February 24, 2024 2:56 PM

How Do We Improve Watson Lake’s Water Quality and At What Cost?

Watson Lake is one of two reservoirs at the Granite Dells formed in the early 1900s when the Chino Valley Irrigation District built a dam on Granite Creek.  The City of Prescott bought the reservoir and surrounding land in 1997 to preserve it for recreational purposes as well as to capture surface water runoff from the local creeks for recharge purposes.

Effective management of Watson Lake is critical to maintaining the recreation and recharge benefits for all who enjoy the beautiful scenic views, recreational opportunities and trails surrounding the Lake.

In 2020, the City completed a comprehensive study of Watson Lake, “The 2020 Watson Lake Reservoir Management Plan”https://www.prescott-az.gov/wp-content/uploads/2020/12/Watson-Lake-Reservoir-Management-Plan.pdf.

Study Findings

The study found Watson Lake to be impaired with pollutants, including:

  • nitrogen
  • phosphorus

The study also identified a number of cost effective measures to improve water quality:

  • Lake aeration
  • herbivorous fish
  • nutrient binding

These measures are incorporated into the City’s fiscal year 2022 budget.

Lake Aeration

Lake aeration is not the typical golf course fountain that comes to mind most immediately.  In this setting several aerators would be located at the bottom of the lake and air would be pumped down to them.  As that air rises through the water column it adds oxygen to the lake and mixes with the lake water.  This oxygen increase greatly improves and expands the fish habitat, while simultaneously preventing the release of phosphorus and ammonia. These are both catalysts for algal blooms which alter the lake’s pH balance.

Preliminary estimates for design and installation: $125,000 – $250,000.

Herbivorous Fish

Adding herbivorous fish, also known as grass carp or White Amur, would help thin out the jungle of aquatic weeds found at Watson Lake.  This biological control method does not have the additional equipment or operational expenses associated with physical weed management.  The City will work with the Arizona Game and Fish Department to add fish to the lake while meeting the applicable state guidelines.

Preliminary cost estimates to add herbivorous fish: $35,000 – $70,000

Nutrient Binding 

Lanthanum impregnated clay is applied to the lake and binds with the phosphorus which then falls to the bottom where it is no longer biologically available.  Phosphorus is considered to be the catalyst for algal blooms (the green icky stuff we see right now).  The costs associated with this are dependent on the water quality (primarily phosphorus loading).

Preliminary cost estimates to be in the range of $96,000 to $192,000.

Why Dredging is Not Recommended


A full dredging of the Lake comes with a big cost, approximately $17 million dollars.  A partial dredging of the lake would be cheaper, but it would only expose an older layer of nutrient rich sediments and do very little to address the pollutants of concern, nitrogen and phosphorus.

As Residents, What Can We Do?

As residents we can help to improve the water quality of Watson & Willow Lakes by:

  • Maintaining septic systems with regular pumping and seasonal visual inspections,
  • Not over – fertilizing our lawns and gardens
  • Picking up after our pets (everywhere, nobody wants to see that!)

These all help reduce the flow of nutrients into the lake and extend the effective lifespan of the lake water quality improvements that the City is installing.

View the Watson Lake Reservoir Management Plan

Questions may be submitted to Matthew.Killeen@prescott-az.gov.


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