I could have been clearer that both descriptions were meant to be or statements.
Fast rivers or wide rivers are challenging, the former as they're inherently treacherous, the latter as they're difficult to bridge.
Narrow rivers can be bridged. Slow rivers can be traversed by boats, ships, barges, etc.
Yes, wide rivers tend to be slow, and narrow rivers tend to be fast, all else constant, but all else is rarely constant such that there are narrow slow rivers and wide fast / treacherous ones.
Fast rivers or wide rivers are challenging, the former as they're inherently treacherous, the latter as they're difficult to bridge.
Narrow rivers can be bridged. Slow rivers can be traversed by boats, ships, barges, etc.
Yes, wide rivers tend to be slow, and narrow rivers tend to be fast, all else constant, but all else is rarely constant such that there are narrow slow rivers and wide fast / treacherous ones.