So far in this series on river fieldwork, we have measured width, depth at points across the width and wetted perimeter, all of which have involved crossing the river from one bank to the other. For our next measurement we will be looking along the river at each site, to find the slope of the river channel.
If you have been following this blog for a while you will have already had a go at measuring slope, either on a hillside or across a beach. Have a look here for the post that I wrote previously telling you how to make some basic equipment and then use it to measure a slope.
We want to find out if the slope of the river is changing from one site to the next. You need to do exactly as described for the hillside and the beach except this time you will be standing in the river channel.
You will need to dig out your ranging poles and clinometer. You will also need your tape measure or 10 metre knotted string and 2 helpers.
Use your tape measure or knotted string to measure out 10 metres along the bank.
Make sure your poles are the right way around in relation to each other so that when they are side by side the tape is at the same height.
Get one helper to hold a pole in the middle of the river at the upstream end of the 10 metres.
Position yourself, with the other pole, at the downstream end of the 10 metres.
You also need the clinometer.
Line up from the top of the mark on one pole to the top of the mark on the other pole and get your other helper to read the scale on the protractor, whilst you are looking through it to keep it in line. For more details on all this, check out the previous post on slope measurement.
It’s worth repeating the measurement a few times and then calculating an average. Compare the results from all your sites. Is the slope angle bigger upstream or downstream?