SALT LAKE CITY — A team of researchers used Juniper tree rings to chart the longest climate history in northern Utah to date, going back 1,200 years to map the wet periods and the droughts experienced by the headwaters of the Bear River.
What the study revealed is that the second wettest period happened on the Bear in the latter half of the 20th century, and four of its wettest years were from 1983 through 1986.
Researchers say the tree rings showed that none of the dry years for the Bear happened during the period of modern instrumental record keeping, from 1943 to 2010, and that should serve as a precautionary theme when it comes to charting future water supplies.
"The ability of local communities to work together to forestall drastic water shortages is reassuring as they are likely to be challenged with much more substantial droughts in the future," the findings warn.
The new study, called "A millennium-length reconstruction of Bear River stream flow, Utah." was published in the Elsevier Journal of Hydrology. It was led by Robert "Justin" DeRose of the Forest Service's Rocky Mountain Research Station and included scientists from Brigham Young and Utah State universities.
Meandering 491 miles, the Bear River begins in Utah and ends just 90 miles from where it started out, concluding an inverted U-shape after passing into Idaho and Wyoming.
The Bear, the largest river in North America that ultimately does not reach the sea, is the biggest tributary of the Great Salt Lake and a signficant source of water for the growing Wasatch Front in northern Utah.
Under the Bear River Compact, Utah gets 275,000 acre-feet of water from the Bear below Bear Lake, of which 220,000 has been marked for development of water supplies in Cache and Bear river valleys, and Weber, Davis and Salt Lake counties.
"I think what this shows is that we have a huge variability in our water supply and we will continue to have it," said Todd Adams, deputy director of the Utah Division of Water Resources.
The study marks the first time tree rings from the Utah juniper were used, and it is the largest in its scope of time, going back 1,200 years to give the fullest picture to date of the climactic history of the northern Utah region.
Researchers collected core samples and cross sections of both dead and living Utah juniper trees to chart droughts and wet periods, finding that the most prolonged drought lasted 70 years from 1210 to 1281. The year 2000 marked the end of a 39-year wet period, according to the study.
The study points out that it is noteworthy for water managers to be aware that the fourth and fifth most extreme wet periods occurred during the modern instrumentation period, extending from 1968 to 1975 and from 1981 to 1987, with the latter causing widespread flooding of the Great Salt Lake.
The study notes that the latter half of the 20th century was the second wettest 50 years in the 1,200-year study period.
"The past century has been relatively wet," said researcher Simon Wang, with Utah State University and assistant director of the Utah Climate Center.
Wang noted that the serious droughts delineated by the tree rings point to recurrence on some scale in the future, but the question will be when.
"If it has happened before, there is a good chance it will happen again, but can we predict when?"
Wang says he fears that the Bear, in extreme conditions, will be challenged by the many water users along the system by the time it ends up in Utah.
"If I am a water manager, how am I to cope with that?"
Tage Flint, manager of the Weber Basin Water Conservancy District and a member of Gov. Gary Herbert's water advisory team, said managers need to build that variability into their water budgets, and use all available information at hand, including studies like these.
"We are constantly trying to change and react to what we know now. Any time we get new studies like this we try to factor that in. It becomes a balancing act as to how much drought we plan for in the future, how much we plan for to sustain and what are the reliable supplies through those periods," he said.
The study warns that banking on water from the Bear for future development along the Wasatch Front — coupled with a changing climate altering precipitation patterns — could present a "pressing" challenge to water managers in the future.
Adams, like Flint, stressed that counting on variability has to be built into water resource plans.
"The difference between the top and the bottom is what we have to plan for," he said.