Duckweed

Upper Posquotank River, NC
N 36 23.139 W 076 17.193

This morning we encountered a new experience. The canal was covered with a solid green blanket of duckweed. Duckweed is not an algae, but rather a small round leaf-like thing about 1/3 mm in diameter. Millions of these float on the surface of the water. Yesterday, there were a few light patches of duckweed in the canal. They looked like wind blown streaks of dust on a shiny floor.

Today, the duckweed was so thick as to form a continuous blanket over the water 1-2 mm thick. It made the canal appear to be quasi-solid rather than liquid. The appearance was surreal. Of course Tarwathie glided through the water with only imperceptibly increased resistance. Since duckweed floats, it did did not get sucked in to our cooling water intake to clog the filter.

The most noticeable effect of the blanket was to totally still any wind caused ripples in the water. Even when conditions are totally still and the surface of the water shines like a mirror, one can see patches of ripples in the distance caused by the stirring of wind currents. The duckweed covered canal by contrast made perhaps the most perfect large-scale flat surface I've ever seen. Of course, optical mirrors and semiconductor wafers are ground much more perfectly flat than this, but the canal's
flat surface extended for miles and miles. It conforms to the earth's surface but otherwise it is flat and level.

The surface tension of the duckweed covered water was obviously very high. It would take a very powerful current or wind to disturb the surface. It occurs to me that it would be nearly impossible to model or to calculate the tension of this surface from first principles, no matter how big a computer one had at one's disposal. It makes a good illustration that no matter how much we learn about nature and no matter how powerful our calculating tools, simple phenomena of nature surround us that defy
our ability to understand in detail.

I wonder how the duckweed affects the ecology. It blocks all sunlight from penetrating in to the water, and that must have a dramatic effect on the flora and fauna below. I notice also that even fall leaves dropping from the trees do not enter the water. Instead they sit on top of the duckweed and don't even get wet.

As we passed, Tarwathie left a wake behind her of water clear of duckweed. However, within 10 minutes of passing, the wake trail closed in and there was no trace left.

As we entered the lock I notice a number of hemispherical bulges in the surface, each about 1 inch in diameter. It appears that bubbles of methane from the muck on the bottom get trapped by the duckweed and can not burst on the surface. As the level of water dropped in the lock as we cycled through, the walls of the lock got covered in a thick coat of duckweed, perhaps as much as 1/2 inch thick.

We were only two boats cycling through the lock this morning. The lock master says that just now is typically the time of peak southern migration of the cruising boats, but so far, not this year. It's curious. Do they all know something that we don't know.

Attention cruisers: The lock master said that the canal is likely to be closed before the first of November because of lack of water.

We are anchored tonight in a wonderfully isolated place. Following a tip in a cruising guide, when the Dismal Swamp Canal met the Posquotank River we turned right instead of following the left turn for the Intracoastal Waterway. We followed the Upper Posquotank River through several bends about 1/2 mile away from the ICW. We saw a fisherman when we got here, but after that we had the whole place to ourselves.

This afternoon it was 96 degrees hot. Instead of sitting still, we got in the dinghy and went for a tour of the river and the swamp. The swamp was alien and beautiful. At least the plant life was beautiful. We saw some fish and some insects, but we saw no birds, no animals of any kind. The vegetation is tinder dry. That's remarkable considering that the ground is covered by swamp water.

Tonight, soon after dark, a flock of Canada geese flew over very low. They were following the river. That's the first flight of geese we've seen so far this fall. Cool.