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[Mr. Palau]

So my Dwarves live next to a river with a nice little water fall, only two z-levels high. Normally they stay well away from the river and its inhabitants (just carp and vermin fish) but I decided to build a bridge (not the raising or retracting kinds but just one made out of wooden floors) across the bridge. The first time this happened my carpenter, for whatever reason, decided that it would be a good idea to build the bridge starting with the floor tiles on the other side of the river. He did not know how to swim, and although there were ramps in the river from worldgen he pathed towards the waterfall, presumably because the water wasn't 7/7 there and that meant he wouldn't need to swim and could just walk over to build the bridge. There was one critical flaw in his plan however, the water could push him off the top of the waterfall and then he would be stuck at the bottom and die. 

Well that’s exactly what happened, so I killed the game and reloaded an auto save, had him start again, and watched him this time, this was when I found out what he was doing. Before this I hadn't known what he was doing, no combat report so nothing pulled him into the river, carp were all on the other side of the water fall so it wasn't them, like I said before it appears he just pathed into the river thinking he could make it out. So I canceled all the build orders for the floor tiles on the far side of the river and forced him to start building it from the near side. He then successfully built the bridge.

I had a couple of trees cut down on the far side of the river, in order to build a bridge across the fork of the river ( the map was divided into three parts separated by a river that forked halfway across the map, the second bridge was to connect the part that was separated by the fork to the other two parts). So the wood cutter went over the bridge like any sane intelligent creature, which clearly dwarves aren't, and cut the trees down. Before I could have my carpenter go across the river to complete the bridge, a stone cutter tried to cross using the waterfall, since he also had hauling turned on presumably to take some of the wood inside to one of the many wood stockpiles.    He promptly fell down to the bottom of the river, and fell of the water fall. Now that guy didn't die, because I saw the guy go into the river, so I was able to act quickly. I had my miners dig a staircase down to where he was and save his life. The question I have, now that all information I believe is relevant has been presented to you, is has anyone else experienced this, and why do the dwarves do this when they could just take the bridge?

EDIT: was wondering how I could post screen shots here, because that would make explaining this a lot easier.

tl;dr, dwarf drowned, didn't  go over bridge, y?

[czolus]

Sounds like there's no prioritization in the pathing algorithm to avoid water.  Or, rather, that all tiles have the same weight (likelihood of being selected) so long as they can be traversed (which 7/7 water can, technically).

[Crioca]

Yeah I had fun with this; dwarves running right along the edge of a waterfall.

Six of my seven starting dwarves chilling at the bottom of a waterfall before drowning to death.

[Mr. Palau]

So what you are saying is I should restrict the waterfall so that the Dwarves don't drown, good idea. wish the pathing algorithm was a tad more advanced in it's calculation in odds of drowning. this is one of the downsides of worldgen including ramps in rivers, the weird thing is however the dwarves only go for the par tof the river where the water is shallow, AKA the waterfall, if the algorithim doesn't account for water, why didn't he go into the 7/7 area (the area by the water all is between 1/7 and 2/7). Oh and another guy is now stuck at the bottom, hopefully I will get to him in time.

EDIT: Yeah with rampws now put in rivers in worldgen the pathing algorithim needs to take into a account the danger of falling of a water fall to dwon to death.

[lemmily]

as a work around could you set the waterfall area to restricted traffic? d-o-r and the bridge to high traffic d-o-h see if they take that as a hint to not go near the splashy cliff of doom

[Mr. Palau]

yeah I did exactly that, and it works perfectly now, they avoid the waterfall and go over the bridge, and the last guy who fell down took the new staircase up and was fine. Its weird you'd think that almost drowning would bother them... but they just go on with thier lives as always.

[Vehudur]

Sometimes you'll see a "Concerned about the lack of air recently" bad thought.

[smeeprocket]

For building, I have heard that the dwarves follow a first in, last out pattern. So lay down designations for floors, etc, in reverse of the order you want them to build.

[guale]

The first in last out issue isn't the problem here. I'm guessing you're on the east side of the river trying to build a bridge to the west? Dwarves prioritize building from the west side of a construction to the point where they will wall themselves out of the fortress which is fairly well known. I've never heard of this problem, I'm guessing it is a result of the new rivers. I know the old work around was to designate a wall and forbid it one the sides of a construction you don't want Dwarves to stand before designating your constructions you want build and they will not stand on those tiles.

[Girlinhat]

Any creature will walk through 3/7 water, it's 4/7 where "swimming" starts, and dwarves won't path somewhere if they have to swim there, even if they're legendary swimmers.  So, what happens is that at the waterfall, you have 7/7 water, then you suddenly get 5/7 and 2/7 at the edge because it's quickly flowing over the cliff.  The very edge of the waterfall remains at 3 or less, and is thus pathable terrain and dwarves will attempt to walk through it.

1: d-o-r and restrict the area around the top of the waterfall.  This makes pathing some 12x more expensive.  So, if you have two routes, one 10 tiles and the other 120 tiles, and you mark the 10 to "restricted" then both paths will be equal pathfinding weight.  This can effectively make dwarves extremely unlikely, or entirely impossible, to go through a certain area.  For best results, cover an area around the top of the falls to make sure it gets proper coverage.

2: Build a bridge over the top of the falls, though this would be a little messy and not look as good, it will cause them to walk over the bridge instead.

[Mr. Palau]

The first in last out issue isn't the problem here. I'm guessing you're on the east side of the river trying to build a bridge to the west? Dwarves prioritize building from the west side of a construction to the point where they will wall themselves out of the fortress which is fairly well known. I've never heard of this problem, I'm guessing it is a result of the new rivers. I know the old work around was to designate a wall and forbid it one the sides of a construction you don't want Dwarves to stand before designating your constructions you want build and they will not stand on those tiles.

yeah I found a work around but for the record I was building a north-south bridge, and it was in a straight line. So I think the dwarf just decided to take the scenic route.

[Skyrunner]

Sometimes you'll see a "Concerned about the lack of air recently" bad thought.

That made me laugh.

[MagmaSolutionsInc]

I lost 10 dwarves in a fort to this.

Then the local vampire also fell in, and sat at the bottom, deeply pissed for all eternity.

I guess there's a bright side to it.

[guale]

The first in last out issue isn't the problem here. I'm guessing you're on the east side of the river trying to build a bridge to the west? Dwarves prioritize building from the west side of a construction to the point where they will wall themselves out of the fortress which is fairly well known. I've never heard of this problem, I'm guessing it is a result of the new rivers. I know the old work around was to designate a wall and forbid it one the sides of a construction you don't want Dwarves to stand before designating your constructions you want build and they will not stand on those tiles.

yeah I found a work around but for the record I was building a north-south bridge, and it was in a straight line. So I think the dwarf just decided to take the scenic route.

Was he building from the north then? The actual priority goes west>north>east>south>NW>NE>SE>SW.

[Mr. Palau]

hey guys sorry to necro this but apperantly this is also happens to traders. When the dwarven caravan came to my map it tried to cross the river through the shallow par tnear the water fall, just like the carpenter and the stone cutter. trouble is mercants don't respect traffic designations so they just stuck trying to cross the shallow part of the water fall, and fell down and drowned. Others have already mentioned ways to solve this, build bridge over water fall, and I have thought of my own, remove ramps near the waterfall. I am merely putting this out there to warn people that if you have a shallow water fall which traders could potentially cross, they will attempt to and die.