At each monitoring site, a gaging station is located. The gaging station measures the height of the water, also known as the gage height, relative to some feature unique to that site. For this reason, the gage height can't be compared between different stations. One station's flood gage height may be another's low water gage height. The image to the left shows one possible instrument that is measuring the water height by measuring the height of water in the instrument that is also connected to the stream. However, this can not be done in all situations so a variety of different methods can be used. 
The next step is to measure velocity. Velocity is difficult to measure because the water flow in the channel is not laminar as it interacts with the stream channels. The way around this is to measure the velocity in a series of sections, and using these various measures to find velocity. With stream water velocity and the the size of each section, the discharge, or the amount of water flowing through the stream channel over a period of time can be calculated. 
Of course, it is not possible to measure velocity and calculated discharge all the time, so discharge measurements are combined with gage heights. With enough discharge measurements and gage heights a stage-discharge curve, also known as a rating curve, can be created. The rating curve allows discharges and gage heights to be predicted. However, rating curves are statistical interpolations, and may not reflect the reality of the stream. The are normally plotted log-linear so the rating curve is straight. The final terminology used is flood stage. Flood stage is best defined as when the river fills its stream channel and starts to also flow on the surrounding floodplain. What a flood actually is (most places don't look like the lower Mississippi River) is a subject for another post.
