ALLEGHANY, a river of the United States, which rises in the north of Pennsylvania, and after flowing about 300 miles, first in a northerly, but for the greater part of its course in a westerly direction, during which it passes for a short distance into the state of New York, unites with the Monongahela at Pittsburg to form the Ohio. The country through which it flows is mostly hilly, and large numbers of pines, white oaks, and chestnuts grow upon its banks. It is navigable for small steamers for about 290 miles above Pittsburg.