Rumor among the locals that stopping the spill way could potentially cause damage to the levee system.

Levees contact or expand depending on water levels. When the water level is high and they want to lower it. They do so over a period of time to allow levee to adjust.

Levees are currently full or filling up. With the spill way stopped water levels downstream in some of the levee systems may drop dramatically with out time to adjust. Potentially comprising the levees.

Don't know how accurate this is but anyone familiar with the Sacramento Valley knows how important the levee systems are.