ST. GEORGE — After the volley of rainstorms Southern Utah has experienced, has it changed the water levels of the local reservoirs? The answer, the county’s top water manager says, can be “complex.”
“It comes up a lot,” Zach Renstrom, the general manager of the Washington County Water Conservancy District, said of people asking him about reservoir water levels following heaving rains or flooding on the Virgin River.
“It’s a very, very complex and dynamic system that we have,” he said while sitting down to speak with St. George News on Tuesday afternoon.
Particular factors come into play on whether recent storms will add more water into the county reservoirs and just when a rise in water levels may be noticed.
Currently, there is a “noticeable difference” for reservoirs like Gunlock that are built “on-stream,” Renstrom said. Gunlock and Baker reservoirs are built on the path of the Santa Clara River, so people have commented on seeing some rise there corresponding with increases in the river following the rain.
Conversely, people aren’t likely to notice any major difference at the county’s two largest reservoirs – Quail Creek and Sand Hollow – until the end of May, Renstrom said. This is due to how the reservoirs were built, how they receive water and how they are operated, he said.
Both reservoirs were built “off-stream” and require diversions that pump water through a pipe system to them. The delivery capacity of these pipes also is limited due to their size, which dictates how fast a reservoir may be refilled.
“We are pumping (water) into those reservoirs, but they come up very, very slowly over a multi-month period,” Renstrom said. “For Quail and Sand Hollow reservoirs, you’ll never see a dramatic change (following a storm). It’s a slow process. Come back by the end of May and you’ll see an increase.”
However, the possibility that there will be more water in the reservoirs from the storms is also subject to change and circumstance. As previously reported, the snowpack southwest Utah currently enjoys – which stands at 238% of normal as of Thursday – could be significantly diminished or disappear entirely if the weather warms up before April and May when snowpack levels peak and begin to melt into runoff.
“It’s crucial that our current snowpack comes off very slowly,” Renstrom said. “If it melts rapidly, we’ll have little to nothing to refill the reservoirs.”
Renstrom’s explanation referred back to the pumps and capacity of the pipes feeding Quail Creek and Sand Hollow. If there is a slow, consistent rate of runoff, then the water district will be able to capture the water needed to gradually refill the reservoirs. If there is a “big flooding event” caused by rapid runoff instead, then there’s nothing the water district can collect.
With a heavy level of snowpack that hasn’t been seen in years, the water district and local authorities are concerned that it may lead to a major flooding event similar to what the county experienced along the Santa Clara River in 2005.
While some people may see flooding as beneficial to the local water supply, it is quite the opposite, Renstrom said.
When a major flood hits due to rapidly-melting snowpack, the water that runs off the mountains into the valleys and water bodies below becomes a mix of mud and debris it picks up along the way down. When this mix hits the Virgin River, the water district closes off the diversion dams. This is done so the flood waters don’t get into the reservoirs where the quality of the water can be impacted and silt and other debris can clog pipelines.
“The (Virgin) river has to be in a small parameter for us to be able to capture that water,” Renstrom said.
The ever-present threat that the snowpack won’t last till the peak runoff period is why the water district continues to push for water conservation during the winter season when not as much water is otherwise used.
Other factors that play into reservoir water levels involve how they are managed, Renstrom said, still referring to Quail Creek and Sand Hollow, the county’s principal sources of drinking-quality, or culinary water.
Each reservoir and their accompanying dams “have personalities” that water managers need to become familiar with in order to determine the best way to use them, Renstrom said.
“You’ll tweak them throughout the years and see what works,” he added.
In the case of the two reservoirs, the water district has made a practice of bringing up water levels for Sand Hollow first while allowing water levels in Quail Creek to drop in order to accomplish this. The district also likes to keep more water in Sand Hollow longer so it can seep into the sandstone and recharge the aquifer.
The process then switches as the year progresses and allows levels to drop at Sand Hollow while Quail Creek is replenished. This process also appears to have an impact on the quality of drinking water produced by the water district.
“With that (practice) we get more water and quality of water,” Renstrom said.
According to the water district’s website, Quail Creek currently sits at 61% capacity, while Sand Hollow sits at 75% capacity. While these numbers have fluctuated somewhat through the previous year, they also have remained relatively stable.
Overall storage capacity for all of Washington County’s reservoirs sits at 48%.
Photo Gallery
At Quail Creek Reservoir, Hurricane, Utah, Jan. 25, 2023 | Photo by Mori Kessler, St. George News
A view of a snow-covered Pine Valley Mountain from Sand Hollow Reservoir, Hurricane, Utah, Jan. 25, 2023 | Photo by Mori Kessler, St. George News
A shot of some of the pipes that feed water into Sand Hollow Reservoir, Hurricane, Utah, Jan. 25, 2023 | Photo by Mori Kessler, St. George News
At Quail Creek Reservoir, Hurricane, Utah, Jan. 25, 2023 | Photo by Mori Kessler, St. George News
At Sand Hollow Reservoir, Hurricane, Utah, Jan. 25, 2023 | Photo by Mori Kessler, St. George News
At Quail Creek Reservoir, Hurricane, Utah, Jan. 25, 2023 | Photo by Mori Kessler, St. George News
A view of a snow-covered Pine Valley Mountain from Sand Hollow Reservoir, Hurricane, Utah, Jan. 25, 2023 | Photo by Mori Kessler, St. George News
At Quail Creek Reservoir, Hurricane, Utah, Jan. 25, 2023 | Photo by Mori Kessler, St. George New
At Sand Hollow Reservoir, Hurricane, Utah, Jan. 25, 2023 | Photo by Mori Kessler, St. George News
At Quail Creek Reservoir, Hurricane, Utah, Jan. 25, 2023 | Photo by Mori Kessler, St. George News
Zach Renstrom, general manager of the Washington County Water Conservancy District, speaks on how people may not see a dramatic difference in water levels at some of the county’s reservoirs until the snowpack in the mountains melts into runoff, St. George, Utah, Jan. 25, 2023 | Photo by Mori Kessler, St. George News
At Sand Hollow Reservoir, Hurricane, Utah, Jan. 25, 2023 | Photo by Mori Kessler, St. George News
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