The forebay is the large steel tank above the indoor water wheel which provides a uniform rate of water to the water wheel when the mill is in operation.  The waste gate provides the means to allow the mill race water to bypass the water wheel and to drain the forebay for repairs.  After years of oxidation, the original waste gate flume had deteriorated, no longer properly directing the water exiting the forebay.   It was replaced by a new flume shown in the first two photos, and is seen in operation in the third photo.  

Waste Gate Flume Replacement