Two people were injured around 5:30pm Thursday when an eastbound motorcycle on the Gordon Drive viaduct collided with another eastbound vehicle and then crossed the center median.
Sioux City Police say the cycle was then hit by an oncoming westbound vehicle resulting in serious injuries to the male motorcycle driver.
A female passenger received minor injuries.
Both of the victims were wearing helmets.
Their names have not been released.
The whole viaduct was closed to traffic for over an hour because of the accident and the westbound lane opened around 9pm.