To make a waterfall, Mother Nature and Father Time need three ingredients:
Waterfalls most often visited by turists are concentrated in the Blue Ridge and along the Fall Line, though there are waterfalls of various height across the state. An early writer in 1676 noted that the waterfalls on the Fall Line were dependent upon the rivers flowing east from the Blue Ridge:1
Cascades (Giles County)
Virginia has plenty of water for waterfalls. On the average, over 40 inches of rain falls each year across the state. Some is absorbed by vegetation and evaporates (actually, "transpires") from leaves and needles. The cooling effect of evapotranspiration is what keeps temperatures from reaching above 100 degrees on most summer days in Virginia.2
Rainfall that evaporates will not reach a waterfall. Normally a Virginia waterfall is less impressive in August than in April because evaporation and transpiration reduces streamflow, but waterfalls can reach impressive peaks after a summer thunderstorm or hurricane.
The presence or absence of topographic relief determines where Virginia waterfalls are located. The resistance of the rock determines the type of waterfall.
Tidewater east of Interstate 95 is the flattest portion of the state, but there are still a few bluffs over 20 feet high. Even on the Eastern Shore north of Kiptopeke State Park, bluffs provide enough of a drop in elevation to create a waterfall wherever a stream is present and sediments are consolidated enough to resist erosion.
The bedrock of the Coastal Plain consists of weakly-consolidated sediments. Streams crossing the bluffs quickly etch a valley into the soft rock, rather than form an impressive waterfall. Tidewater Virginia has gullies, rather than waterfalls, where streams cut through clay/sand sediments.
At the Fall Line, however, all three ingredients are present for making waterfalls. Over about a mile, Virginia rivers drop 50-80 feet in elevation from the Piedmont to sea level. The bedrock of the Piedmont (unlike the Coastal Plain) is hard, metamorphic rock which is resistant to erosion.
Great Falls, on the Potomac River
The James River has eroded into the bedrock from the I-95 bridge upstream to the Huguenot Bridge, so the James River crosses the Fall Line in a series of rapids rather than in one giant Niagara-like waterfall. The Potomac River has etched into the bedrock from the Teddy Roosevelt Bridge in the District of Columbia, carving a gorge upstream. The rock at the upstream end of the gorge is particularly resistant, so nearly all of the elevation drop occurs at Great Falls now. (Great Falls is 11 miles upstream from the contact between Piedmont and Coastal Plain, so the creation of the gorge demonstrates the power of the Potomac River to erode even the metamorphosed bedrock.)
Not all Virginia rivers cross the Fall Line, and not all riffles in a river are high enough to qualify as a "waterfall." Boaters on the downstream section of the Shenandoah River are well aware of the tiny limestone ledges that require some skill in "reading the water" to avoid turning over a canoe or running a bass boat aground. Bull Falls near Harpers Ferry is the most significant ledge on the Shenandoah, but the Shenandoah enters the Potomac River far upstream of the Fall Line at Great Falls.
limestone ledges in Cedar Creek under Natural Bridge (Rockbridge County)
The New River, the Big Sandy, the Holston, the Clinch, and the Powell as well as other streams on the western side of the Eastern Continental Divide flow to the Mississippi River, never coming near the Fall Line in Eastern Virginia. There are waterfalls along those rivers too. The New River flows through McCoy Falls (called "Big Falls" in the DeLorme Atlas and Gazetteer) where it cuts through Walker/Sinking Creek Mountain downstream from Radford. Further upstream, where the New River drops over a small ledge, a textile factory was built to take advantage of the waterpower. That waterfall stimulated development of the town of Fries.
Virginia's tallest waterfalls are in the Blue Ridge, because there is plenty of water, hard bedrock - and the elevation differences are greatest there. There is nearly 2,000' of potential topographic relief between the top of the mountains and the base of the Blue Ridge in the valley floor to the west, or the Piedmont to the east.
The smaller streams in the mountains can drop spectacular distances, creating impressive scenic attractions in Shenandoah National Park and along the Blue Ridge Parkway. In the Blue Ridge, streams have not finished the physical process of eroding the hard volcanic or granitic bedrock and levelling the mountains into a flat plain.
Falls of the Little River (Floyd County) and Dismal Falls (Giles County)
artificial waterfall: dam at Gatewood Reservoir (Pulaski County)