Low water levels interact with reservoir aging to increase the 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 analyze 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, United States of America. The most supported model suggests that declining water levels interact with reservoir age, such that an older and lower elevation reservoir leads to more metalimnion dissolved oxygen consumption, with larger spring snowmelt inflows furthering dissolved oxygen declines. We also conducted incubations to understand how sediment source, monsoon inputs, and water temperature affect dissolved oxygen demand and nutrient cycling. Incubation oxygen demand varied significantly by sediment source, exhibiting modest temperature dependence at the non-monsoonal sites. We observed the highest oxygen demand from monsoonal inputs, and substantial phosphorus release from two of three sediment types. Our findings underscore how reservoir aging and dynamic hydrology can combine to reduce dissolved oxygen availability.