Step 1. Determine the stream cross sectional area. The first step in determining cross sectional area is to measure and calculate the average stream depth. In the table below, record depth measurements at one foot intervals along the tape measure you have stretched across the stream. The depth must be measured in tenths of a foot (ie 1.7 feet equals one foot and seven tenths). DO NOT MEASURE DEPTH IN INCHES.

Interval Number	Depth in Feet	Interval Number	Depth in Feet	Interval Number	Depth in Feet
1		11		21
2		12		22
3		13		23
4		14		24
5		15		25
6		16		26
7		17		27
8		18		28
9		19		29
10		20		30
Sum		Sum		Sum

The average depth is calculated by dividing the sum of depth measurements by the number of intervals at which measurements were taken.

Sum of Depths (Feet)

÷

Number of Intervals

=

Average Depth (Feet)

The final step in calculating the cross sectional area is to multiply the average depth (in feet) by the stream width (in feet) at the point where the tape measure is stretched across the stream.

Average Depth (Feet)

x

Stream Width (Feet)

=

Cross Sectional Area (Feet)2

Step 2. Determine the average velocity for the stream.

For a stream less than ten feet in width, select three points in the stream approximately equal distances apart for velocity measurements. For streams greater than ten feet in width, no fewer than four velocity measurements should be taken at approximately equal distances across the stream. For example, if the stream were eight feet wide, then velocity measurements would be taken at approximately two foot intervals across the stream in order to derive three measurements. If the stream were sixteen feet across, then the velocity measurements would be taken at approximately three foot intervals across the stream in order to derive four measurements. This method of measuring the stream velocity will insure that velocity measurements are recorded for the slow and fast portions of the stream.

Once you have determined the number of velocity float trials you need to complete, measure the water's surface velocity in the following manner: Select two points located equal distance upstream and downstream from the tape measure you have stretched across the stream. Determine the distance between these two points and record this value (in feet) in the distance box below. Count the number of seconds it takes a neutrally buoyant object, such as an orange or an apple, to float this distance. Record this time (in seconds) in the table below for each float trial you complete.

Velocity Float Trials
Trial Number	Time (seconds)
1
2
3
4
5
6
7
8
9
10
Sum

Distance Floated (Feet)

The next step in calculating the surface velocity is to determine the average float time. Average float time is equal to the sum of float times (in seconds) divided by the number of float trials.

Sum of Float Times (seconds)

÷

Number of Trials

Average Float Time (seconds)

The final step is to divide the distance floated (from the box above) by the average float time.

Distance Floated (Feet)

÷

Average Float Time (seconds)

Average Surface Velocity (Feet per Second)

Water in a stream does not all travel at the same speed. Water near the bottom travels slower than water at the surface because of friction (or drag) on the stream bottom. When calculating stream discharge the water's velocity for the entire depth (surface to bottom) needs to be determined. Therefore, you must multiply the average surface velocity (from above) by a corretion factor to make it represent the water velocity of the entire stream depth.

Choose the correction factor that best describes the bottom of your stream in the area where of the stream that you measured the surface velocity.

Correction value based on Stream Bottom Type:

Rough, loose rocks or coarse grave - use a correction value of 0.8
Smooth, mud, sand or hard pan rock - use a correction value of 0.9

Multiply the cross sectional area (feet)² from Step 1 by the corrected average stream velocity (feet/second) from Step 2.

Cross Sectional Area

x

Corrected Average Stream Velocity (Feet per Second)

=

Stream Discharge (Feet)3per Second or Cubic Feet per Second (CFS)