A rare kind of mussel has suddenly exploded in population this year in two Anne Arundel rivers, coating bridge pilings with its brown shells and filtering the water to its clearest condition in years.

The dark false mussel, about the size of a pistachio nut, is usually found only in small concentrations on submerged oyster bars.

But this year, tens of millions of the bivalves have appeared on pilings, jetties and piers in sections of the Magothy and South rivers. In some places, scientists estimate that their shells are packed 30 or 50 per square inch.

In previous years, said Christopher C. Judy of the Maryland Department of Natural Resources, "you would have had to look hard to find them."

"Now, when you go to the area, every piling, almost every wooden bulkhead" is covered, said Judy, who is director of the shellfish program.

The apparent reason for the mussel's growth is a two-year period of heavy rainfall, which has put more fresh water into the rivers, Judy said. The saltiness of the river water has been lowered to a level at which the mussels thrive, he said.

There is no shortage of food. The mussels eat microscopic algae, which is unnaturally abundant now in the Chesapeake Bay and its tributaries because of pollution. As the mussels have used their tiny filters to extract algae from the water, the rivers have gotten markedly cleaner, scientists say.

In the Magothy, underwater visibility has increased to nearly double its normal level. Surveys by the state Department of Natural Resources have found that, in most years, the water is so clouded by algae in July that objects disappear at a depth of about .9 meters.

But this July, a survey found that visibility had reached 1.6 meters, said Peter J. Tango, a scientist for the Department of Natural Resources.

The improvement in water clarity may not be entirely caused by the mussels. Scientists have noticed clearer water all over the upper bay region this year -- and in many places where there are no mussels. Tango said that large colonies of algae may be responsible for some of the filtering.

On Monday in the South River, the water was clear enough that a blue crab could be seen scampering across the bottom in three feet of water.

"Normally in summer, you wouldn't be able to see more than a foot," said Judy, staring into the water near the Riva Road bridge.

Nearby were large concentrations of the mussels -- called "false" because they are more closely related to clams than the ocean mussels that seafood lovers eat.

Judy stepped out of his small boat near one jetty and said he felt them under his sneakers.

"Ooh," he said. "I feel that soft, crunchy mussel carpet."

What will happen to the mussels is uncertain, Judy said. If the rains ease, and the river water becomes saltier, the mussels may not be able to reproduce as well. But the existing mussels are still likely to live for three to five years, he said.

On the Magothy, the clearer water has brought more swimmers and kayakers to the river than in previous summers, said Paul Spadaro, president of a residents' group called the Magothy River Association.

"Scientists have been working for years to see some sort of an improvement," Spadaro said. "In one six-month or nine-month period, it's about a thousand percent better."

One unexpected consequence of the clearer water, Spadaro said, is that his children are afraid to swim in Cattail Creek, a Magothy tributary that runs by their house in Severna Park.

They could see the bottom for the first time -- and were scared of stepping on the rocks they saw, Spadaro said.

"I keep on telling them the water's supposed to be clear," he said.

These rare mussels are from the South River, one of two local waterways where the dark false mussel is proliferating because of two years of heavy rainfall.