The main affluent , Mill Creek , joins from the left within the Prado Basin .
The last portion of the creek is predominantly marshland , situated within the Prado Flood Control Basin behind the Prado Dam , and mostly in Riverside County .
The main affluent , Mill Creek , joins from the left within the Prado Dam   .
The last portion of the creek is predominantly marshland , situated within the Prado Flood Control Basin behind the Prado Dam , and mostly in Riverside County .