River Flow Changes Over Time


River Flow Changes Over Time

The amount of water moving down a river at a given time and place is referred to as its discharge, or flow, and is measured as a volume of water per unit time, typically cubic feet per second or cubic meters per second. The discharge at any given point in a river can be calculated as the product of the width (in ft or m) times the average depth (in ft or m) times average velocity (in ft/s or m/s).

The vast majority of rivers are known to exhibit considerable variability in flow over time because inputs from the watershed, in the form of rain events, snowmelt, groundwater seepage, etc., vary over time. Some rivers respond quickly to rainfall runoff or snowmelt, while others respond more slowly depending on the size of the watershed, steepness of the hillslopes, the ability of the soils to (at least temporarily) absorb and retain water, and the amount of storage in lakes and wetlands.

Video: How to Measure a River (8:35)

Click here for a transcript of How to Measure a River.

Good morning. I'm Barry, I'm Ben. We're the Geography Men. 

Ben: Now today I'm going to be showing you how to measure the discharge of a river. Now for this what you're going to need is a tape measure, a meter stick, a flowmeter, a couple of stakes to help you out, and a recording sheet to record your data.

So the first thing you're going to want to measure is the width of the river. Now as I said before, for this you're going to need a tape measure, preferably let's say a 30-meter tape measure. Now from the left-hand bank, you want to have your 0-end of your tape measure. The easiest way to do this is to tie it to something or to use a stake in the ground. Now here I'm going to tie it to this root just to help me out. Now you want to stretch the tape measure across the river, making sure that it is tight across the surface of the water. You do not want to allow it to go slack, otherwise, that tape is going to get carried off by the river and you are going to get a false measurement of the width of your river.

Now that you have your tape set across your river, you want to record where the river begins, where the water meets the bank. Record on the other edge, on the other bank, where the water meets the bank, and then work out that distance from one bank to the other.

With this river here, our left-hand bank starts at 1 meter 60 and our right-hand bank ends at 5 meter 60, giving us a width of our river of 4 meters. Now the next thing we need to do with that width is we need to divide it by 11, in order to work out the intervals at which we need to work out the depths of our river. The reason we divide it by 11 is because we're going to take a measurement at each of the banks. This will give us 10 intervals across our river to take our depth.

Now for our depth, we're going to want to use a meter stick. Now with the meter stick, there's some very simple things that you need to remember. Number one, make sure the zero end of the stick is at the bottom of the river. You don't want to have it upside down and be getting readings of 80 or 90 centimeters. You want to turn the meter stick parallel to the flow of the water, so as that meter stick does not block that flow of the water giving you two false values on either side. Starting at the bank, place that meter stick into the water until it reaches the bed of the river. Now you want to take a reading and you want to convert that reading straight into meters, as you want the same units for each of your measurements. So here we have 25 centimeters, so we have naught .25 meters. Find your next interval on the tape and do exactly the same again. We have naught .21. Naught .24. And you would then follow that across the river until you reach the right-hand bank.

Now the final measurement you want to take at your site, to work out the discharge of the river, is a flow reading. You want to work out how fast that water is rushing past your feet. Now for this, the flow meter is the best option. However, if you do not have a flow meter, you can use a float and a tape measure and work out how fast that float flows down 10 meters of your stream. You can then convert that into a speed. With the flow meter, the propeller on the end spins as the water rushes past it and you get a reading in meters per second. As we take three readings across our river, you want to do it a quarter of the way across, a half of the way across, and three-quarters of the way across channel, making sure that you or anyone else in the group are with you, are not stood directly in front or behind the flow. You want to place the flow meter into the river 1/3 of the way down and record the flow in meters per second from the electronic box, every 10 seconds for one minute you. So here our first reading is naught .94 meters per second. Now we leave it another 10 seconds. Our next is naught .78. And you would then repeat this every 10 seconds for one minute, giving you six readings for the left-hand bank, one-quarter of the way across the river. You then repeat this at the halfway mark. So you're halfway across your river again, you want to place that flow meter a third of the way down into the channel. And again, every ten seconds for one minute record how fast that water is flowing in meters per second. You then repeat that on the right hand back three-quarters of the way across the river.

Now that you've got your measurements done, the next step is to work out some calculations. The first calculation you're going to need to work out is your cross-sectional area. For your cross-sectional area, you need to times your width by your mean depth. For our calculations, we got a width of 4 meters and our average depth worked out at 0.2 meters. Now this gives us a cross-sectional area of naught .8 meters square. Now with our cross-sectional area we can now use our velocities and work out a mean velocity from our six at the left bank, our six in the middle, and our six at the right bank, and used both of those calculations to work out the discharge of our river in meters cubed per second or cumecs. Now we know that our cross-sectional area is 0.8. And we've worked out that our average velocity, our mean velocity, is one meter per second. This quite simply gives us a discharge of 0.8 cumecs or meters cubed per second.