Flow:	The volume (amount) of water The greater flow, the more power obtained
Head:	The height of the fall of the water The greater the height, the more power obtained
Efficiency:	The measure of how well a waterwheel captures the weight (from the flow) and force (from the head) of the water The greater the efficiency, the more power obtained

	Low flow			High flow (more water)
	Low head			High head (more height)
	Low efficiency			High efficiency (more directed)

Water can only do its work when there is both head and flow.  The earliest mill sites in New England were those where there was a natural fall of water which created head and flow.  If there was no natural fall, head was created by building a dam.  The kind of dam was determined by the type of stream bed and by the resources available.  There were many options and dam builders could mix and match features. Dams often created ponds behind them, which stored water and effectively increased flow as well.

	A crib dam is an interlocking framework of timbers filled with stones. This was the earliest and simplest type of dam, easy to build where wood and stones were plentiful. It was the most common dam built in the 1700s.
	For some sites earth-filled dams with stone facing were more practical. Two stone walls were built about 12 feet apart and the space in between was filled with earth. This type of dam was built were river bottoms were either rocky or a smoooth flat ledge. These dams were the most common type built in the 1800s.
	Decisions also had to be made about the downstream face of the dam, the spillway.  It might be either a sheer wall or a series of steps. A sheer wall worked well on ledge, but constantly falling water could dig a hole in a sandy river bed.  This weakened the foundations of the dam, which could be swept away in a storm. A step dam diverted water from the base of the dam. This type was a common form of factory dam built on soft stream beds.