Is Zeigler Reservoir Snowmass Co an old lake?

Ziegler Reservoir, located in Snowmass Village, Colorado, is the site of a geological anomaly located high above glacial valleys by 200 to 300 feet. The collection of water is situated on a plateau of Mancos shale just below the sub-alpine elevation of 8880. There are aspens to the west and scrub oak and sage to the east growing on a circular moraine. Granodiorite is evident here.
It is likely that this pool was part of a greater body of water during glaciation. My field work reveals granodiorite in adjacent bedrock and superficial deposition from higher elevations in the Elk Mountain range. However, an ice sheet in this location would prevent sufficient grass and other material to sustain megafauna. Therefore, a better reference than lake that would reflect on megafauna being brought into place through catastrophic flooding would be catch basin.