Use a map to make sure that all the sites are on the same river. In other words, you could put a float in the water at the most upstream site and it would eventually pass through all the other sites. You don’t want to have to divert up a tributary.
The most upstream site will be Site 1. Number your other sites so that the river passes through them in number sequence.
For the first measurement you just need one of these (the longer the better),
…some of this and a helper.
Measure off 10 metres of string and mark it to show each individual metre, with permanent marker or by tying a knot.
We are going to begin with a really simple measurement – the width of the river. Except it is not quite as simple as it sounds.
Take a look at this site. Where exactly do you measure? The width isn’t the same at every point.
If you just pick a place, your results are going to be influenced by your personal choice, so instead, stretch your 10 metres of string out along the bank.
Take 3 measurements of width – one by each end of the string and one by the knot in the middle. Add the 3 measurements together and divide by 3 to find an average.
If you want to take more than 3 measurements, using the other knot positions, go ahead. Your results will be even better. Just remember to calculate the average by adding up all the measurements and dividing by the number of times that you measured.
You are probably expecting the river to get wider as it goes downstream from site 1, to site 2, to site 3 etc. Tributaries will be adding in water, but that doesn’t happen gradually. There may be very little change for several miles and then a tributary empties into the river and it suddenly becomes much wider. And sometime the river becomes narrower too.
Measure the width of the river at all of your sites. Can you explain your results?
Hint: If your tape measure is quite short and your river quite wide, the tape might run out in the middle of the stream. A second knotted string can be used to help measure the width. Count the knots to measure the number of whole metres and then just measure the remaining part metre with the tape measure.