1. Making measurements
Crosssectional area and wetted perimeter vary uniquely at each stream or
channel. In order to calculate these values, first you must make measurements
of the stream bed. These measurements will require the use of a string,
a ruler, and a level. Unless the stream is very narrow, you will also
need to wade out into the stream to take the measurements, so you should
have a partner present and should practice stream safety.
First, stretch a string from the top of one bank to the top of the opposite bank. Use the level to ensure that the string is precisely horizontal. You may need to bring the string down from the top of one bank to level it with the lower bank.
Next, measure the distance between the string and the bottom of the channel at regular intervals. We chose to measure the depth of our stream at every 5 inches across the channel's width. Based on the width and crosssectional complexity of your channel, you may choose a different measurement interval.
The depth measurements from our stream are recorded in the table below:
Distance

Depth (in.)

0

0

5

2

10

7

15

10

20

17

25

19

30

17

35

17

40

21

45

24

50

24

55

25

60

25

65

24

70

21

75

16

80

9

85

4

90

0

Using this chart, crosssectional flow area can be estimated by counting the number of squares in the streambed and multiplying this number by the area of each square. On the chart above, there are 56.5 squares within the streambed. The scale of each square is 25 in.^{2}, so the total crosssectional flow area is: