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Is it reasonable for a river to be shallow enough to ford in one location, while nearby (say within 5 miles or less), be at least 12 feet (3.6m) deep? The same river can have shallow sections and deeper sections, right?

The reason I need it to be that deep is because, in the fantasy novel I'm working on, there is a scene where a character jumps in from a partially collapsed bridge. Because the bridge is collapsed, the nearby town can only be reached via the ford.

If it matters, this is in hilly territory, i.e. the foothills of a mountain range.

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    $\begingroup$ Not only possible, it's quite common. And a 5 mile distance is much more than necessary. For instance, one swimming hole I used as a kid was an 8-10 foot deep pool under an old (but not collapsed) bridge, yet in summer you could wade across 100 yards up or downstream from it. This was in fairly hilly country, between the Catskills and Finger Lakes. $\endgroup$
    – jamesqf
    Commented Nov 24, 2016 at 17:54
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    $\begingroup$ 12 feet is too deep to ford. You should caulk the wagon and float it across or wait to see if conditions to improve. $\endgroup$
    – Mazura
    Commented Nov 24, 2016 at 18:02
  • $\begingroup$ @Mazura based on OP's question, the 12ft deep area isn't the fording point. $\endgroup$
    – Chris Charabaruk
    Commented Nov 25, 2016 at 18:08

2 Answers 2

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It's certainly possible, as long as you allow the width of the river and/or the speed of the river to vary appropriately. Let's say the deep portion is 10 feet deep and 10 yards across. Then the area across the river is 300 square feet. Since water is incompressible, the flow through any area of the river must be constant. So, if the river widens out to 100 yards, the depth must be 1 foot deep for the same flow rate. And anything in between will do. For instance, if the river is running at 1 mph at your shallows, for a narrow section, say, 5 yards across and 5 feet deep (area is 75 square feet) the flow rate must be 4 mph.

So you can mix fast, narrow and deep as you like, as long as you keep the overall flow rate the same.

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    $\begingroup$ It goes well beyond possible, and into the realms of utterly commonplace in the real world. In terms of storytelling, it's about as surprising to have a river do this, as contain water. Upvoted for the discussion of how to figure the width needed to get desired depth though. $\endgroup$
    – Leliel
    Commented Nov 24, 2016 at 19:03
  • $\begingroup$ I'll add that a river can change greatly not just with position but with time. I was once cut off for a day in a small town in Arizona when the river went from a trickle to a torrent raging across the road in the space of an hour. A large thunderstorm many miles away, a flash flood that did not surprise the locals who had seen it happen many times before! $\endgroup$
    – nigel222
    Commented Nov 25, 2016 at 17:59
  • $\begingroup$ Great answer, very helpful about the constant flow rate. I also realized that the flow rate could change, i.e. have more flow further downstream if it is added to by tributaries (that wasn't really applicable to this question though, so your answer is perfect). $\endgroup$
    – Michael Lucas
    Commented Nov 26, 2016 at 13:44
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Yes, absolutely

There is no problem in that, in 5 miles a river can completely change.

The depth of a river depends of how fast the river is , and the composition of the ground, both things can change rapidly in no time.

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