If it's an ill wind that blows no good, it's also an ill flood that lacks some benefits.

So it was with the deluge of early November that caused havoc to upstream communities and turned the silt-laden Potomac River to the color of a Hershey bar.

For those of us who live near the tidal Potomac, the bad news is that the floods uprooted and washed downstream about 75 percent of the resurgent submerged mass of various aquatic plants. The good news is that the wash-away included a similar proportion of the voraciously fast-growing and rival-crowding hydrilla. The generally good news is that 25 percent of the plant growth remains, although even a sprig of hydrilla is bad news.

We have at hand a report on these matters from the U.S. Geological Survey, whose Reston-based scientists Virginia Carter and Nancy Rybicki did surveys of the river in September, then again on Nov. 21 after the flood.

The submerged aquatic plants, most of them (but not the hydrilla) regarded as good, began returning to the river in 1983 after an absence stemming from the 1930s. By October 1984 they were well established from Hains Point to Marshall Hall in Maryland.

The return of the aquatic grasses reflects the improved health of the river, permitting fish and other wildlife to spawn or use it for feeding grounds.

"Fishing has improved dramatically," said scientist Carter, "a fact that I can attest to personally."