A PALEOHYDROLOGICAL MODEL FOR THE CANYON OF THE ANCIENTS REGION, SOUTHWEST COLORADO, EXAMINING RELATIONSHIPS BETWEEN DRINKING WATER SUPPLIES AND ANASAZI SETTLEMENT PATTERNS FROM 600 TO 1300 AD

This paper discusses the development and testing of a paleohydrogeologic model for the Canyon of the Ancients Region, Southwest Colorado, to help understand and examine the relationship between prehistoric drinking water supplies and settlement patterns. The broad research perspective is to conceptualize and characterize the modern hydrogeologic system by: developing a solid hydrogeologic block model to visualize and analyze the 3-Dimensional framework of the ground water flow system; developing and testing numerical models to determine ground water flow paths and water quantities on a watershed-ground water basin scale; developing an interpretation of the paleohydrologic system based on paleoclimate data (tree ring record); and comparing paleohydrologic variations with prehistoric population shifts.

The study area has three classes of hydrologic systems: regional, subregional, and local. The N aquifer and lower Morrison Fm. sandstones are the regional aquifer with geographically limited, though constant, sources of prehistoric drinking water supplies, and are exposed in canyon bottoms or in the core of regional uplifts. By comparison, the Eolian/Dakota/Burro Canyon (D) aquifer is both subregional and local, based on topographic continuity, and was used extensively as a drinking water source by the prehistoric people.

Steady-state and transient (50- and 5- year intervals from 600 – 1300 AD) numerical simulations of the hydrologic system were conducted using the MODFLOW code. This model simulates, the dynamic Eolian/Dakota/Burro Canyon aquifer, with a constant hydraulic conductivity of 0.2 m per day, at a 200m by 200m model grid cell resolution. Results illustrate that time-lagged fluctuations of spring discharge occur in response to long term prehistoric precipitation variation. Also that the amplitude of spring discharge variations over time are related to the size of the ground water recharge catchment area as well as the landscape position of springs within the ground water flow system. Future work will entail refined calibration of spring discharge with cultural population distribution over time.