Watch Out For Variable Depth!

Yes, that’s right, river depth varies and it’s not always deepest in the middle. Let’s try some measurements.

Again, you need…

…but this time you need 2 helpers.

At each site find a spot where the width is close to the average that you calculated from last week’s measurements.

Station a helper on each side of the river and stretch your tape measure (or knotted string) directly across.

Firstly, you need to make a note of the width at this point. (If you have signed up for worksheets, check your inbox for a recording sheet ready for you to fill in.)

Mine measures 1 metre and 20 centimetres, so that 120 centimetres.

We are going to take some regularly spaced depth measurements. From these we can calculate an average depth and also use them to draw a cross-section diagram of the river. I’ll show you how to do that next week.

So, take your width measurement and divide it by 10. (Mine is 120 ÷ 10 = 12cm)

This will divide the width into 10 sections and will give you 9 places from which to measure the depth (as the last section will end at the side of the river).

So how to actually measure the depth. You could grab hold of your tape measure and just stick your arm in, down to the riverbed but ideally use a dipper as you’ll get a more accurate measurement and you really need to leave the tape measure stretched out across the width, to show where to dip for the depth.

A garden cane makes a good dipper but any straight stick will do. You just need to make sure that you are holding it vertically and that it touches the riverbed.

Then lift the dipper out of the water and measure the length of the wet bit. That’s the depth at that point.

You need to measure depth at each evenly spaced point across the river.

Add the results together and divide by the number of measurements to get an average depth for the site. Do the same for the other sites and compare the results. Is it as you would expect?

Keep your results safe and next week we’ll use them to draw cross-sections for each site.