How did the James river serve as a natural defense during The US Civil War?

Question

In a text I read it the following was stated:

“Although Richmond's proximity to Washington, D.C., was hazardous strategically, Virginia's topography—the Appalachian Mountains and rivers, such as the James and Rappahannock, that flowed east to west—served as a natural defense against invasion.”

But how did rivers act as a natural defense to prevent the North from invading into Virginia?

Answer 1

Any river can provide a natural defence if it is difficult to cross it with an army. In these cases, defenders can concentrate their forces at bridges and fords and so deny an enemy access.

The James River was navigable by ocean-going ships as far as Richmond at the time of the Civil War, and this made it an effective barrier against the North. The Rappahannock River had several fords where crossings could be made, and some of these were the scenes of some of the fiercest pitched battles of the American Civil War.

Answer 2

Any attack across any significant body of water, at any time, puts the attacker at a stiff tactical disadvantage.

1. Only a few points where crossing is possible (fords, bridges, boat landings), allowing defenders to focus on those points
2. Crossing is slow (wading a ford, or funneling your troops through available boats / across a bridge), leaving you exposed and giving the enemy time to bring in reinforcements
3. No cover while crossing (both from being spotted and being shot at)
4. Troops in vulnerable disarray for some time after the crossing
5. If opposing bank is held in force, uphill fighting against prepared positions with no room for maneuvering
6. No real avenue of retreat (see points 1-3 above)

Depending on the time frame / geography, you might also be out of effective range of your own archers / artillery / ... on the far side of the body of water, while being in effective range of the enemy's.

This makes even medium-sized rivers like the Rappahannock tricky to cross. As this video states, the crossing of the Rappahannock was...

...the first riverine crossing under fire in American military history.

All this makes a river a "natural defense". Easier to defend than open ground, in any case.