Overturn in Upper Pond November 16, 2024
An unusual (but not abnormal) event happened in Upper Pond on Narrow River on Saturday, 11-16-2024. It was an overturn (also called a ventilation) of the bottom waters. This drone video by Steve Carey shows the effect.
If you were near or overlooking Upper Pond, you saw the milky colored water and smelled hydrogen sulfide (the rotten egg smell). The bottom waters in the deep ponds of Narrow River are naturally anoxic, meaning without oxygen. Overturns occur in the deep ponds of Narrow River when there are certain weather conditions: a dry fall, cooler temperatures, and blustery winds. This occurred Saturday because the dry fall meant less freshwater entered from Gilbert Stuart Stream and the King Tides brought more seawater from Rhode Island Sound flowing into Upper Pond. Seawater is colder and saltier, which is also denser, so when it flows into Upper Pond it sinks to the bottom and pushes the bottom water up towards the surface. The winds help mix the water. So the anoxic bottom waters come to the surface and when they come in contact with air, the sulfur is exposed and makes the water look milky and smell like rotten eggs. These conditions may last for hours to weeks. NRPA is sampling and we’ll keep you posted on what we find out.
*Scroll down this page for more photographs and information on the science of overturns.*
Email us your questions, comments and photos.
Also, I wanted to thank Laura Reed and her husband Howard who saw it early Saturday and contacted me. We were able to go profile Saturday (thank you MERL for the YSI!) and confirm the low oxygen (0.70-0.45 mg O2/L) and high salinity (22-24o/oo) throughout the water column of Upper Pond. There were normal oxygen values in Lower Pond, though salinity was also high there (24-27o/oo).
There were no visible dead fish, but Howard (who fishes) suspected any fish had moved out as the sulfur came in. There were people fishing in Lower Pond.
~ Veronica Berounsky
So what’s going on?
Narrow River is actually not a river, but a tidal estuary. Salt water from Rhode Island sound flows into the estuary and reaches Gilbert Stuart stream at high tide.
Most of Narrow River is a typical two-layered estuary, with fresh water on the top and denser salt water on the bottom. These layers normally mix where they meet.
Upper and Lower ponds of Narrow River are very deep. Upper Pond has a maximum depth of 42 feet (12.8 meters) and the deepest point in Lower Pond is 60.4 feet (18.4 meters) below the surface. These deep basins are home to a third layer of water, which is a nearly permanent anoxic (oxygen-free) zone. This is a natural condition and NOT indicative of any problems. This condition is very unusual on planet Earth and contains organisms that are adapted to the oxygen free and sulphur rich environment. The presence of these rare anoxic basins is one of the reasons that scientists come from all over the world to study Narrow River.
From time to time in such an estuary, an ‘overturn’ occurs. It takes a dry fall, cooler temperatures and blustery winds. Put simply. the anoxic layer rises to the surface. In Narrow River, this occurs about every 10 to 15 years.
An overturn is easy to identify by the milky color of the water (created by the sulfur from the bottom layer reacting with the air), the presence of a ‘rotten egg’ smell (hydrogen sulfide) and the presence of dead fish and observation of crabs escaping to shore to avoid the hydrogen sulfide. As you might think, an overturn is disruptive to the animals, phytoplankton (microscopic plants) and microbes living in the Ponds!
More details about the science of an overturn can be seen by clicking here.
The most recent overturns occurred in December 2020 (see details and photos below), November 2012 (partial overturn), November 2010 (partial overturn) and October 2007 (widespread and persistent overturn). The October 2007 overturn persisted for about seven weeks.
December 2020 Overturn
On the morning of October 10, 2020, Chris Gouveia, a volunteer water monitor with NRPA’s River Watch Program, noticed a milky color in the water in Upper Pond of Narrow River.
This is indicative of an overturn! An overturn is a unique, natural phenomenon that occurs in Narrow River every few years.
This overturn only affected about half of the Upper Pond and the milky water appears to have dissipated by October 11, so this is considered a partial overturn. A second partial overturn was observed on October 15 in the lower part of Upper Pond, through Casey’s Sill and into the northern most part of Lower Pond.
MANY MORE DETAILS BELOW!
Please email your photos of the overturn to us at nrpa@narrowriver.org. Please indicate if we may post your photos to our website, newsletter and social media and how you, as the photographer, would like to be credited.
