Red River Discharge Field Trip
Earth Science Extras
by Russ Colson
At 28800 cubic feet per second at the crest in Fargo ND, the flood of 1997 on the Red River was, at the time, the largest flood in history. It seriously threatened Fargo and devastated Grand Forks.
Field Measurements of River Discharge and Sediment Load
This lab introduces you to important ideas of surface water by engaging you in taking measurements in a real river. In this on-your-own field trip, you will measure the discharge and sediment load of the Red River at Fargo ND-Moorhead MN (or other river if you are not near Fargo). You are responsible for finding materials like tape measures, rope, and weights (to withstand current, weights need to be at least a few pounds--do not lose them in the river!). You will need to have a river-water collection container, and either an Imhoff Cone, or some clear container where you can measure how much sediment settles from a water sample (at least a liter, ideally more). To do this, the sediment from a large volume needs to settle over a day or two into a small volume (thus, the value of a cone). The volume of sediment will be quite small, so you need a means to measure that small volume. Be creative, but also careful with details. You will also need to get access to a crossing of the river so that you can measure water depth and velocity in the middle of the river, and so you can measure the width of the river (e.g. bike bridge?). You can see that there are a number of field/measurement/logistical challenges that you need to solve--key aspects of any authentic experimental or field investigation.
Discharge is how much water a river carries per unit time. For example, we used units of cubic feet (a volume) per second (a time) in our problems above.
If we imagine a stream channel with a rectangular outline, we can portray discharge as shown in the diagram below.
where D = depth of the stream, W = width of the stream, and V= the distance that the water travels in one second (that is, the velocity).
Notice also that D x W = cross sectional Area of the channel (=A).
So we can write an equation for the discharge where
Discharge = Q = D x W x Distance/time = D x W x V = A x V.
notice that all of these expressions say the same thing, expressing discharge as a volume per unit time, such as cubic feet per second (cfs).
From the equations above, we see that discharge is the amount of water carried by the river per unit time.
Discharge = cross-sectional area of the river x mean river velocity.
For our measurements of discharge of the Red River, we are going to assume that the channel is triangle shaped, as shown below (not exactly correct of course). You will need to know the cross-sectional area of the river (which we may estimate as shown below (which requires that you measure the width and depth of the river—use units of feet): We will measure the depth of the water by measuring the distance from the bridge banister to water level, then measure distance from bridge banister to river bottom-and take the difference to get water depth) (or use whatever creative method you can think of). I will let you figure out how to get the river width, pacing, string, etc. We will have a shared tape measure or two.
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Area of a triangle = ½ width x height
You will to measure the river velocity (Use units of feet per second—measure more than one value and average). You will need to figure out a method for measuring the velocity.
You can compare your value for discharge value to the official river discharge rate (typical values for late Sept to early October in Fargo are 60-600 cfs) - you can check actual values at http://waterdata.usgs.gov/nd/nwis/uv/%3fsite_no=?station=05054000
Sediment load is the amount of sediment being carried by the river past a given point in the river. We are going to calculate how much the river would transport over an entire year if the sediment load averaged the value that we will measure on our field tirp. The sediment load per year can be calculated by multiplying the sediment per cubic foot of water times the discharge in cfs, times the number of seconds in a year.
We will measure sediment load by collecting 1 liter of sediment-laden water and letting the sediment settle in an Imhoff Cone. This will give us the volume of sediment per liter of water (in milliliters = cubic centimeters).
Create a written report of your field work results: Your field trip report should include
1) the key measurements that you made, any repetitions or averages of measurements,
2) calculations of river discharge
3) calculations of sediment load per year
last updated 9/14/2022. Text and pictures are the property of Russ Colson.