Velocity is mentioned briefly in the AASF Water Quality Monitoring material; discharge is not. Velocity is the speed, often measured in feet/sec, the water moves. Discharge is the amount of water moving past a point on the river. It is often measured in cubic feet/sec. The discharge varies throughout the year. Both affect the dispersion of pollutants, but are also important for other reasons. Together, velocity and discharge determine the shape and pattern of the river.
The average values for velocity and discharge are important, but understanding the river also depends on how much the values vary and the patterns in which they vary. In some rivers, such as those around here, the maximum flow comes with the spring melt. In other areas of the country the maximum flow may come at other times of the year. For example, in the south where there is little snow to melt, the maximum discharge coincides with spring rains. In the more mountainous regions of the west maximum discharge comes in the late summer, when the melting of the glaciers and snowcaps on the mountains reaches its peak.
Another aspect of discharge that should be considered is the extent to which it is "flashy". In some areas, like the desert southwest, there is little vegetation and soil humus to act like a sponge and slow the runoff of water into the river. Consequently, after a rainfall the river reaches its peak discharge very quickly, then returns to a lower level very quickly - these are "flash" floods. In New England in general the soil and vegetation slow the release of the water into the river and the peak in discharge is less pronounced. However, New England watersheds vary and some rivers are considered to be relatively more flashy than others.
The upper elevations of the Boquet and Ausable River watersheds are steep and have little soil depth. When the rain is heavy or prolonged there is little material to act like a sponge and retain the water. Sometimes the soil slides off the sides of the mountains. This lack of ability to absorb water causes high spikes in the river's discharge and so the river is considered relatively flashy.
Velocity and discharge not only tell us about the shape and patterns the river will be likely to assume, they also can tell us about the consequences of trying to alter that shape. Artificially narrowing a river beyond its natural width may make it more prone to flooding or more likely to have ice jams in the spring. Changes that speed up the river may cause more erosion in that section and/or greater flooding and deposition downstream.
The field studies Determining Velocity and Discharge Volume describe simplified procedures, suitable for use by students, for determining velocity and discharge.