Sampling Pebbles

The stuff a river carries along in the water is called its load.  Some of the load may have been put there by people (shopping trolleys and other random stuff that has been dumped), while some of it you won’t even be able to see because it is dissolved in the water.  We are going to concentrate on some of what you can see – pebbles!

So how do you think the pebbles will change as you move from site to site down your river?  How will they change in size?  How will they change in shape?

To find out you are going to need a sample of pebbles from each of your fieldwork sites.  You need a labelled bag for each site and your tape measure or knotted string…let’s go!

But how are you going to get your pebble sample?  Are you going to be random or systematic? 

What you can’t do is look and choose, because if you deliberately select the ones that you like the look of you are going to be affecting the results.

So, if you want to be random, pick up a stick, go into the river and without looking put the end of the stick down to the riverbed.  Then have a look and pick up the pebble at the end of the stick.  (It is safer to use a stick than to randomly put your hand in, as there may be broken glass).  Then move a few paces and put the stick in again and repeat until you have a sample of 10 pebbles.

If you want to be systematic you need to have system or a plan before you start.  Use your tape measure or knotted string.  Stretch it out in the river and pick up a pebble say every 50 cm or by every knot…you decide the rule and keep going until you have a sample of 10 pebbles.

It doesn’t matter which method you use.  Try them both if you like.  Visit each site, collect 10 pebbles, store them in the correct bag and bring them home.  We will do some measurements on them next week.