Journal Staff Report

Situated at the bottom of Morefield canyon deep within Mesa Verde National Park lies an ancient water diversion project built and operated by the ancestral Puebloans, researchers say. First analyzed in 1967, the strange mound raised the curiosity of a team of archaeologists, engineers, and geologists who organized a two-year project to study it in 1995, discovering in the process evidence of a 1200 year-old-dam.

"It’s called paleo-hydrology, or the study of ancient dams," said Erik Bikis in a presentation at the annual meeting of Colorado Water Officials in Durango last month. "We found that the Morefield Mound was an active reservoir at the turn of the last millennium, not a ceremonial platform or geologic anomaly. It was operated by the Ancestral Puebloans for around 350 years and probably supported a population of around 500 people. We know that the society in Morefield canyon was great because nearby is the largest kiva ever found at Mesa Verde."

Bikis, of Wright Water Engineers, the firm conducting the study, relayed the story of the mound and explained the results of the project. The way the dam was built and the problems of sediment build up are the same problems water developers face today, Bikis said, "Water use and management problems were similar, as well as the successes."

The problem of sediment build-up was a large one for the 200-foot earthen dam that blocked Morefield Creek during wetter years. It rises 16 feet above the valley bottom with a 1400 foot "handle" stretching up the valley. In fact, because the mound was so high off of the valley floor, some scientists in 1967 dismissed claims that it could be a reservoir since there would be no way for the water to flow uphill into the basin.

"They did not have pumps back then," Bikis said. "We wanted to solve this enigma, and we found that it was an ancient dam used for domestic water purposes. But, it was not large enough to support agriculture."

What puzzled engineers and archaeologists was the mounded handle that protruded from the mound like a long finger stretching uphill parallel to the stream. The answer turned out to be sediment build-up, which eventually filled up the reservoir, finally rendering it useless. Ancestral Puebloan water engineers overcame the problem by continuously moving the intake channel further and further upstream in order to counter the rising build-up of sediment, thus allowing water to continuously flow downhill into the reservoir.

Excavators, with permission from Mesa Verde, cut a 140-foot-long trench, 16 feet deep through the mound in 1997, which was stepped back gradually, marked and surveyed. What they found was layers of different sandy soils that revealed, potsherds, and debris that coincided with different time periods when subjected to carbon dating, pollen studies, and soil analysis.

Doug Ramsey, a soil expert with the Natural Conservation Resource Service office in Cortez, conducted studies of soil samples taken from different levels within the mound. He found iron staining in the soil, evidence of oxygen deprived soils being saturated for long periods of time, such as in a reservoir, Bradley concluded.

Other proof found in the mound by Bradley and others were the existence of ripple marks on sandstone rock caused by wind and water, ancient charcoal from slash and burn agricultural practices that washed into the basin from upstream, other organic debris consistent with long-ago flooding, and evidence of berms.

The reservoir was initially constructed "on-channel," and then was moved off-channel over time. Researchers documented that as the man-made lake began to fill up with debris and silt it was laboriously cleaned out by hand, with sediment dumped over the side. Seven reservoir stages, or cleanings, were recorded, Bikis said, with slag piled up as berms.

"But when the reservoir was cleaned, which must have been a very difficult, time-consuming job for those people, not all of the sediment was disposed over the side. Suddenly the bottom of the reservoir is higher than it was originally, stopping water from flowing in via gravity," Bikis said.

"So they began to construct a feeder canal just as any irrigator today would do. They continued to use the diversion channel but as they did they were unwisely farming upstream and cutting timber for wood in their houses."

This poor land-use practice denuded the land and destroyed its natural filtration properties, which contributed to more and more erosion, Bikis explained. Powered by flooding and storms, the sediment kept filling up the lake. As it was filled and then not cleaned out sufficiently, the bottom rose, and the intake canal had to continuously be moved further upstream.

"When it was all said and done, the intake canal measured 1400 feet long, with a mound that measured 16 feet high. We were also able to determine, by excavating the mound trench to the original stream bed of Hesperus soil that when first damned, there was a depression here to catch water," Bikis said.

In conclusion the study found that ancient cultures used creative water engineering tactics to store water for domestic use, and dealt with similar problems that plague our reservoirs today, mainly sediment build-up. But it also revealed that those practices produced irreversible damage to their surrounding, possibly contributing to the Ancestral Puebloan mysterious demise.