Blogs > Boat Blog > UPDATED: Mysterious White Chalk Material Along the Croton River

UPDATED: Mysterious White Chalk Material Along the Croton River


View more images on our Flickr site

Update 4/12/13 – John Lipscomb

We have heard from Westchester County DOH as well as our Water Quality partners at Lamont-Doherty that the white material consists of the remains of Diatoms, an algae. Apparently Diatoms are “everywhere” and have a silica/glass casing which they construct from silica taken from the water. They are fresh water algae and thrive when conditions are suitable – water, light and nutrients (including silica). Nutrients can come from a wide range of sources including sewage (both treated and raw), fertilizer, animal waste, etc. When they die they leave the white silica casing.

Diatomaceous earth

Diatomaceous earth

We have now had reports that a similar white layer such as we found on the Croton River has also appeared – all at the same time – at the Sing Sing Kill in Ossining, the Pocantico River in Sleepy Hollow and at a stream in Putnam Co and another in Orange County. Everyone reports that they have “never seen this before” so while we now know it’s a “natural event” it is a complete MYSTERY to me why it seems that it has never been observed before in recent memory in this area.

One theory from Dr Andy Juhl at LDEO:
So my guess is that this phenomenon is unlikely to be directly related to anything man-made, but rather something to do with the regional weather. Diatoms like it colder than many other kinds of algae, so maybe they did better during this cold spring than the usual community, that would help create a lot of diatoms growing on the rocks. The cold spring was accompanied with really dry conditions, so that water level dropped rapidly, exposing the diatoms. To see something like this would be fairly rare, because you would need the combination of factors to see the phenomenon.

Early on in this investigation some felt that the algae was bleached by the sun. I wonder if that is correct as the white layer was just as thick, opaque and bright white on the north side of the rocks along the bank as the south side. I am trying to check this by drying rocks (with live algae covering them) in both sunny and shaded conditions. This will help me understand whether sunlight plays a part and also whether the whitening can take place over 48 hours, as observed by the watchdog who initially reported this event to Riverkeeper.
I’ll revisit the Croton R after today’s rain and see how condition have changed.


Update 4/8/13 – John Lipscomb

Below are some add’l photos taken late yesterday, Sunday 4/7, by a second public Watchdog

4/7 – Croton Dam. White material is on east wall at bottom and on west shore bank. Fisherman on left side provides scale.

4/7 – fisherman and white material just below the Dam

4/7 – west shore just below Dam

4/7 – fisherman in white material just below Dam.

4/7 – bridge support just below Dam. shows how high the pulse of water was earlier this spring. I have checked rainfall records and find no rain early last week.


Update 4/6/2013 – John Lipscomb

I visited the Croton R today and the white material is still very evident today, Saturday 4/6.
This report was submitted to NYS DEC spill response and enforcement as well as Westchester Co. Department of Health which all began investigating.

Note that the Watchdog was in the same area on Tuesday 4/2 (between 1:00 and 3:00) and saw no white material then. So it seems that the white material appeared between Tuesday 4/2 and Thursday 4/4 (from 3:00 on) – when he first saw the material and took his initial photos (attached at the end of this thread). He has fished the Croton for years and has never seen anything like this.

Photos from site visit today. 4/6. Refer to map “Substance locations” for reference – all my photos were taken in vicinity of “photo 1 and 2”

Two maps from Watchdog
Map of trails to the river
Substance locations

4/6 – Silver Lake Park, Croton. Condition seems worse here than farther downstream (south). This area is above the tide.

4/6 – All areas/rocks have a layer of white material up to about one foot above the current water level.

4/6 – Silver Lake Park

4/6 – Silver Lake Park

4/6 – There is a small town beach and a small dam just north of where the previous 4 photos were

4/6 – Silver Lake Park dam

4/6 – Silver Lake Park. Looking upstream (north)

4/6 – Farther south, between Mayo’s Landing Park (west side) and Deer Island (east side) there is less white material evident. I spoke with some local residents who believe they noticed the material about the middle of last week – which coincides with the Watchdog’s observation. I am told that this part of the River is the head of tide.

4/6 – White material on the gravel bar at the north end of Deer Island

4/6 – Rocks and shore at Mayo’s Landing

4/6 – Rocks and shore at Mayo’s Landing

4/6 -Looking north along west shore from Mayo’s Landing. White watermark visible.


Original/first Report – received 4/5/13, about a condition on the Croton River observed yesterday, 4/4.

Hey Riverkeeper,
I have a question for you. I am an avid paddler, I kayak an canoe the Croton River several times a week and have for years. I have never seen this before and can’t figure out what it is, I was wondering if you had any insight. The entire shore especially the rocks had a white chalky something covering it. The white stuff only went to the top of the waterline and got thicker the higher upstream I went. I only go half way up the river so I don’t know if it is coming from the dam or something before it. This was just yesterday so it definitely wasn’t frost or anything. I don’t paddle as often in the winter but I still do once every week or two and have never seen this. Thanks,



Tell Gov. Hochul to block invasive species at the Erie and Champlain canals
Become a Member