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Introduction

If you are looking at erosion or deposition, you may want to calculate the load transported by the river. Load is measured in kilograms per cubic meter but, obviously, you don’t want to be dealing with a cubic meter of cold river water full of sediment!

The solution is to take a much smaller volume of water, such as 1 litre, and then scale up the result to represent one cubic meter. This works well because 1 litre of pure water has a volume of 1,000 cubic centimetres and weighs 1 kilogram. There are 1000 litres in one cubic meter, so take the result for 1 litre, multiply it by 1000 and you have a value for the load carried by one cubic meter of water!

Equipment

Digital scales that measure in grams.

Container to hold 1 litre of water, that fits on the scales

Thermometer (optional)

Bottle containing a little more than 1 litre of tap water or pure water.

Method

Place your bottle of pure water in the river and let it reach the same temperature as the river. Leave it there for an hour or use a thermometer to check it.

Place the scales on a flat, horizontal surface, and switch them on.

Place the 1 litre container on the scales and fill it with 1 litre of tap water / pure water.

Record the mass shown by the scales.

Empty the container and re-fill it with river water.

Again, record the mass shown by the scales.

Deduct the first mass (pure water) from the second mass (river water).

The result is the mass of the load.

For example, the first mass is 1000g and the second mass is 1006g.

1006 – 1000 = 6, so the load has a mass of 6g

To find the load suspended in one cubic meter, multiply the 1 litre value by 1000.

For example, the load in 1 litre is 6g.

1000 x 6 = 6000g, or 6Kg.

The load suspended in one cubic meter of river water is 6kg.

Notes

1. Unless you use very expensive scientific scales (and the science department probably won’t let you carry them off into a field!) you will need to use digital kitchen scales. These are not accurate. Your 1 litre of pure water might weigh only 975g, or perhaps 1015g according to the kitchen scales, even though you know it *should* weigh 1000g. Don’t worry.

1. The difference between what you know is correct and what the scales tell you doesn’t matter, because we have weighed a litre of pure water and a litre of river water on the same scales under the same conditions. We need only the difference between the two measurements. The degree of error applies to BOTH measurements and is largely cancelled out.

1. The density of water changes with temperature and pressure. Luckily for us, the changes are very small. At 4C a litre of water has a mass of 999.973g, and at 20C it is 998.203g, only 1.77 g difference. If your river water was 4C and your pure water at 20C you’d have an error of 1.77g. However, buy making sure that the pure water and the river water are the same temperature you eliminate this potential error.

1. The mass of the container used for the water samples doesn’t need to be found because we use the same container for both samples, then find the difference between. The container mass was constant, so it can be removed from the problem.

1. Both river water and tap water may also contain dissolved load, minerals and gasses. The dissolved load has such a tiny mass that its of no relevance to our calculations. If you are curious about the actual mass, at 4C the density of pure water is 1g/cm3 and the density of typical tap water at the same temperature is 0.99999g/cm3. Not much difference is there!