Rivers, Streams and Sediment
Written by Carson Pruitt, Intern with Phinizy Center for Water Sciences
Rivers and streams do much more than just transport water downhill. They carry anything in their paths along with the water, many times all the way to the ocean. One of the most important substances that streams carry is sediment. In fact, excess sediment has been identified as the number one pollutant of waters in the United States. There are two main ways that water transports sediment—by rolling sediment along the bottom, called bed load, and by sediment that is suspended in the water column, called suspended load. We also refer to the clarity of water as turbidity. Generally, when you look at a stream that is ‘muddy’ during or following a rain event, we say that it is very ‘turbid’.
As part of The Phinizy Center’s research on rivers and streams in the area, we measure the amount of sediment that travels downstream. Above is a graph showing actual water levels of Rock Creek over time during a particular storm. You can see the water level (black line) go up as the rain (blue line) falls with the red dots highlighting times when we took water samples.
The picture below shows the water samples that were taken during the storm that correspond to the labeled red marks on the graph. The sample labeled ‘Bl.’ is drinking water that is used as a reference. It is easy to see the progression of turbidity in the samples over time; sample 3 is clearly the most turbid. If you look at sample 3 on the graph, you’ll notice that it is the sample where the water level is the highest. This makes sense because, as the water rises during a storm, the velocity and shear stress of the water increases, enabling it to carry more sediment. As the water level comes back down after sample 3, the turbidity also decreases.