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My players have manipulated events in my game so that they will encounter the BBEG (a 7th-level rogue) during his overland travel on foot rather than in his stronghold. Those players are clever like that. Their ambush site will be where he and his minions ford a small river. I have a few days before the game session to draw a map of the ambush terrain.
I cannot find terrain rules for wading in knee-deep moving water. The rules cover swimming a river, instead:
Currents and Flowing Water
Ocean currents, flowing rivers, and similar moving water are difficult terrain or greater difficult terrain (depending on the speed of the water) for a creature Swimming against the current. At the end of a creature’s turn, it moves a certain distance depending on the current’s speed. For instance, a 10-foot current moves a creature 10 feet in the current’s direction at the end of that creature’s turn.
I have to decide whether the shallow ford counts as difficult terrain, greater difficult terrain, or uneven ground.
Uneven Ground
Uneven ground is an area unsteady enough that you need to Balance (see Acrobatics on page 240) or risk falling prone and possibly injuring yourself, depending on the specifics of the uneven ground. You are flat-footed on uneven ground. Each time you are hit by an attack or fail a save on uneven ground, you must succeed at a Reflex save (with the same DC as the Acrobatics check to Balance) or fall prone.
The party has two 4th-level rogues, so terrain that leaves the enemy flat-footed, especially an enemy with Deny Advantage to ignore flanking and surprise attack, would be extremely useful to them. Everyone in the party has good ranged attacks. I would adapt the uneven terrain rules so that falling moves a creature 5 feet downstream rather than injuring them, which could sweep the creature into deeper water.
Would uneven terrain be too extreme? It won't last long, for the ford would be only 25 feet wide, unless the target is knocked prone and swept into deeper water.
Uneven terrain sounds fun, and it cuts both ways. I'm a fan of the environment being a big part of a fight. Especially here, if the villains have a reason to be smart about tactically using the river it could be a much more difficult fight for the party.
I say go for it, and be clear where deeper water is (if someone slips and is brought down river or if they just want to try and swim), and maybe add some cover here and there to mix things up for the ranged party members.
They'll remember this fight.
I would also increase the movement cost of moving through the ford. So it would be both (greater) difficult terrain and uneven ground.
So moving through the water is slow going, and you risk losing your balance and falling into the water and being swept downstream. Sounds about right for a river crossing.
Sounds like this BBEG got seriously outmaneuvered.
There's also the potential to give vision penalties (e.g. concealment) for creatures prone in the river due to the kicked up sediments. Such penalties might increase each round of fighting and go down over time based on the speed of the current, or you could treat areas as effects (like Obscuring Mist) that move downstream at the rate of current* and get kicked up every time someone moves (for every square they move through) or attack (their square). Depending on how much work you're willing to put in (obviously digital makes this easier).
*Or some multiple thereof, as the "current move speed" is how far it can push a human body and light silt will move much faster. Similarly, however far you decide that the silt moves each round is how far downstream the initial placement will go from the source. Eg. if you decide that the silt move downstream 15 feet a round, then when someone moves, they're going to kick up silt in a 15 foot wide area.
If the water is still (slow moving enough that it can't push characters) then the effect would be more "billowing cloud" that starts in the squares people move through and spreads outwards in all directions and won't settle for minutes.