The size of streams can be measured in many different ways; the length of the stream from headwaters to mouth, the area that the stream drains (called the drainage basin), the number of tributaries (small connecting streams) that feed into the main stream, etc.  Another measure of stream size is the discharge.  Discharge is defined as the volume of water moving through the strream in a fixed amount of time (volume per time).  The units of discharge are (typically) gallons per minute (aka gpm), cubic feet per second (cfs) and cubic meters per second (cms).  Rock Creek, a small stream in southwest Washington state, has a typical discharge of a 2 cubic feet per second.  The Mississippi River at Baton Rouge, Louisiana, has a typical discharge around 500,000 cubic feet every second.

    Thinking for a moment, you will readily see that measuring discharge is a problem.  Rock Creek has a discharge of 2 cubic feet per second; that's two boxes of water each second, where each box is a foot long, a foot wide and a foot deep.  There is no way you could grab two boxes of water every second out of the creek, to see how much the discharge really is.  And forget about the Mississippi.  Discharge cannot be measured directly, except for very tiny flows.

    So how is discharge measured?  Examine the units of discharge for a moment, concentrating on the metric unit "cubic meters per second" or m^3/sec.  This unit of discharge can be writeen:  meters * meters * meters / second.  In other words, discharge is a length times a length times a velocity (length per time).  So if we measure the (correct) two distances and a velocity, we can multiply them together to get discharge.

    We measure the width of the stream, from one edge of the water to the opposite bank.  And we measure the mean (average) depth of the water along this same profile or transect, by pulling a tape measure across the stream.  We get the mean depth by measuring many depths along the transect (using a stadia rod), then average those depths.  The width and the mean depth are our two lengths.  Multiplying the two lengths together gives the area of the stream in cross-section (a vertical slice down through the stream) along our transect.  The area of the slice through the stream is called the cross sectional area.  And finally we measure the mean velocity of the stream along the transect, using a velocity meter.  Multiplying width times mean depth times mean velocity gives us the discharge (flow) of the river at that point at that time.

    Clearly, the discharge changes with time.  In western Washington, discharge is very high in the early spring (March-April) and very low at the end of the long summer drought (September-October).  Discharge is very high during storm events.  The maximum discharge on the Raging River (one of the rivers we will measure) is over 4000 cubic feet per second.  The minimum discharge on the Raging River at the end of the summer drought is about 5-10 cubic feet per second.

    The discharge also changes as you go along the river.  Near the headwaters, the river is small, and discharge is low.  As you go downstream, the river collects more groundwater, seeping into the stream from the banks and from the bed of the river.  As you go downstream, you also get contributions from tributaries.  The discharge at the mouth of a river is usually way higher than the headwaters, unless people are extracting the water for irrigation, industry or municipalities.

    Knowing the discharge of a river, and especially its discharge history or variation, is very important for understanding the river as an ecosystem.  Many salmon, for example, are dependent on a certain amount of discharge in order to spawn successfully.  If too much water is extracted from a river at the wrong time or times of the year, salmon reproduction can be diminished.

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