The Pocono plateau, nearly all of the central and south-east provinces and the north-east portion of the Alleghany plateau are drained by the Susquehanna and Delaware river-systems into the Chesapeake and Delaware Bays; the greater part of the Alleghany plateau is drained by the Allegheny and Monongahela rivers into the Ohio river; the extreme southern portion of the central province and the extreme western portion of the south-east province are drained by tributaries of the Potomac; the Erie plain is drained by short streams into Lake Erie; and a very small section of the Alleghany plateau, in the northern part of Potter county, is drained by the Genesee river into Lake Ontario.
The few small lakes of the state are mostly on the Pocono plateau, where they were formed by glaciation; here, too, are some streams with picturesque cascades.
Except on some portions of the Pocono plateau, Pennsylvania was originally well forested, and, although most of the merchantable timber has been cut, about one-half of the state is still woodland.
Oil and gas are of importance in the Lower Carboniferous Pocono sandstone of West Virginia and in the Berea grit of Ohio, where brine also occurs.
The Pocono plateau, into which the central province merges at its north-east extremity, is a continuation of the Catskill plateau southward from New York and covers Wayne, Pike and Monroe counties and the east portion of Carbon county.
Perch, sunfish, trout, bass, pike and pickerel abound in many of the streams. Yellow perch are especially plentiful in the lakes on the Pocono plateau.
The song and insectivorous birds - thrushes, flycatchers, vireos and woodpeckers - of this latitude, are well represented, and the high plateaus (particularly the Pocono plateau) have especial ornithological interest as the tarrying-places, during the migratory seasons, of many species of birds whose natural breeding ground is much farther north.