only once, in 2012, did results trigger the need for more action. That history led to the Lead Reduction Program, which includes replacing customer-owned lead service lines. The 2025 tap results:

• January–June 2025: 90th percentile of 4 ppb across 337 homes, with 1 above the 15 ppb action level. The highest result was 126.1 ppb.
• July–December 2025: 90th percentile of 3.1 ppb across 245 homes, with 3 above the action level. The highest result was 98.8 ppb.

Those homes were chosen because they're the ones most likely to have lead. The utility offers free test kits. If yours comes back high, a filter certified to NSF/ANSI 53 for lead covers you until the line is replaced. Our filter selector can help you pick one.

Moderate minerals, almost no PFAS

Total hardness in the distribution system averaged 105 mg/L in 2025 (about 6.1 grains per gallon), with a range of 55 to 124 mg/L. Calcium averaged about 30 mg/L and magnesium 7.5 mg/L. Fluoride at the Foothills plant entry point averaged 0.61 mg/L (range 0.55–0.70). Disinfection byproducts stayed well under their limits: the highest locational running averages were 35.5 ppb for total trihalomethanes and 19.5 ppb for HAA5.

Under EPA's UCMR 5 program, all four plant entry points were tested for PFAS between 2023 and mid-2025, and every result was below the minimum reporting level. Lithium showed up at trace levels at two plants in 2023, averaging 5.3 ppb at Foothills and 9.2 ppb at Marston. For hardness at this level, a softener is a convenience rather than a must. Run your numbers in our softener sizing calculator before you buy.