The drainage of Loudoun can be divided into two provinces.  One is the Potomac province, which is drained by a system of small tributaries of that stream.  Its elevations are quite uniform and are referable to that master stream, whose grade is largely determined by its great basin beyond the “Catoctin belt.”  The second province is the region drained by smaller streams, chief of which is Goose Creek.  In this province the drainage lines head entirely within the “Catoctin belt,” and the elevations are variable according to the constitution of the rocks in the belt itself.

The tributaries by which the drainage of the two provinces is effected are Catoctin Creek, North Fork Catoctin Creek, South Fork Catoctin Creek, Little River, North Fork Goose Creek, Beaver-dam Creek, Piney Run, Jeffries Branch, Cromwells Run, Hungry Run, Bull Run, Sycoline Creek, Tuscarora Creek, Horse Pen Run, Broad Run, Sugarland Run, Elk Lick, Limestone Branch, and as many lesser streams.

The general slope of the county being to the northeast, the waters, for the most part, naturally follow the same course, as may be readily perceived by reference to maps of the section.  The streams that rise in the Blue Ridge mostly flow to the eastward until they approach the Catoctin Mountain, where they are then deflected more toward either the north or south to pass that range by the Northwest Fork and Goose Creek, or by the Catoctin Creek which falls into the Potomac above Point of Rocks.  East of Catoctin Mountain the streams pursue a more or less direct northern course.

Goose Creek, a right-hand branch of the Potomac River, is a considerable stream, pursuing a course of about fifty miles from its source in Fauquier County to its junction with the Potomac four miles northeast of Leesburg.  It once bore the Indian name Gohongarestaw, meaning “River of Swans.”  Flowing northeastward across Loudoun, it receives many smaller streams until passing the first range of Catoctin Mountain, when it claims a larger tributary, the North Fork.  Goose Creek represents subsequent drainage dependent on the syncline of the Blue Ridge and dating back at least as far as Cretaceous time.  Its length in Loudoun is about thirty miles, and it has a fall of one hundred feet in the last twenty-two miles of its course.  It drains nearly one-half the county and is about sixty yards wide at its mouth.

Catoctin Creek is very crooked; its basin does not exceed twelve miles as the crow flies, and includes the whole width of the valley between the mountains except a small portion in the northeastern angle of the County.  Yet its entire course, measuring its meanders, would exceed thirty-five miles.  It has a fall of one hundred and eighty feet in the last eighteen miles of its course, and is about twenty yards wide near its mouth.

The Northwest Fork rises in the Blue Ridge and flows southeastward, mingling its waters with the Beaver Dam, coming from the southwest, immediately above Catoctin Mountain, where their united waters pass through a narrow valley to Goose Creek.