Measuring Discharge (A Review)

During and following a storm, surface runoff will accumulate within a stream channel. Many streams are perennial, which means they flow all the time. Groundwater seeps in from the banks and bed during times when rainfall is low or absent. Other streams are more or less temporary and flow only during or after rainfall. These ephemeral streams will go dry during some part of the year, usually the dry summer months.

The question to answer is "how much water is the stream moving?" The volume of water moved over a given time interval is called Discharge. Discharge is usually high during and shortly after storms and also during periods snowmelt. It is usually low during dry periods.

Discharge at any location along a stream is the product of the velocity of water (ft/sec) and the stream's cross-sectional area (in square ft). Discharge has units of cubic feet per second (cfs). The picture shows that the stream's speed varies from top to bottom and from side to side because the channel depth also varies from side to side.

The technique of measuring discharge can be very challenging.