Lake levels have taken a noticeable dip this year.
As of Wednesday, Dorena Lake was reported holding 37 percent of its maximum conservation storage for summer with Cottage Grove Lake stood at 27 percent.
“I’ve worked here for 16 years, and this is the lowest I’ve ever seen it this time of year,” said U.S. Army Corps of Engineers Park Ranger Christie Johnson.
Johnson noted that this is also the first year she has seen the Wilson Creek boat ramp at Cottage Grove Lake closed at the start of the season.
“And my supervisor has worked here even longer than me, maybe 20 years. And she’s never seen that happen,” she said. “So, it’s pretty historic.”
This year, lake levels dipped lower than levels of years past starting around the beginning of April.
The Army Corps of Engineer’s “water control diagram” sets the ideal water levels throughout the year.
From mid-May to the beginning of September, water is to be maintained at the lakes’ highest conservation levels. Fall requires a gradual drop and, through the winter, water levels are maintained at their lowest point. On Feb. 1, lake levels are allowed to begin climbing back along a curve up to the maximum pool.
This year, both Cottage Grove and Dorena lakes were able to follow the refill curve back up until April when each fell short and continued to decline.
Congressional edict sets the water management requirements of dammed lakes.
“And that’s why when sometimes people say, ‘Well, why don’t you fill it earlier?’ or, ‘Let it be full later,’ we will say, ‘Well, this literally would take an act of Congress to change the way we operate the dam, because that’s how they were authorized to be operated,’” said Johnson.
Tom Conning, a public affairs specialist for the Portland district of U.S. Army Corps of Engineers, further pointed out that recent weather history illustrates why these curves were designed the way they are.
In 2019, for instance, there was an April rainstorm. April stands at a point where the recommended levels are still rising on the water control curve.
“If we had adjusted because we thought, ‘Oh, it’s looking dry, let’s just refill and not release any water,’ then we would have had that storm event come in and we wouldn’t have been able to reduce some of the downstream flooding from that,” he said. “So, there’s a recent event that illustrates why these curves are in place.”
Still, reaching ideal levels each summer has been a challenge.
In the past seven years, Cottage Grove Reservoir has only filled to its recommended summer maximum pool twice — in 2017 and 2020.
And while the Army Corps of Engineers maintains lake levels by controlling outflow through its dams, the rate of outflow for both area lakes has been set to the minimum since late March — 75 cubic feet per second at Cottage Grove Lake and 100 for Dorena Lake, meaning water levels cannot be controlled any more than they already are.
Minimum outflows are set with downstream ecology in mind. Low water levels can negatively impact wildlife and water quality, a necessary concern especially considering Dorena Lake serves as source of water for Cottage Grove.
Policy restrictions aside, this year’s drought conditions are largely to blame for the low lake levels.
The Willamette Basin relies heavily on rainfall to refill its reservoirs. Dorena and Cottage Grove lakes are no exception and cannot depend on snow melt to meet desired levels.
“Snow is not a significant contributor for many of our reservoirs, but especially the ones in the Coast Fork watershed,” said Johnson.
For the rain-driven system, this year has produced exceptionally low levels of precipitation.
“We had some good rain in February, but then in March, it started tapering off,” said Johnson. “And so we really rely a lot on that inflow in April and May to get that final fill. And we didn’t get that.”
Conning added that fires can contribute to lower lake levels as well.
“Some of these watersheds, with these massive fires, are probably even worse in the sense that snow melt might not even make it to the reservoir for inflow just because it’s being absorbed more quickly,” he said. “So, there are a lot of other factors that contribute to that, too.”
As a broader trend, the nation’s continental West is overall getting warmer, seeing more varied rain events, experiencing a rise in the snowline and seeing earlier spring snow melts.
The National Oceanic and Atmospheric Administration’s National Centers for Environmental Information shows a marked rise in average summer high temperatures between 1991 and 2020. In the long term, annual average temperature trends in the region have also seen a gradual increase between 1895 and 2020.
With the heat, the area is getting drier as well.
Though drought recordings in Oregon dating back to 1895 show that swings between dry and wet periods are frequent, March 1 through May 31 this year marked the driest meteorological spring on record.
Over the past 45 years, many areas have had increases in the average time without precipitation events, which means not only drier summers, but during the other seasons as well.
Natural phenomena have had an effect this year, too. The relatively weak turnout this last season of the climate pattern La Nina, for example, impacted the lack of precipitation this year while a stronger presence may have meant a wetter period.
Johnson pointed out that the low lake levels could pose safety risks as well. Exposed shoreline forces people to walk out farther than they would normally in some areas and engage with some potentially hazardous topography.
“And when the water level is low, people in boats need to be more cautious about potential obstacles, things that they might not normally see this time of year, like trees down for a shallow area,” Johnson said. “Or there could be a little island exposed that’s not normally there, and people really need to watch out for that.”
Additionally, the public is encouraged to wear life jackets as there may be greater traffic density on lakes as they shrink.
Those looking to use boat ramps should plan ahead to make sure they are accessible.
Daily updates to the Willamette Basin water levels can be found at www.nwd-wc.usace.army.mil/nwp/teacup/willamette.
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