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Talking dirty with a nephelometer

So I was reading the Mail Tribune Fishing Report this week and saw something I haven't seen before — NTUs. I gathered it has something to do with how dirty the water is, but what exactly is an NTU?

— J.T., Medford

Well, J.T., regular readers of this column know it doesn't take much to get us talking dirty at Since You Asked Central, especially when it comes to NTUs.

NTU is an acronym for the measurement of turbidity, or how much dirt is in water such as the Rogue River at any given time, and that time is most prevalent in the winter when storm runoff flushes sediment into streams.

NTU stands for nephelometric turbidity units. Certainly, it's hard enough to spell and tough to say, which is why you won't hear this explanation in a Mail Tribune podcast.

And just how do you think you get NTUs? By using, what else, a nephelometer. It's a little gizmo that shoots light through a column of water and uses sensors to count how much of the light is deflected at 90-degree angles from hitting particles.

The water nerds at Since You Asked Central recall when NTUs were actually called JTUs, or Jackson Turbidity Units. The Jackson part came from the use of a Jackson candle to measure the amount of light that passes through water, but the nephelometer took over, and the old candle got placed back in the drawer for the next power outage.

And with it came the name change. Kind of like when Mount Pitt suddenly became Mount McLoughlin.

In low and clear conditions between storms on the upper Rogue, the river's turbidity will be as low as 1 NTUs, which is commonly called "gin clear" to anglers. In a rip-roaring storm, the caramel-colored Rogue can contain water of several hundred NTUs.

The Rogue's salmon and steelhead fishermen like the turbidity no lower than 4 NTUs and generally no higher than 14 NTUs. But that's a guess throughout most of the Rogue because the only stream gauge where NTUs are measured on the Rogue these days is at the Grants Pass water treatment plant. You can find that data daily at www.grantspassoregon.gov/354/Water and scroll down to "River Data," where the Rogue's turbidity was listed Wednesday at 1 NTU.

— Send questions to “Since You Asked,” Mail Tribune Newsroom, P.O. Box 1108, Medford, OR 97501; by fax to 541-776-4376; or by email to youasked@mailtribune.com. We’re sorry, but the volume of questions received prevents us from answering all of them.