Morefield Mound

Morefield Mound sits in the middle of the wide valley at the bottom of Morefield Canyon in Mesa Verde National Park. It served as a water supply for ancient Native Americans a thousand years ago, making it one of the earliest known domestic water-supply works in the United States. The reservoir mound was one of four ancient water catchments at Mesa Verde National Park named collectively as a National Historic Civil Engineering Landmark by the American Society of Civil Engineers in 2004.

The reservoir mound has a diameter of 220 feet at its base, with side slopes of 3:1 (horizontal:vertical) that rise up sixteen feet to the flat top of the mound, which is 130 feet in diameter, like a truncated cone. The mound is made up of layers of water-deposited sediment―sandy layers interspersed with dense clay-silt layers. The original pond that the Native Americans excavated into the natural soils—now silted in—is located beneath the mound. A canal leading to the mound extended upstream for a quarter mile.

Morefield Reservoir began where the valley floor was 500 feet wide and the tributary drainage basin area was 4.1 square miles. Wright Paleohydrological Institute excavations showed that the pond was approximately four feet deep in about 750 CE, with a diameter of fifty feet. A seasonally high water table likely provided it with a variable water pool, even without any surface flow. However, because the pond was in the canyon bottom, all the runoff from a storm would flow into it along with any sediment; it would not have taken long for the pond to become silted. Cleaning out the pond with digging sticks and baskets was a labor intensive but necessary operation. Nevertheless, with each succeeding flood, the pond would again fill with fine sand, silt, and clay. Over the years, the pond rose in elevation due to accumulated sediment and, eventually, a canal became necessary to deliver water for storage.

At first, the canal was short. As the reservoir silt accumulated and the water level rose, the canal was raised and extended upstream. The canal banks were lined with shaped stones to help guard against erosion. As with other Stone Age civilizations, hammerstones were utilized effectively for stone shaping.

Research at Morefield Mound

The Wright Paleohydrological Institute performed research at Morefield Mound from 1994 to 1998, including the excavation of a trench to reveal sediment layers. Team member Dr. Jack Smith had also excavated at the site twenty years earlier. Smith was the chief archaeologist of Mesa Verde National Park and had studied extensively the Ancestral Pueblo people who lived there. Smith thought that the Morefield Mound was the remains of a reservoir, but he could not prove it. Other scientists speculated that the site may have been an ancient dance platform while some judged it to be a natural terrace deposit.

The Wright team conducted excavations that were deeper than Smith’s excavations in the 1970s. Evidence found in the exposed sediment layers proved that the mound had once been a reservoir and the original, undisturbed soil under the mound allowed the team to define dredged sand deposits and embankments above it. Buried in the deposits was an ancient digging tool dated to AD 860 by the University of Colorado Physics Department.

Study of the sediment layering and its characteristics provided evidence for environmental conditions, activities, and some of the problems faced by Ancestral Pueblo engineers during the building and operation of the Morefield Reservoir structures. Three continuous soil profiles were taken down the south trench wall. One complete profile was analyzed by the Natural Resources Conservation Service Field Office in Cortez for listing in its national database.

The soil profile data provided solid evidence that the reservoir was watertight because of the dense clays deposited in the reservoir―the type of clay that modern engineers might use for solid waste site designs to preclude leachate movement. The data also showed that over the life of the reservoir, about fourteen forest fires resulted in ash deposits, as evidenced by continuous thin layers of carbon about one-sixteenth of an inch thick. The sediments provided evidence of twenty-one periods of flooding in the canyon bottom, as indicated by thick, sandy sediment deposits, though many of the thickest deposits were, in turn, highly stratified, indicating successive independent inflows of high water. The shape of the layering told of reservoir cleaning operations, where the dredged sediment was cast to build berms—flat strips of land bordering a canal—or where it was wasted outside the berm. It was additionally noted that sometimes one part of the reservoir was used for storage while the other was not.

Because the excavated trench depth was limited to sixteen feet, due to the consolidation of the soil, the team was able to excavate only to the bottom of the mound, not to the original pond bottom. Then, by using a hand auger in the trench bottom, the team logged sediment deposits to an additional depth of five feet until the auger encountered the original natural soil. By exposing the natural, undisturbed soil surface in the west and east ends of the trench and having the pond bottom defined, the team could sketch the likely original shape of the excavated pond that lay under the sixteen-foot-high mound, for a total reservoir height of twenty-one feet.

Interpretation of Evidence

By studying the Morefield Mound excavation, the Wright team learned more about the ancient people of Mesa Verde and what they were doing in Morefield Canyon. The sediment deposits could be read like an open book because the evidence had not been disturbed during modern times. Findings indicated, for instance:

1. Morefield Reservoir began as a hand-dug pond in the canyon bottom to capture seasonal runoff. A later supply to the reservoir was surface water carried by a stone-lined canal. A sequence of canals was outlined in the sediment, one above the other.
2. Sediment from the upstream drainage basin was carried to Morefield Reservoir, sometimes at a high rate. Total volume of sediment carried into the reservoir was about 430,000 cubic feet (0.0067 acre-feet per square mile per year). Abandonment of the reservoir likely occurred when dredging became too inefficient or when the Morefield people began thinking about moving away to cliff dwellings.
3. The dredged sediment used for the Morefield dam embankments was a mixture of clay, silt, and fine sand, which created a nearly impervious berm area.
4. Based on potsherd analyses, Morefield Reservoir was used for approximately 350 years, during the 750–1100 CE period of the Pueblo I and Pueblo II people.
5. Prehistoric agricultural fields in the Morefield basin and occasional forest fires likely allowed enough runoff for Morefield Reservoir to store up to 120,000 gallons of water at one time.

The team also deduced that the ancient people of Mesa Verde were organized, industrious, and good water managers. The monumental task of building and maintaining the Morefield Reservoir could not have happened without the people’s diligence and organization.