"Head" is the vertical drop between an upstream location and downstream location.
A water fall's head is easy to measure, and is simply the vertical drop from water top to water top. This can easily be measured with a rope or a long tape measure. The head of a reservoir is measured in the same way.
To measure the head of a stream that travels a long distance is much harder. Before the computer age, many measurements had to be taken and put into trigonometric functions to calculate the head. Today however, there are two easier alternatives.
The first is GPS or Global Positioning System. Hand held GPS units can accurately determine the altitude of a location and are affordable. Simply find both elevations and subtract to get the head.
The other alternative is to use Google Earth. Google Earth is a free program distributed by Google.
To find the head of a stream simply move your cursor over the desired location and the elevation in feet will be displayed at the bottom of the screen. To get the head, simply subtract the two elevations.