Since the February 6 storm dropped almost half a foot of rain on the upper Catawba basin, we have received numerous calls about the color and clarity of Lake Norman. Long time residents told us this was the worst they had ever seen the lake and wondered why it hasn’t cleared up. To answer this question we collected samples and combined them with data from NCDEQ, CharMeck Stormwater, and Duke Energy.

Why is the lake brown?

The water clarity or turbidity of Catawba lakes is determined by the concentration of suspended small particles (like clay) and algae. In the winter it is primarily driven by clay laden runoff. That February 6 storm caused the upstream lake, Lookout Shoals, to spill over for 10 days, discharging billions of gallons of muddy water into Lake Norman. Local creeks also contributed.

Is the lake unusually turbid?

Yes. Based on monthly data from the last 30 years, February had the worse or near the worse clarity recorded. Since 1990 about 98% of samples taken have met the state standard of 25 turbidity units. All sites that Catawba Riverkeeper sampled this week met state standards.

Data and graph provided by Duke Energy. 

How long will it take to clear?

In order for the water to clear, the particles either need to fall out and settle on the lake bottom or be flushed out. The average time for water to move through Lake Norman is several months, so we’re usually depending on it to fall out. The rate of settling is fairly constant but can be slowed by mixing from windy conditions, which we have had. Additionally, every time it rains more sediment is washed in. It is impossible to predict an exact date, but the water clarity should continue to slowly improve. Calm, dry weather will expedite the process, wind and rain will delay it.

Will this happen again?

While this storm was an outlier, climate and land-use changes make this more likely in the future. The latest precipitation models predict that the frequency of large storms will increase in the Southeast. At the same time, the area is rapidly developing which is increasing the amount of runoff generated.

 

Is there anything I can do?

Non-profit groups like the Catawba Riverkeeper are actively working to mitigate current and future impacts on water quality from stormwater. We need your support financially or as a volunteer to successfully execute our mission. If you found this information useful, please consider becoming a member or Water Watcher.

Finally, you can vote for candidates who are concerned about water quality. Most buffer rules and stormwater ordinances are passed at the county level. For Raleigh, look for representatives who will repeal SB 469 (SL 2018-145), which prevents local governments from requiring redevelopment to meet current stormwater codes. Read more about this bill and find a sample letter HERE.