Final answer:
In a hydroelectric power system, a turbine is in place downstream of a dam to generate electricity.
Step-by-step explanation:
In the context of a dam and hydroelectric power, the answer to the question 'What is in place downstream?' would be a turbine.
When water is released from a dam, it flows downstream with a high velocity. This fast-flowing water is directed into a turbine, which is a device that converts the kinetic energy of the flowing water into mechanical energy. The spinning motion of the turbine is then used to generate electricity. So, a turbine is what is in place downstream of a dam in a hydroelectric power system.