Here we will start by measuring and marking 3 locations on both sides of the stream. You can use flags or whatever you have on hand to mark the locations. Your distances can be different for smaller streams (0, 1.5 and 3 ft) verses mid sized streams (0, 10 and 20 ft). Record this length from the upstream to the downstream boundary as the distance. We will come back to this distance.
As for the width, measure the distance across the stream at each of the 3 locations. And then find the average by adding them all up and dividing by 3. This average gives us a good representation of the width of the river!