Playa and Lunette Sedimentation Response to Artificial Water Levels: Case Study of the San Luis Lake Area
San Luis Lake, one of the largest playa systems within the aeolian environment of the San Luis Valley, has been artificially filled and modified, to keep it at levels suitable for fishing and recreational boating, for approximately 80 years. Recently, San Luis Lake has been allowed to fully dry out, exposing the playa sediments for the first time since artificial filling began. This has had a unique effect on the sedimentation patterns within the system. Prolonged high waters levels, that have not been allowed to drop, have cut off the supply of fine sediments to the adjacent lunette system, thereby causing the dunal system to begin to erode away. This is essentially a forced climatic response in the dunes, which respond directly to their playa derived source of fines. At present, the dunes record a prolonged wet climate even though that is not the climatic regime that has been in place over the valley. This is creating a conflicting climatic record in the dunal system, with most dunes recording the semi-arid, drought affected patterns of the region, and the lunettes, in contrast, recording a significant wet period. The artificial filling has also forced a unique response in the lacustrine sediments found at the center of the playa. A thick, black, organic rich layer, not found elsewhere at the site, is found in the depocenter of the playa. This layer is up to 36 cm thick and has a pervasive, strong, sulfurous odor that can be smelled in the groundwater up to 150m away from the edge of the deposit. This effect of this layer, and the by-products it has been creating, on the groundwater, is currently unknown, but the layer does appear to be a response to the artificial fill in the playa. Other wet cycles in the region do not correlate to such layers, leading to the conclusion that forcing the system to remain filled and not allowing the cyclic dry/wet patterns has created a unique, human-related deposit in the center of the playa. This deposit is unique and while thinner versions may exist in other deposits elsewhere in the world, the thickness of this deposit is not known to be within normal ranges. While part of the thickness may be due to San Luis being located at the depocenter, the presence of such a thick layer and the fact it is impacting groundwater a fair distance from the edge of the deposit may be indicating that the deposit may be contributing to the degrading water quality in the valley.
AAPG Datapages/Search and Discovery Article #90193 © 2014 Rocky Mountain Section AAPG Annual Meeting, Denver, Colorado, July 20-22, 2014