Measuring River Gradient
The gradient of a river is a measure of how steeply it loses height. A river with a high gradient loses height quickly and is typically fast flowing and youthful. A river with a very gentle gradient loses very little height and is typically a slow flowing mature river. In fact, a lowland stream may have a gradient which is so gentle that it is impossible to measure without specialised equipment.
A graph to show how the gradient of a river changes along its length is called a long profile. For lengthy sections of rivers this can be constructed using the contours on a good map; for shorter sections, especially where the gradient is noticeable, it is possible to measure the gradient directly with very simple equipment.
The most basic set of equipment required is a clinometer and two surveying poles. Actually, any two poles can be used so long as you can mark them both clearly the same distance from the bottom. Something around 1.5m is usually a good height.
Some way of recording your data is also required, so plan how you will record it before you start measuring anything.
Since you will be standing in the water to take your measurements you will also need sensible clothing, foot wear, a towel and a warm drink.
The most difficult part of the equipment to acquire is usually the clinometer. This is an instrument to measure angles. Clinometers can be bought, or easily constructed at home or school from a few readily available items.
Making a Clinometer
The following items are needed to make a simple clinometer:
- Protractor: The plastic type found in drawing sets and used by maths departments are fine, but larger ones can be easier to read.
- Narrow tube: A thin tube such as the body of a ballpoint pen. This will be the ‘sight’ mounted on the protractor.
- String or thread: About 30cm ( 1 foot) of thin but Strong string.
- Weight: Anything small and heavy which can be attached to one end of the string to keep it taught, such as a metal nut or fishing weight.
- Glue or sticking tape: To assemble your clinometer. Glue must NOT dissolve in water!
1. Glue or tape one end of the string firmly to the center of the protractor where all the lines meet. This will be half way along its flat base. Make sure that the string can hang down freely when the protractor is held with the flat side at the top.
- Attach (glue or tape) the ‘sight’ tube along the straight edge of the protractor.
- Attach your weight to the free end of the string and check again that the string can move freely as you tilt the protractor up and down.
That’s all there is to it. You now have a clinometer which, hopefully, looks something like the one below.
Measuring in the field
Using your clinometer is quite simple.
You need to break up your river into short lengths which you can easily measure.
Measure out a stretch of water where you want to begin your survey. The length you choose is up to you, but a couple of points should be remembered.
- Short sections of a couple of meters will give a very accurate profile, but you will need to take many readings to cover a long stretch of river.
- Long sections of 20m or more lose some detail and can generate difficulties around meanders.
I find that anything from 5m to 15m is a good size stretch of river to survey per measurement, depending on the variation in the river itself and the degree of detail I want to record. There is nothing to stop you from choosing different distances every time you take a measurement, but this can make record keeping very difficult and confusing. Unless you must alter your measuring length, keep it the same all the time.
Right, you’ve measured a length of river.
- Now stand at one end of the stretch with a pole and your clinometer.
- Get a helper to stand at the other end with the other pole.
- Make sure that both poles are vertical and the right way up (It happens, people do hold it upside down sometimes!)
- Place your clinometer against the height mark on your pole and look through the sight, adjusting the angle of the clinometer until you are looking directly at the height mark on the other pole.
- Taking care not to move the clinometer, look to see what angle is shown by the string which will still be hanging vertically down.
- Record the value NOW. Don’t rely on your memory, get it written down.
- Pick up your pole, walk to where your helper is standing.
- Your helper now moves the same distance again along the river and you take another reading.
- Keep repeating this until you have surveyed the whole of your study area.
Diagram shows positions of surveying poles and clinometer when measuring the river gradient.
To produce your long profile, you need a sheet of graph paper large enough to plot all your data at whatever scale you have chosen. The scale will have to be decided based on the size of paper you have and the length of your river survey.
For the moment, let’s assume that one large square on your graph paper represents 1m along the river. If you took measurements over 10m lengths of the river, you will be entering data on your graph every 10 squares.
Start at the bottom left of your graph paper. This represents the lowest part of the river.
To show the gradient on your graph you can use one of two methods. Firstly, you can use a protractor to draw lines at the correct angle from one measuring point to another. Thus, if you had a gradient of 5 degrees you would draw a line going up at 5 degrees from the start of the measurement to its end.
This is the easiest method, but it is hard to get the lines at exactly the right angle and you can soon get way off course, adding a little more error every time you draw a new line.
The alternative way is to use the length of the measured section and the angle to find the change in height between the ends of the measured section.
The triangle below represents our problem:
The letters on the triangle are:
A The angle of gradient that you measured with the clinometer
B A 90 degree angle
C 90 degrees – the angle of the gradient
a The difference in height, which we want to find
b The hypotenuse of the triangle
c The distance between your two poles
To find the height (a) we use the Sine Rule, which goes like this:
By substituting the values we know (A, B and c) we can find the height change represented by ‘a’.
Once this value is known for each section, we can plot the changes of height on the long profile and join the points together. Although it’s more work than using the protractor the results are much better.
Once the profile has been drawn, remember to add a title, key, and both horizontal and vertical scales.