DENVER, Feb. 17 (UPI) -- A large aquifer that supplies Denver's water was made possible by geological forces at work in the region millions of years ago -- including the brief appearance of a tropical rainforest -- researchers reported Monday.
The rainforest spread along the front of the then-upwelling Rocky Mountains, giving rise to the most diverse plant variety known among North American fossil sites, and "arguably the oldest one known on Earth," said Kirk Johnson, curator of paleontology at the Denver Museum of Nature and Science.
At the annual meeting of the American Association for the Advancement of Science, Johnson described how he and fellow researchers mapped an alluvial fan in the Denver area. An alluvial fan is a type of flat basin that collects the flow from streams.
The fan discovered by Johnson and colleagues spreads out from an ancient canyon that formed as the Rockies rose at about the end of the Cretaceous period about 65 million years ago -- after the disappearance of the dinosaurs. Beneath the fan are the remnants of the ancient rainforest, located at depths of several hundred to 1,000 feet, which created an aquifer that today holds a large quantity of groundwater.
As Colorado and the western United States suffer a fifth year of drought, this groundwater is being consumed by Denver's Front Range communities, which lie beneath the Rockies' eastern slopes.
"We are fairly confident that the water is not being recharged -- the water is being mined," Johnson told United Press International. "The water fell as rain about 18,000 years ago on the backs of mammoths and camels," animals that were common in the basin at that time.
During the life of the rainforest -- between 64 million and 63 million years ago -- research suggests rainfall levels were about 100 inches per year. The rainfall estimates are based on a "log of leaf size," Johnson said, so the potential error in the estimates is fairly large.
"In general," Johnson said, "larger leaves mean more rainfall."
Paleontologists have identified 170 species of broad-leafed and flowering plants, making it the largest diversity of ancient plants discovered in North America.
Rapid growth of the Denver metro area has led to an explosion of fossil finds. The rainforest fossils originally were discovered by state highway department paleontologist Steve Wallace in 1994.
Wallace told UPI he was intrigued when he discovered large fossil leaves of unknown species.
The most important of the bedrock aquifers is the Arapaho aquifer, which supplies groundwater to Douglas County south of Denver, one of America's five fastest growing counties during the 1990s, according to the 2000 U.S. Census.
Johnson said he and colleagues "have placed fossil sites and the aquifers into a stratigraphic framework based on an understanding that the basin was filling as the Rocky Mountains were uplifting.
"This understanding of ancient landscapes allows predictions about the location and amount of groundwater available to the expanding population," he said.
