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