Hydrographic (?!?) question: artificial river port

[Caenwyr]

If the rocky area is relatively small, I'd think the river would flow around it, and not over/through it. There's no reason for a river to start eroding a rocky outcrop if it can avoid it: water generally follows the path of the least resistance ;-).

You could however have rocky islands in the middle of a stream as remnants of e.g. glacial erosion (large rocks deposited there after the retreat of a glacier, and just now being uncovered by the eroding of the river). This would result in rocky islands in a still relatively smooth flowing river, eradicating the need for an artificial port altogether.

There's one restriction to that theory, however. Such huge boulders could only have been taken there through glacial erosion, which means it should be in an area that once sported a glacier, an ice field or an ice cap (any of the three would do). Which in turn means the topography should still reflect that in some way. Check out the various post-glacial landscapes if you want to know if one fits the area your city is in.

[feanaaro]

If the rocky area is relatively small, I'd think the river would flow around it, and not over/through it. There's no reason for a river to start eroding a rocky outcrop if it can avoid it: water generally follows the path of the least resistance ;-).

The idea is that the river flowed as normal through the path of least resistance, made of regular soil. However, by eroding the soil at a certain point it exposes the rocks underneath, which are not smooth but have "spikes", thus the rocky bottom and the small islands. This entire rocky formation relatively near to the surface intersects the flow of the river more or less perpendicularly, thus the river does not go around it but through it, after having eroded enough soil to uncover it at a certain point.

[Caenwyr]

The idea is that the river flowed as normal through the path of least resistance, made of regular soil. However, by eroding the soil at a certain point it exposes the rocks underneath, which are not smooth but have "spikes", thus the rocky bottom and the small islands. This entire rocky formation relatively near to the surface intersects the flow of the river more or less perpendicularly, thus the river does not go around it but through it, after having eroded enough soil to uncover it at a certain point.

Actually a river meeting an obstacle perpendicular to its original path would, at first, just pool up behind the obstacle in the form of a lake. Eventually it would, by pooling up, reach a height where it would again be able to flow freely, which would be either:

• a low point in the rocky structure, in which case this "overflow" would gradually (in the course of several thousand years) be eroded into a smooth channel, eliminating the lake behind it, or
• a low point next to the rocky structure, in which case the river would divert its entire flow to that point and, after enough erosion, eventually drain the lake.

In short: the river would first flow over the rocky structure with a lake behind it, then flow through the structure in a relatively smooth channel, or around the structure altogether if the obstacle proved too great.