MORMON LAKE AND THE POSSIBLE FUTURE CLIMATE OF THE SOUTHWEST COLORADO PLATEAU AND PLATEAU WOODLANDS

This research relates long-term changes of water level to the climate of the Mormon Lake region. Generally, changing lake levels are indicators of sustainable water and hydrologic change. Mormon Lake (ML; elev. 2,166 m; area 29 km² in a 105 km² basin) is in the ponderosa pine forest of north-central Arizona. The basin is closed without substantial diversion into or out of ML. Thus, climate—mostly precipitation and temperature—modulates lake level and shoreline elevation. In shallow lakes, like ML, temperature is the main control on water level as it exponentially affects atmospheric evaporative demand.

Elevation of prehistorical and historical shorelines, expressed as water depth, was measured with a survey-grade GPS receiver with vertical precision of 5–15 cm. Using these data, a lake-level chronology was developed ranging from prehistorical (> 0.25 to < 2–4? ka BP) to the present. Prehistorical water depths were 9–23 m, historical depths of the early 1900s were 5–8 m, and modern (post-1941) depths were 0 to < 3 m. The prehistorical level consists of < 30 cm of clay-rich lacustrine sediment typically deposited on benches beneath subvertical ledges formed on Pliocene basalt flows. The age of the prehistorical sediment is poorly constrained, although it is relatively young as suggested by the lack of soil, interbedded lightly weathered basalt clasts, and modest patination of basalt ledges. This high stage likely resulted from reduced temperature and perhaps increased moisture during a regional late Holocene neoglaciation that also affected San Francisco Mountain (elev. 3,854 m) 50 km north of ML. Historically, ML was highest from 1905–41. ML then was only 22% of its highest prehistorical level, even though the historical levels occurred during the Early 20th Century Pluvial, the wettest and coolest episode of the century. Frequent desiccation and low levels were typical of the modern era; the result of recurrent droughts and increasing temperatures averaging 1° C/100a since 1895, as demonstrated by analysis of the ML and regional instrumental records. This warming trend, which is coincident with global warming, essentially desiccated ML while promoting regional hydrologic drying. The ongoing drying of the SW Colorado Plateau and tree mortality in plateau woodlands will likely continue in the present warming climate.