CHAMBERSBURG, PA. -- Falling Spring, one of the four premier trout streams in south-central Pennsylvania, long has been revered by mid-Atlantic fly fishermen for its wealth of big fish and the insects they feed on.

But the little stream's future is imperiled by changing land-use practices in the Cumberland Valley, where housing, industry, farming and other forms of economic development charge ahead, even in hard times like the present.

Dennis LaBare and a band of compatriots with several Washington-Baltimore area Trout Unlimited chapters hope to reverse the degradation of Falling Spring, stretches of which are nearly devoid of trout as silt and other pollution creep in.

The goal of his nonprofit Falling Spring Greenway Inc. is to raise $3 million to buy land along 3 1/2 miles of the banks and protect a narrow strip. The little meadow valley would be kept pristine so fly fishermen and nature lovers could enjoy it forever, regardless of how much development takes place.

So far, the fledgling project has raised about $15,000 in start-up funds, but LaBare said well-heeled foundations have expressed interest and he has high hopes of succcess. For details, write Falling Spring Greenway Inc., 8903 Flagstone Circle, Randallstown, Md. 21133.