Although many archaeological sites are located along lakeshores across the country, little is known about how changes in water levels affect these sites. Jonathan Van Hoose, one of the District’s archaeologists, set out to change that.
The opportunity arose when the District was planning a deviation from the normal water control plan at Cochiti Lake: the temporary detention of water at the lake during the spring runoff to create a slightly higher peak spring runoff discharge downstream to facilitate spawning of the endangered Rio Grande silvery minnow and help maintain habitat for the endangered Southwestern willow flycatcher. The detention of water at Cochiti Lake – even though only for a short period – causes a rise in lake level, flooding shoreline areas and potentially impacting nearshore archaeological sites. Federal agencies are required by law to consider the effects of their actions on historic and cultural resources.
“There was a significant potential for adverse effects to 115 archaeological sites during such operations,” Van Hoose recalls, “To mitigate these impacts through surface collection and excavation at such a large number of sites would have been very costly. In addition, these activities would have told us nothing about how fluctuating reservoir levels affect archaeological sites.”
So in 2008, Van Hoose and former District archaeologist Lance Lundquist proposed an agreement with the State Historic Preservation Office with funding from the District’s Lake Operations Branch to study whether fluctuating lake levels redistribute artifacts along the shoreline by actually tracking the movement of artifacts at the lake over a five-year period under controlled conditions. But, along the way, they encountered their first hitch: they could not use real prehistoric artifacts in the study because to do so would require destroying archaeological sites to obtain the artifacts; losing some of those artifacts due to movement or burial; and potentially having the artifacts cluster downwind on the shoreline, creating a “new” site with real artifacts.
“To begin with,” Van Hoose explains, “we found a spot along the shoreline without a documented archaeological site. Then we created an artificial site using small aluminum disks as stand-ins for stone and ceramic artifacts. We picked aluminum because it is similar in density to stone tools and ceramic sherds,” the main artifact types found on archaeological sites. Items with similar densities and shapes are likely to be moved by water in similar ways, making aluminum disks a good analog for artifacts.
“The aluminum disks were each stamped with a unique number, and laid out in a series of four rows at our ‘site.’ The original location of each row was marked with a series of rebar stakes driven into the ground. Each row was roughly perpendicular to the shoreline, running from the average lake elevation to above the expected elevation of the lake during the deviation.”
In 2009, there was enough water in the river that a deviation action was not conducted; however, water was temporarily detained behind the dam in an emergency action to enable the retrieval of a drowning victim, resulting in a temporary lake rise of seven feet.
When Van Hoose and colleagues visited the site the following fall, the aluminum “artifacts” had not moved much. Some had been buried, and those close to the shore had moved a little bit. In 2010, though, a wetter winter allowed the District to retain more water at the lake to enable overbanking flows downstream. The lake temporarily rose 17 feet, completely inundating the site. No deviation occurred in 2011, due to drought.
According to Van Hoose, “When we went out in October 2011, it was very clear how high the lake had risen during the 2010 deviation. A line of driftwood and other objects had accumulated at the high-water mark.” At the site, some of the aluminum artifacts within five feet of the high-water mark, within the wave zone, had moved 15 to 20 inches from their original location; others could not be found at all, even with a metal detector.
“There was lots of disturbance, considering the site had only been in the wave zone for a month or two at most,” Van Hoose observed. Interestingly, artifacts lower on the landscape – between the ordinary lake elevation and the wave zone associated with the high water mark – were not affected, although inundated by the rising lake.
From his vantage point halfway through this study, Van Hoose thinks results will show that: a) Most of the impact to archaeological sites from reservoir operations will occur to artifacts and architectural features on the surface close to the shoreline. Buried materials are unlikely to be impacted. b) Most of the damage is a result of wave action when water levels are constant. Damage resulting from lake rise and fall is relatively minimal. c) Inundated archaeological sites lying below the wave zone are likely to be minimally affected by reservoir operations.
Van Hoose is looking forward to revisiting the site in mid-2012 and again in 2013, after the spring runoff. In 2013, he wants to relocate every one of the original artifacts to see how far it has moved or how deeply it has been buried. He is confident that “by nailing down the impacts of lake levels on surface artifacts, the study will help the Corps meet the federal mandate to properly consider how our actions affect archaeological sites.” Van Hoose is looking forward to turning on the metal detector and listening for the satisfying beep that indicates another artifact has been found.