This metric classifies a sample site with regard to the size of the river and its gradient. The width of the river can be estimated in the field. Gradient can be measured in feet per mile from a USGS topographic map. To do this, find the contour lines that cross your river nearest your station, both upstream and downstream of your station. Be sure to use only contour lines that cross the stream. Ignore the ones that go along it. Figure out what elevation is implied by each contour line and subtract to get the vertical drop. For example, if the downstream contour line is 580 feet and the upstream contour line is 590, your vertical drop is 10 feet. Now measure along the stream from one contour line to the other. Be sure to measure along the channel of the stream itself, not in a straight line. Convert your measurement to miles. On a standard 1:24,000 USGS topographic map, 1 inch equals 2,000 feet, or 0.3788 miles. To continue our example, if your upstream and downstream contours were separated by 3.6 inches on the map, this would be equivalent to 3.6 x 0.3788 miles, or 1.36 miles. To convert this to gradient in feet per miles, you simply divide the 10 feet vertical drop by the 1.36 miles, or 10 / 1.36, or 7.33 feet per mile. If your river is 25 feet across, this would be considered a moderate gradient.
It is most meaningful to treat River/Watershed Size and Gradient as a matrix in which the score is assigned based on your estimates of the two parameters. This matrix, along with the scores, is shown below: