Download the latest version of Google Earth and zero in on the your stream of interest. If you do not have access to Google Earth, obtain the United States Geological Survey topographic quadrangle map for the area containing your stream.
Place the cursor on the upstream end of your stream in Google Earth and record the elevation showing at the bottom of the screen. If you don't know which end is upstream, don't worry; you'll know in a moment.
Place the cursor on the downstream end of your stream and record its elevation. Whichever elevation is greater is the upstream end of your stream.
Subtract the downstream elevation from the upstream elevation (lower from higher). This number is the total drop in elevation over the stretch of stream you have measured.
Measure the length of your stream by clicking on the "Tools" menu at the top of your screen and then click on "Ruler." Place the cursor at the beginning of your stream segment and click; this produces a yellow box for your cursor and a pop-up window for "Ruler."
Click the other end of your stream, if the stream is fairly straight; a yellow line will appear whose length is recorded in the "Ruler" pop-up. Select miles for your units and record this number. If the stream is not straight, repeat this "Ruler" step in several straight line segments down the entire stream. Record the length of each segment and add them together when finished. This process gives you the total approximate length of your stream.
Now divide the total drop in elevation in feet by the total length of the stream in miles. This calculation reveals your stream's gradient in units of feet of drop per mile of distance.
If you are using a topographic map with 20-foot contour intervals, use a ruler and the map's scale to determine the length of your stream. The Rule of "V"s states that contour lines crossing streams form "V"s that point upstream. Count the number of contour intervals or "V"s that cross your stream and multiply by 20 to get the total drop (or rise) in elevation for your stream. Divide that change in elevation by the length of your stream, and the result is your stream's gradient expressed in feet per mile.
