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Wed, June 19

Full to the brim: Prescott Willow, Watson lakes overflow their spillways

Because of differences in the watersheds that feed Prescott’s two Granite Dells-area lakes, Willow Lake fills up less often than does nearby Watson Lake. But with the runoff from Prescott’s massive snowstorm on Feb. 21 to 23, Willow Lake began overflowing at its spillway on March 3, 2019. (Cindy Barks/Courier)

Because of differences in the watersheds that feed Prescott’s two Granite Dells-area lakes, Willow Lake fills up less often than does nearby Watson Lake. But with the runoff from Prescott’s massive snowstorm on Feb. 21 to 23, Willow Lake began overflowing at its spillway on March 3, 2019. (Cindy Barks/Courier)

The liquid dividends from February’s monster snowstorm keep streaming down Prescott’s creeks.

Fueled by the runoff from the more than 2 feet of snow that fell in Prescott Feb. 21 to 23, both Willow and Watson lakes began overflowing at their dam spillways by late February/early March.

And that has already started paying off for Prescott with recharge of the aquifer.

Prescott Public Works Director Craig Dotseth reported that Watson Lake topped its spillway and began overflowing on Feb. 28, while Willow’s dam began overflowing on March 3.

Because of earlier snowstorms that began with the New Year’s Eve storm on Dec. 31, 2018, the lakes already were fairly full by the Feb. 21 storm, Dotseth said.

His numbers show that both lakes rose by more than 8 feet during the months of January and February.

On Jan. 1, Willow was 8.9 feet below its spillway, while Watson was 8.44 feet below the top of its spillway.

“Watson was up pretty close even before (the Feb. 21 storm),” Dotseth said. By Feb. 1, the lake’s level stood at 4.57 feet below the spillway, and by Feb. 20 – just prior to the start of the big storm — it was 0.39 feet below the spillway.

Willow’s level was at 3.3 feet below the spillway on Feb. 20.

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The canyon below the Watson Lake Dam is currently roaring with water overflowing the spillway. City officials say Watson Lake began overflowing on Feb. 28, 2019. (Cindy Barks/Courier)

GROUNDWATER RECHARGE

Based on Prescott’s practice of using surface water from Willow and Watson lakes to augment its groundwater, water from both of the overflowing lakes is now being directed to the city’s recharge field near the airport.

Dotseth said the Salt River Project (SRP) notified the city on March 1 that the Phoenix-area utility company’s chain of Verde River-fed lakes was full and overflowing.

That triggered the city’s ability to begin recharging water into the aquifer, he said.

By the terms of a decades-old court stipulation that settled a legal dispute between SPR and the Chino Valley Irrigation District (the previous owners of Willow and Watson lakes), Prescott normally can take water from the lakes for recharge only from April through November.

But the stipulation allows the city to begin recharging sooner if SRP’s lakes are overflowing.

“SRP contacts us (if its lakes are overflowing) prior to the April 1 date,” Dotseth said. “They did contact us on March 1.”

Because there is plenty of flow over both dams, Dotseth said the city is not withdrawing any additional water from either of the lakes. Rather, it is directing some of the overflow to the city’s recharge field.

After receiving word from SRP, city wastewater workers opened a valve that diverts water from Granite Creek into an underground channel, and on to Prescott’s recharge field at the Airport Wastewater Treatment Plant.

There, the water is allowed to seep into the ground and replenish the aquifer. The recharged water results in water credits for the city, which can then be allocated to new development as “alternative water.”

Dotseth said about 4 million to 5 million gallons (12.25 acre-feet to 15.3 acre-feet) per day is currently being diverted to the recharge field.

WEATHER-DEPENDENT LEVELS

Depending on the levels of the lakes from April through November, city policy calls for withdrawing water from the lakes to augment its water recharge. The policy was set by the city in 1998 during the purchase of the lakes.

In the past, the city has withdrawn water from the lakes — mostly from Watson — throughout the spring, which tends to bring the lake levels down. The city typically halts its withdrawals in about May to allow for fish spawning to go on undisturbed.

It would be impossible to predict how long the lakes will continue to flow over the spillways, Dotseth said, noting that the levels are “completely dependent on the weather.”

Recreation Services Director Joe Baynes said the last time both Willow and Watson lakes were overflowing was in the winter/spring of 2017.

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