The whole aggregate surface of the three sub basins of James, York and Rappahannoc, are com prised in Virginia, of which political section they constitute the south-eastern, lowest, and warmest part, if we except a small section of James river basin above Blue Ridge. This comparatively tro pical physical section of Virginia is followed by the important basin of Potomac, which forms con nexions physically and politically deserving of more attention than the limits of this head will admit. The Potomac derives the far greater part of its water above the Blue Ridge, and the passage of its volume through that chain constitutes so striking a point of division, as to justify separating in de scription the two sections of the basin.
Potomac, above the Blue Ridge, is formed by the North Branch or Potomac Proper, Patterson South Branch, Cacapon, Back creek, Opequan and Shenandoah rivers, from the south-west,and a series of comparatively small confluents from the north. The stream to which the distinctive name of Poto mac has been given, rises in the Alleghany chain opposite to the sources of Cheat and Youghiogany, N. Lat. 39° 10', and Lon. 30' W. from W.C.; and flowing about 30 miles to the north-eastward, receives Savage river from the north, and inflecting to the south of east, in a distance of 10 miles, tra verses one or two minor chains, and resuming a north-eastern course of 18 miles receives Wills creek at the town of Cumberland. The Potomac fed by mountain torrents, has at the influx of \Vills creek become a river of considerable volume, and again breaking through two mountain chains in rapid succession, through which the channel is so tortuous, that in one place the course is turned back wards to south-west, but quickly winding round to a course of a few degrees south of east, unites with the South Branch by comparative courses, 15 miles below Cumberland.
What is called the South Branch, is the real and main Potomac, having its remote source in Pendle ton county, Virginia, as far south as Lat. 38° 22'. The numerous mountain streams which, by their final union form the South Branch, originate in the mountain vallies, between the Kittatinny and Alle ghany chains, and from thence. by a general course of north-east, they unite near Moretield, in Hardy county, and also near the intersection of N. Lat. 39° and 2° NV. Lon. from W.C. Below the union of its principal constituents, the South Branch, with out receiving much additional water, continues its north-eastern course over Hardy and Hampshire counties, 40 miles to its union with Potomac. The
enlarged volume formed by both streams, traverses a mountain chain immediately below their junction, and again winding round to a course of north-east, continues that direction by a very crooked channel, but comparative distance of 25 miles, to Hancocks town in Washington county, Maryland, having re ceived several creeks from the north, and beside some others of less consequence, Great Cacapon from the south. At Hancockstown the channel of Potomac has reached its extreme northern point, N. Lat. 39° 41' nearly, and less than two direct miles from the southern boundary of Pennsylvania; but again inflecting to a course of S.S.E. 20 miles to the influx of Conecocheague, from the north, hav ing traversed the Kittatinny and entered the fine valley between that chain and the Blue Ridge. Be low the mouth of Conecoeheagne, the course is west of south 5 or 6 miles to the influx of Opequan river from Frederick, Berkeley and Jefferson coun ties, Virginia, but having received the latter stream, the Potomac bends to a general course of S.S.E., 20 miles to its junction with the Shenandoah, and passes through the Blue Ridge at Harper's Ferry.
The real length of the channel of Potomac will be shown partially in the subjoined tables, but it may be observed here, that in order to preserve consistency between the actual area drained, and the channel, only the general length can be ex pressed in the verbal description.
Shenandoah brings to the Potomac little less volume than the main stream above their union. The former rises in the Kittatinny valley, as far south as 55', and in the southern angle of Au gusta county, Virginia, two degrees W. of the me ridian of W.C., interlocking sources with the North, or Great Calf Pasture Branch of James river, and on the opposite side of Blue Ridge from the sources of Rivanna. The entire valley of Shenandoah is comprised within and forms a part of the great Kittatinny valley. The main stream is formed by two branches, which, taken together, drain the far greater part of Augusta, all Rockingham and She nandoah counties, and finally unite on the southern border of Frederick; the main or south-eastern branch having flown at their point of union, by com parative courses, to the N.E. 85 miles. Gradually inclining upon the base of Blue Ridge, the Shenan doah below the union of its two constituents con tinues north-eastwardly 35 miles to its junction with the Potomac, at Harper's Ferry, after an en tire comparative course of 120 miles.