Nov. 14, 2000 Reno, Nev. -- Major cycles in the size and depth of Utah's Great Salt Lake are known from as far back as the 19th century, but now a Penn State researcher suggests an explanation for the seemingly odd behavior of the lake.
"In the 1980s, the Great Salt Lake was very high," said Dr. Christopher J. Duffy, associate professor of civil and environmental engineering. "Twenty years earlier, in the 1960s, the lake was so low that there was talk of it drying up."
It appears that the long-term fluctuations of the Great Salt Lake do not directly match the fluctuations of the rainfall and snowfall, since rain and snow in the Wasatch Mountains move rapidly downhill to the lake each season.
However, the Great Salt Lake rises and falls over time scales of decades. According to Duffy, to explain how the mountain-front stores the water, is to explain the cycles in the lake.
"At the highest part of the Wasatch Mountains, runoff from rain and snow forms perennial streams with little storage underground and rapid downhill movement," Duffy told attendees today (Nov.13) at the annual meeting of the Geological Society of America in Reno, Nev.
The middle and lower parts of the mountain slope occupy what hydrologists call the "losing stream zone." This is where topology and geology interfere with simple gravity-driven runoff.
When streams cross the losing stream zone, the fractured bedrock and deep alluvial deposits can store significant amounts of water, carrying stream water underground into deep reservoirs. These groundwater reservoirs have a long residence time before the water re-emerges and flows into the lake.
"The huge underground reservoir smooths out the seasonal variations of climate signal, leaving only the long-term cycles, on an approximate 11- to 22-year period," says Duffy. A second groundwater effect that enhances the long-term cycles of the lake has to do with the fluctuating position of the water table relative to the stream level. High water table conditions limit the storage of stream low by groundwater and flooding may result. Low water table conditions allow the groundwater reservoir to temporarily store part of the flood. During drought years, even large amounts of rain may not cause an increase in lake levels until the underground reservoirs are fully recharged.
"The long cycle of filling and draining mountain-front aquifers is in response to very weak cycles in the climate record, where groundwater acts as a noise filter and amplifier," says Duffy.
The Penn State researcher has developed a dynamical model of this behavior, which seems to explain how low-frequency cycles dominate the Great Salt Lake level. He hopes to look at the tree ring data, a record of rainfall and the water table, to correlate with his findings.
Further information is available at http//www.cee.psu.edu/dynsys/
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