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Operation and Maintenance The modeled operational scenarios to meet the NPS desired flows would require increased use of bypass valves, spillway gates, spillways, and stilling basins as compared to the baseline. The original design for these structures did not contemplate routine annual operation or extended use. These structures were intended to operate on a limited basis when needed to pass high flows attributed to unusual hydrologic events. Prior to implementation of any of the proposed scenarios, it would be necessary to conduct a thorough analysis of the structures and the associated design and maintenance parameters to determine if the structures would function properly under the new operational scenarios. Part of this analysis would be to identify costs to modify structures and to provide additional maintenance. These structures must not be compromised in any manner that might weaken the structural or operational integrity of the dam. Failure to ensure the structural and operational integrity of these structures.and dams would put downstream, populations at risk. In addition, frequencies of inspections and maintenance would increase under higher spillway uses. This type of work often entails interruptions of river flows and affects river stability. Review of modeling results in the table below shows changes in spill frequency at Blue Mesa, Morrow Point, and Crystal. For example, Blue Mesa spilled in 5 of the 24 years analyzed under baseline conditions, and could spill up to 16 times under the NPS desired flow conditions. Spillway Use Frequency (years of 24 year study period) Dam Base Run . NPS Desired Flows Blue Mesa 5 5-16 Morrow Point 5 5-7 Crystal 8 22 Similarly, increased bypass use can increase maintenance costs. The following table presents information on bypass use frequency. Bypass Use Frequency (years of 24 year study period) Dam Base Run NPS Desired Flows Blue Mesa 9 22 Morrow Point 7 7-24 Crystal 18 20 - 24 8