Windrows and Scumlines

What is that crud on my lake?! You may have said this to yourself after seeing rows of scum on the surface of your lake. You may have noticed that these rows appear to be parallel and pointing in the same direction as the wind. These scum rows, also called windrows, scum lines or windlanes, are caused by a rather complex water movement known as Langmuir circulation. A specific blend of wind speed and wave movement is required for Langmuir circulation to be established, but it happens rather often on many Missouri lakes.

If you want to know how this works, keep reading. Otherwise, turn the page quickly while your sanity is still in tact. It takes some 3-dimensional visualization, but I’ve put together an illustration that might help. Here goes: As the wind blows across a lake, a unit of water is moved from point A to point B. As this unit of water leaves point A, more water rushes up from beneath to occupy the space left behind. This net movement of water creates an upwelling. At point B there is now more water than before. A downwelling occurs as the excess water pushes downward.
As this continues to happen, spiraling ‘cells’ are established in the water. In the illustration the cells are the things that look like fruit roll-ups. The arrows indicate the direction of the water movement within each cell. Wherever the cells touch the surface, scum resting on top of the surface tension is pushed from the upwelling point to the downwelling point.

When you see a scumline, look around for another. The distance from one scumline to the next is equal to the width of 2 cells. If you put your boat on one of these scumlines, the water beneath you is more or less moving down and downwind. If you park your boat in between two scumlines, the water beneath you is coming up and moving downwind. When you collect your sample, be sure to position your boat over an upwelling point as opposed to over a scum line. This will help keep the nutrient-rich surface scum out of the sample.
There are other forces working in a lake that influence water movement. Otherwise, all of the water would be in a big heap on the downwind shore of the lake! We’ll discuss these in future issues of The Water Line.

(Originally published in the Winter 2002 Water Line)

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