The James River has experienced a return of the rings.
River lovers this month found mysterious white rings running horizontally on rocks, rock walls and concrete bridge supports just above the water line.
The chalklike rings have popped up before, most recently in February 2011.
Scientists are stumped.
"As of now, we don't know the cause of the rings," said Leonard A. Smock, a Virginia Commonwealth University stream ecologist.
William Taylor, a Richmond truck driver who visited the river Thursday near downtown, said, "There's no telling what's in this water."
Most experts believe the rings are harmless calcium deposits.
Ralph White, manager of Richmond's James River Park, finds the rings beautiful.
"Maybe they are not as lovely as the flowers in spring, but they are part of the natural cycle," he said.
During dry spells, some Western lakes and canyons sport similar "bathtub rings," caused by mineral deposits.
The James River rings seem to show up only in late winter, but no one knows why. The warming of the water could play a role in helping the deposits to form, Smock said.
Paul Bukaveckas, a VCU algae expert, said the rings could be caused by calcium or other minerals in algae that attach to the rocks when the river level is fairly high and stable for perhaps a couple of months.
Then, if the water level drops — and it has been fairly low, for winter, during the past few weeks — the exposed algae would dry up, leaving the minerals behind.
The algae would have grown in a band or ring on the rocks near the surface, where the light is best. Winter is the perfect time for the algae to do that because in summer, the water level tends to jump up and down with storms.
Of course, Bukaveckas said, "it's just a hypothesis."
This year's rings could be short-lived. If they haven't disappeared by the time you read this, they could go soon because the James is expected to rise from weekend rains.
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