What are existing environmental rules for fighting on a raging river?

Question

I want to set up a climatic battle for my players battling opponents on multiple rafts while everything rushes down a raging river.

Has someone already set up environment rules that would cover this? How do I structure this?

In more detail, I picture something like this:

<-- River flow - <-- A  <-- PC  <-- B  <-- River flow.

• The Player Characters start by trying to escape with a bunch of treasure on a raft at the point labeled PC
• One set of opponents start on a raft downstream from the players, at A, a hundred feet away
• One set of opponents start on a raft upstream from the players, at B, a hundred feet away
• Each square raft is big enough to hold six Medium sized creatures. Maybe fifteen feet by fifteen feet. They're not fancy white-water kayaks designed to handle rapids; they're makeshift square rafts and they're unstable.
• The river is three hundred feet wide, more or less
• The river has a "fast" current.
• Lets say the PCs are all third level and there are six of them.
• A set of opponents with ranged weapons races on the river bank to get to a good point to take a shot at the PCs from the shore.
• If the PCs can get to reinforcements in some number of rounds (twelve?) they'll be safe and the risk from the encounter is over.
• My goal is to create excitement and tension. My goal is not to kill a PC automatically.

What I'm specifically looking for here is a list of the environmental threats I should track for this encounter, and how those should be implemented as Pathfinder rules. I could make something up, and I probably will if no one says, "This has already been done in (insert module or Adventure Path or source book here) "

I've put forward an answer which seems complicated and arbitrary to me, which is why I'm looking for precedence.

Answer 1

You can use chase rules: place cards for each segment of your river, tokens (or miniatures) for the different rafts... Here your finish line is the point where your PCs will get your reinforcements. The PCs have a Head Start, since the bad guys are trying to catch them.

Your obstacles can be anything that would make the ride dangerous or just slow down the raft: big patches of algae, violent flow... be creative! The algae could for example require either a DC 20 in Swim to go off the boat and push it by yourself or a DC 15 in Survival to cut them from the boat. Of course you can accept other solutions, like if one of the PCs is a Druid she should have plenty of solutions through his spells, or maybe a PC could substitute Survival with Profession(lumberjack) if she happens to have opened this skill.

As the PCs are all on the same boat, you will have to change a bit the rules here. I suggest you to make different roles on the boat, with only one being able to move the boat with chase rules, and let the PCs switch every turn. Example of what these roles could be:

• Driver (the one using the chase rules)

• Fighters (they can do a melee attack toward someone in the same case, or a distance attack)

• Helmsman (makes a charisma check and give a bonus to one other member of the crew based on the result)

• Oarsmen (can use the help another action toward the Driver)

• Sentry (gives a bonus against arrows)

• Shipwright (can make a check to repair the boat)

As you can see there is no need for the PCs to fulfill all these roles and some of them could be more or less adapted (for example as long as the raft is fine you don't need to repair it). It's up to you to decide which ones you will permit and if there is a limit on the number of characters on each one. There will also probably be some character who wants to do something you haven't decided a role for, so be ready to improvise a bit.

Depending on how many opponents there is you can apply the same rules for them, or simplify them.

Answer 2

Against the river

The rules for Flowing Water can be found on the core rulebook (page 432):

Flowing Water: Large, placid rivers move at only a few miles per hour, so they function as still water for most purposes. But some rivers and streams are swifter; anything floating in them moves downstream at a speed of 10 to 40 feet per round. The fastest rapids send swimmers bobbing downstream at 60 to 90 feet per round. Fast rivers are always at least rough water (Swim DC 15), and whitewater rapids are stormy water (Swim DC 20). If a character is in moving water, move her downstream the indicated distance at the end of her turn. A character trying to maintain her position relative to the riverbank can spend some or all of her turn swimming upstream.

Swept Away: Characters swept away by a river moving 60 feet per round or faster must make DC 20 Swim checks every round to avoid going under. If a character gets a check result of 5 or more over the minimum necessary, she arrests her motion by catching a rock, tree limb, or bottom snag—she is no longer being carried along by the flow of the water. Escaping the rapids by reaching the bank requires three DC 20 Swim checks in a row. Characters arrested by a rock, limb, or snag can’t escape under their own power unless they strike out into the water and attempt to swim their way clear. Other characters can rescue them as if they were trapped in quicksand (described in Marsh Terrain).

Do note that the surface of water has generally the same speed, while undertowns (faster movement under the surface) and riptides (ripples caused by wave movement near the bank) will not affect the surface currents.

Vehicle movement

If they are all on rafts or logs going downwards the river, they will follow the river current's speed, regardless of their skill checks. The rules for this can be found on the vehicle combat rules from Ultimate Combat:

Current: From canoes and large ships to winged gliders, vehicles propelled by currents typically manipulate an already existing power source within or outside of nature—an air current, a water current, or more exotic currents, like conduits of magical energy. Usually, manipulating a current-propelled vehicle requires a skill like Fly, Knowledge (nature), Profession (sailor), Survival, or even Acrobatics or Knowledge (arcana), depending on the nature or makeup of the vehicle and the current the vehicle is manipulating.

Water Current: Vehicles that only rely on currents of water for their propulsion are somewhat limited. These vehicles can only move in the direction and at the speed of a current unless they also employ some other means of propulsion or manipulation, and thus often have an additional form of propulsion, such as muscle in the case of a canoe, and wind in the case of a galley. A current-driven ship such as a river barge with a crew of two or more creatures requires either a Profession (sailor) or Knowledge (nature) check for the driving check, as ships require precision, discipline, and knowledge of the natural world. Smaller water-current vehicles, like canoes, use the Survival skill as the drive skill, as reading the terrain is a very important aspect of maintaining control over those types of vehicles.

If it moves with the current, a water-current vehicle's maximum speed depends on the speed of the current (often as high as 120 feet). The acceleration of a water-current vehicle is 30 feet.

A Raft vehicle can move following the river current's speed with no acceleration from the vehicle, or have muscles properling it forward to increase the acceleration up to 30 feet per turn. So, if the river currents are at 60 ft/turn, you can propel it further to 90 ft/turn going with the current, or attempt to go against the current and reduce the vehcile speed to 30 ft/turn.

Futher reading, we see that the driver of a vehicle can propel the vehicle forward or backwards by using vehicle specific actions (Accelerate, Decelerate, Keep Going, Turn, etc), or simply leave the vehicle Uncontrolled, which will make it move foward with a decreasing speed of 10 ft per turn. So in 3 turns left uncontrolled, a Raft vehicle would have acceleration 0 and move according to the river current's speed.

Jumping ships

Attempting to jump from a raft to another can be done with Acrobatics checks. Since the terrain slighly unsteady the DC increases by +2 for every check attempted.

If one raft is adjacent to another, the jumping distance is 5 feet (DC 5). However, since you do not have a running distance, the DC is doubled (DC 10). So jumping from your raft to another that is adjacent to yours should be a DC 12 Acrobatics check.

If the rafts are further than 5 feet away, the DC increases considerably (10 feet = DC 22, 15 feet = DC 32, etc). Additionally, the character who fails his check by 5 or more falls into the river, but if he fails for 4 or less, he can make a DC 20 Reflex check to grab hold on the destination raft.

Crashing

Crashing your raft against rocks or another raft could be a problem aswell, on top of taking damage, the vehicle takes -2 to AC, the DC to control the vehicle also increases by +2. If it happens to be destroyed, then it sinks and all characters will have to make the proper Swim checks against the current.

Sudden Stops: When a vehicle comes to a sudden stop—its movement is reduced to 0 in some way other than the driver using a drive action to slow the vehicle, both creatures and items on the vehicle are violently pushed toward the vehicle's forward facing a number of squares equal to 1/2 the vehicle's current speed before it came to the sudden stop. This movement does not provoke attacks of opportunity. At the end of this movement, creatures and objects take 1d6 points of damage, and creatures must succeed at a DC 20 Reflex saving throw or be knocked prone. If the movement pushes creatures or objects into solid objects, that creature or object takes an additional 1d6 points of damage for each 5-foot square the push was reduced by the solid object.

For instance, if a vehicle with a movement of 60 feet makes a sudden stop due to hitting a brick wall, its driver is thrown 30 feet toward the brick wall. If the brick wall was only 5 feet away from the driver at the point of impact, the driver moves forward 5 feet, hits the wall, and takes 5d6 points of damage. She then takes the original 1d6 points of damage, after which she makes a Reflex saving throw to see if she falls prone for the sudden stop.

Additional Rules

Additional rules can be found on the People of the River Player Companion book (page 24):

Catching a Creature Floating Downstream: As long as you are on a riverbank, water vessel, or overhang such as a dock or tree branch, you can attempt to catch a creature floating downstream as long as its path takes it through a space adjacent to yours. In order to successfully grab the creature, you must succeed at a Strength check (DC + 15 + 1 for every 10 feet per round the creature is traveling downstream). For example, catching a creature traveling 60 feet per round downstream would require a successful DC 21 Strength check. If the floating creature is helpless or unconscious, the DC increases by 10. If you are standing on an uneven or unstable surface, the Strength DC increases according to the Acrobatics Modifiers table on page 89 of the Core Rulebook. If you fail your Strength check by 4 or less, you simply fail to grab the creature and it continues downstream; failure by 5 or more means you are potentially dragged into the water as well, and must succeed at a Reflex save (DC = Strength DC above + 5) to avoid the same fate as the creature you tried to help.

You can also use a long, sturdy object such as a pole, loose tree branch, or reach weapon to pull someone out of the water from up to 10 feet away, though in this case you merely brace yourself as best you can and the creature drifting downstream must grab the object, requiring the floating creature succeed at a Reflex save (DC = Strength DC above). You don’t risk being pulled into the water when using an object to catch a creature drifting downstream.

The book also brings another rule related to fighting near a river, that could be useful to your encounter:

Swinging from Vines: As a full-round action, you can swing using a rope, vine, or similar aid within reach toward an opponent and make a single melee attack. You must move at least 20 feet (4 squares) and you must start on elevation that is equal or higher than that of your opponent. Your movement provokes attacks of opportunity as normal. This action is otherwise treated as a charge attack.

Third-party material

Finally, the Cerulean Seas is a third-party campaign setting that focus on underwater campaigns, and it has several rules for both underwater combat, fighting currents and underwater hazards. It's worth checking it out if you are interested on that kind of enviroment play.

Answer 3

The encounter should track the following details:

1. The rafts turn and spin. They do not stay straight. There's a 1 in 10 chance they will turn each round. Roll a d20; on a 1 the raft turns 90 degrees left, on a 20 the raft turns 90 degrees right. This doesn't change the raft's speed; it only changes where the players face.
2. The rafts are slippery. There's a 1 in 8 chance they will tip. There's a chance the PCs will have to make Reflex saves (DC 10) to stay on the raft.
3. The rafts move side to side. Roll a d4 and a d6, on a 1 the raft moves left, on a 4 the raft moves right; the d6 tracks the number of five-foot increments the raft moves (so on a 6, the raft moves 30 sideways)
4. The rafts move down the river. (How fast should the raft move each round? I'm going to pull a number out of thin air and say 70 feet, so they can outpace a human making a double move down the bank.)
5. There are rocks in the river. The rocks have a chance to halt (or damage!) each raft, but not every round. The rafts are AC 15. The rocks get a +1 Attack to hit the raft every other round, and do 1d6 damage. Each raft has 10 hp and hardness of 4 (softened wood). On a 20 the rock has initiated a grapple and the raft is stuck until the PCs spend a standard action damaging the raft or the rock, or freeing the raft.

Answer 4

There are concentration rules for spellcasters that would apply. Being on the raft should be basically at least vigorous motion with a chance per round of increasing to violent or even extremely violent motion because of rapids.

For ranged attackers you can use the mounted combat rules to approximate, taking -4 to attack when the above chance for violent motion comes up and -8 for extremely violent.

The rules for vehicles give survival* as the skill to pilot rafts so one player making a successful survival check should be able to keep the raft from moving violent.

*more exactly the row boat uses survival and all bigger craft use profession (sailor) so I assume that rafts should use survival, too.

If you plan on having your PCs make acrobatic checks to prevent falling the following could be important:

If you take damage while using Acrobatics, you must immediately make another Acrobatics check at the same DC to avoid falling or being knocked prone.