What is the mechanism created near the top of the dam for the passage of excess water from the reservoir is known as?

A dam is a structure built across a river or waterway to control water flow and consists of: a foundation, abutments, crest, spillway, outlet works, embankment, and a protective layer. 

A spillway is a structure built into a dam that provides a controlled release of excess water from a reservoir. When the water level in the reservoir rises to a certain level, it flows into the spillway, which typically consists of channels, gates, or weirs, and then into a downstream channel or river. This helps to prevent overtopping of the dam and potential damage or failure. Spillways are an important safety feature of dams and play a crucial role in managing the water level in the reservoir.