City says lake treatments have reduced harmful algae; more work planned at Willow and Goldwater

City of Prescott water managers told the Council Subcommittee on Water Issues on July 1 that recent treatments have reduced harmful algae and improved conditions at city lakes, but they said additional work is planned at Willow and the Goldwater lakes to address low oxygen, high pH and sediment inflows.

The update focused on results from a June 1 phosphorus‑binding treatment at Watson Lake, ongoing muck treatments at Upper Goldwater Lake and an emerging Willow Lake management plan. Jeremy Cordova, Environmental Project Manager for City of Prescott Public Works, summarized early monitoring, said the city removed more than 220 pounds of phosphorus from Watson Lake and reported a 45% phosphorus reduction and 25% clarity improvement at Upper Goldwater after a month of treatments.

The work matters, Cordova told the subcommittee, because the lakes are popular recreation and recharge features and because low dissolved oxygen and high pH can create conditions that trigger public‑health actions and reduce fish habitat. "We get a large bit of aquatic weed and harmful algae blooms," Cordova said, adding that those blooms can create body‑contact prohibitions and block recreational use.

Staff presented dissolved‑oxygen profiles from mid‑June showing that the deeper portions of three of the four city lakes fall below 65% saturation — conditions staff called "anoxic" and that they said could qualify as impairments. Cordova said Arizona Department of Environmental Quality (ADEQ) recently visited Watson following the June treatment and told staff the state does not plan further monitoring visits and had stepped down to city self‑reporting, a sign staff described as encouraging.

Managers described a mix of short‑ and long‑term tactics. In the short term the city has used products described as phosphorus binders and beneficial bacteria to digest bottom organics (muck) and reduce nutrients; Cordova said Upper Goldwater Lake treatments produced measurable phosphorus reductions and improved clarity. The city has also deployed solar‑powered circulation units ("circulators") and a remote‑controlled aquatic weed harvester. Public Works said the harvester removed about 1,000 pounds of weeds from the South Boat Launch at Watson during training runs, and that users reported immediate improvements.

For longer‑term control, staff described plans to construct a fish barrier so the city can stock grass carp (where permitted) in Watson and Willow to consume invasive vegetation, pending Arizona Game and Fish Department permitting and final barrier siting. Cordova said Game and Fish has identified a single suitable barrier location that could serve both Watson and Willow, reducing cost and design complexity. Staff also said they plan a larger sediment (bottom) treatment at Watson in October.

Council members and staff discussed monitoring and safety. When asked whether the circulators alone would be enough, Cordova said they appear helpful but are "more of a backup to the other treatments" and that manufacturer guidance and staff monitoring indicate it may take a season or two for circulators to show long‑term benefits. The subcommittee heard that aeration, sediment treatment and watershed stormwater work are complementary tools that together are expected to reduce the need for repeated large‑scale lake treatments.

Members urged public communication. One member asked staff to publicize harvest schedules and treatment results; staff said they would coordinate with Recreation Services and provide public updates (including planned July‑4 and August operations for the weed harvester). Cordova said the fisheries manager at Arizona Game and Fish has requested higher stocking levels at Goldwater lakes following the water improvements.

Next steps identified by staff include continued monthly water quality monitoring, an October sediment‑treatment campaign at Watson, continued weed harvesting through the summer, design and construction of a fish‑barrier site acceptable to Game and Fish, aerator upgrades at Upper Goldwater and implementation of natural sediment filters in lake drainages to reduce incoming nutrient loads. Staff said they will continue to coordinate with ADEQ and Game and Fish on permitting and monitoring.

City staff emphasized that while progress is evident, the lakes still show seasonal anoxic zones and that sustained watershed work and public cooperation will be needed to secure longer‑term improvements.