Is it safe to follow a equestrian trail and assume I can ford where the horse-trail does? Or should I expect equestrians to ford rivers too deep or swift to ford on foot? I'm thinking of the situation in Iceland, where the topographic map may suggest routes for hikers or equestrians, and some of those may ford quite sizeable rivers.
Actually, it is often the opposite--many fords which are fine for humans are unsafe for a horse and rider, because the ground is too rocky, or the area is too narrow or steep, or the ford relies on some tool like stepping stones or a safety rope that the horse can't use. On the other hand, horse-safe fords can be muddier and deeper than those meant for humans...not because it's not safe for humans, but just because most hikers don't like to get that wet. But realistically, most riders would prefer not to cross on horseback anything that they couldn't cross on foot if they really had to...because you never know when you might have to (if your horse goes lame for some reason). So you will probably be able to cross most (but not necessarily all) horse-safe fords, but it may not be very pleasant for you and there may be another crossing nearby that would be better suited for a human.
(Side note, you made a comment about asking someone else for a ride on their horse to get across--I think you were joking, but just in case, don't do this.)
Horses are bigger and stronger and than humans and because of the square cube law, they have more mass to surface area which makes them better suited to fording deeper/faster rivers.
As to whether or not you could cross where the horses do, that will depend on the individual crossing. You probably couldn't cross at all of the places horses can, but that doesn't mean that you would be able to cross at none of them.
I live in New Zealand, land of many rivers. Whether a river is crossable depends completely on depth and flow rate.
Whether a horse or 4WD could cross it is irrelevant, because the flow rate and depth vary depending on rainfall in the high country 2-12 hours ago.
I've seen river crossings that can vary from "Where's the water?" (because it's flowing below the ground) right up to "Where's the far side?", because it's 50 metres wide, and you can't see the far side for the rain.
Rule of thumb for a crossing:
- If you can see shingle/rocks poking through the water, it's fine to cross.
- If the water flow seems "fast" then stop and find another crossing point.
- If the flow is "none-slow" you can cross up to hip depth.
When in doubt, roping up is safer. Not crossing here/now is safest.
And being tempted to try and balance on rocks to keep completely out of the water? That's a cue to get out the camera - rocks in a riverbed can be unstable and/or slippery.