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Talk dirty to me in NTUs

I read the Mail Tribune’s Fishing Report every Friday, and once in a while I come across a reference to NTUs. I know it has something to do with water clarity, but what exactly is an NTU?

— Tom, email submission

Well, Tom, when it comes to NTUs we’re talking dirty in the Fishing Report.

Dirty, as in turbidity, or dirtiness of the water.

One of the things we at Since You Asked Central have discovered over the years is that science guys have a measurement for everything, even how dirty streams like the Rogue River can get during periods of high water.

NTU stands for nephelometric turbidity units.

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 more seasoned 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 passed through water, but the nephelometer took over, and the old candle got placed back in the drawer for the next power outage.

In low and clear conditions between storms on the upper Rogue River, the river’s turbidity will be around 2.5 NTUs, aka “gin clear” to anglers. In a rip-roaring storm, the caramel-colored Rogue can contain water of several hundred NTUs.

The only good spot on the Rogue to get a daily update on NTUs is at Grants Pass’ water treatment plant. The morning NTU measurement, as well as the Rogue flow at Grants Pass, can be found online at www.grantspassoregon.gov/354/Water.

The Rogue’s salmon and steelhead fishermen like the turbidity no lower than 4 NTUs and no higher than 14 NTUs.

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@rosebudmedia.com. We’re sorry, but the volume of questions received prevents us from answering all of them.

I read the Mail Tribune’s Fishing Report every Friday, and once in a while I come across a reference to NTUs. I know it has something to do with water clarity, but what exactly is an NTU?