LOW WATER LEVELS AND RESERVOIR AGING INCREASE SEVERITY OF SUMMERTIME METALIMNION DISSOLVED OXYGEN MINIMA IN LAKE POWELL, DESERT SOUTHWEST USA

Water level drawdowns are common in reservoirs and can affect dissolved oxygen dynamics via several pathways.  In large storage reservoirs, inflow deltas are often important sites for sediment deposition, with some sediment-laden rivers forming highly dynamic delta regions as they enter the reservoir.  As water levels change, deposited sediment may be remobilized and affect pelagic dissolved oxygen dynamics.  Here, we interrogate a long-term set of dissolved oxygen profiles to ask how water levels have interacted with both reservoir age and spring inflow volumes to affect metalimnion low dissolved oxygen events in Lake Powell, desert Southwest, USA.  The most supported model suggests that declining water levels interact with reservoir age to promote summer and fall metalimnion low dissolved oxygen events, with larger spring snowmelt inflows furthering dissolved oxygen declines.   We also conducted incubations to understand how both sediment source, monsoon inputs, and water temperature affect dissolved oxygen demand and nutrient cycling.  Incubations showed that oxygen demand was highly temperature dependent in the two major inflows to the reservoir (Q₁₀ of 3.3-3.7), the highest oxygen demand was from monsoonal inputs, and there was substantial phosphorus release from two of three sediment types.  Our findings underscore how reservoir aging and dynamic hydrology can combine to reduce water column dissolved oxygen availability.   

Primary Presenter: Bridget Deemer, USGS (bdeemer@usgs.gov)

Authors:

Caitlin Andrews, U.S. Geological Survey  (candrews@usgs.gov)

Bryce Mihalevich, U.S. Bureau of Reclamation  (bmihalevich@usbr.gov)

Robin Reibold, U.S. Geological Survey  (rreibold@usgs.gov)

Thomas Sabol, U.S. Geological Survey  (tsabol@usgs.gov)

Charles Yackulic, U.S. Geological Survey  (cyackulic@usgs.gov)