Alexandria, Fredericksburg, Richmond, Petersburg, and Roanoke Rapids developed at the Fall Line of the Potomac, Rappahannock, James, Appomattox, and Roanoke rivers
Source: Library of Congress, A general map of the middle British colonies, in America (Lewis Evans, 1755)
Transporting Virginia products to and importing products from Europe, the Caribbean, and South America required ships to stop at the waterfalls on the Fall Line and unload into wagons (and later trains) for travel inland. Small boats from the interior could go downstream to these Fall Line
cities, but carry the agricultural excess from only a few farms (or bars of pig iron from a furnace).
Fall Line cities, such as Petersburg, Richmond, and Fredericksburg, developed where a geologic barrier blocked shipping from going further upstream on the Appomattox, James, and Rappahannock River. In 1732, William Byrd II had lots sureveyed and "platted" Richmond and Petersburg. He knew that the Fall Line locations on the James and Appomattox Rivers were destined to grow into transhipment points. In addition, the water power provided by the rivers allowed manufacturing to develop at those locations.
On the Potomac, Alexandria is slightly east of the geologic barrier, Little Falls in the PotomacRiver, but it's deepwater harbor allowed it to compete with Georgetown to become the market center for the Piedmont and valleys west of the Blue Ridge. Alexandria had less of a natural advantage for growth, since it is not directly at the falls, but the energy of its merchants for building a transportation network into the agricultural regions upstream, and even in the Rappahannock and Shenandoah watersheds, drew agricultural trade from the backcountry to Alexandria.
Why didn't large cities develop on the Occoquan, Matta, Po, Ni, North Anna, or South Anna rivers where they cross the Fall Line?
Occoquan got its initial start when the Lee and Carter families disputed access to the Potomac River at Great Falls. The Carters thought they could develop a copper mine on their land at today's Frying Pan Park in Fairfax County. The Lees quickly patented the lands along the Potomac River, forcing the Carters to build a "ox road" to the Occoquan for shipping the copper downstream. The copper deposits were insufficient to justify further development, however, and shipping via the Occoquan was always limited in scope.
Look at the Occoquan, Matta, Po, Ni, North Anna, or South Anna on the map, and you can tell why no large cities evolved on them:
As a result, there were fewer barrels of flour, fewer hogsheads of tobacco to be shipped downstream for commercial sale, and less of an upstream market for iron tools, clothing, glass, or other items not manufactured in the local area. Some small communities developed on the Fall Line, such as Ashland, but they were limited in size because the topography limited the size of their natural "backcountry" or "hinterland" market. That natural boundary could be overcome by financing turnpikes and canals and railroads - but small communities had access to less capital, so they were usually unable to match the investments made by the cities like Alexandria.
There's one small Fall Line city in Virginia south of Petersburg, Emporia on the Meherrin River. In addition to being located in a small watershed, the Meherrin lacks easy access to the Atlantic Ocean. It drains into the Albemarle-Pamlico Sound in North Carolina, where ships struggle to traverse the shallow, dangerous inlets between the barrier islands to reach the ocean. In contrast, shipping from cities in the Chesapeake Bay watershed could easily sail or steam between Cape Henry and Cape Charles with far fewer concerns regarding channel depth or currents threatening to drive them ashore.
The existence of a physical barrier does not by itself cause cities to develop. The next transportation barrier to the west of the Fall Line, encountered by Virginia settlers in the mid-1700's, is the Blue Ridge. There are no cities on the crest of the mountains, because there was no requirement for shippers to shift from one form of transportation to another.
And the existence of a river does not by itself cause cities to develop. You'll look in vain for an urban center on the Nottoway, the Powell, or the Clinch. And while, at first glance, it may appear that every Metropolitan Statistical Area (as defined by the Bureau of Census) is centered on a river, the development of Roanoke had little to do with water transportation or even waterpower.
The Fall Line cities developed into manufacturing as well as transportation centers because the falling water provided mechanical energy for powering equipment. Today we rely upon chemical energy from coal, gas, oil, or nuclear power to create steam and electricity. Until the 1900's, however, almost all manufacturing in Virginia was located at places where the facilities could rely upon waterpower. Flour mills at Richmond and Fredericksburg, tobacco stemmeries at Petersburg, and iron furnaces at Falling Creek and Neabsco and then throughout the Valley and Ridge province relied upon the power of water to turn equipment such as stone grinding wheels or the bellows at the iron furnaces. Danville and Fries were able to develop as centers for textile processing because of the waterpower from the Dan and New Rivers.
Lynchburg is not on the Fall Line or at the Blue Ridge. Instead, it is located where the trail east of the Blue Ridge (now US29) crossed the James River. The James River and Kanawha Canal and the South Side Railroad both spurred growth at Lynchburg, and it developed into a substantial tobacco processing town prior to the Civil War.
from Alexandria to Petersburg, cities developed on the Fall Line where ocean-going ships were forced to unload cargoes
Source: Virginia Department of Mines, Minerals and Energy (DMME), The Clay Deposits of the Virginia Coastal plain (1906)
Rappahannock River Fall Line - from I-95, headed north
Great Falls has migrated upstream as the Potomac River etched into the metamorphic bedrock, but Little Falls blocked colonial shipping and determined where Georgetown/Alexandria would develop
Source: ESRI, ArcGIS Online