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[Loud Whispers]

*Shock horror*
I used the search bar, and it appears no one has found a reason for river stagnation yet :/

Whilst happily going about my fortress business, the brook running through my fortress froze. I chose this time to dig out a reservoir and collect some ice - so dug out a big portion of the river. A season later, the river thaws and I happen to notice that every tile in the brook that I mined out was no longer marked as a brook tile, but instead a normal muddied tile which carried the stagnant water tag.
It's a bit hard to show, but pics anyways:

<sup>Near the river source

The boundary where I dug out the river</sup>

There's no easy way to replicate the results without playing spending a bit of time in a temperate biome, but I'm going to take a guess and say that the stagnation bug is caused by rivers no longer being treated as river tiles, whether it be through freezing, player meddling or sudden magma flooding.

[peskyninja]

I'm playing in a temperate biome, I'll try to replicate that.

[Loud Whispers]

Well, then all we'll have to do is wait for the paint to dry...

[Draximo]

I have also noticed after i mined out some ice from the brook for my awesome ice tower of death, that the brook water when it melted and started flowing again was stagnant. I never messed with the water spawn tiles at all. its been that way ever since.

[wierd]

Toady needs to put a simple flag check on the stagnant flag code.

If the water is moving, by definition the water cannot be stagnant. It can be nasty, but not stagnant.

For moving water, he could have a contaminant, like "festering filth".

But a stagnant brook? Contradiction of terms.

[Loud Whispers]

In his defense DF has a long way to go before it meets everyone's standards

On the bright side, this bug may be entirely player-caused, and avoidable.

[NW_Kohaku]

It is not player-caused. It is a bug.  It occurs on maps before changes have been made by the player, although player changes can still cause stagnation.

See the bug report here.  It is caused by the fact that "stagnant" status is placed in the damp soil surrounding a murky pool, and if that soil is adjacent to a stream, that stream then also becomes stagnant. 

[white_darkness]

Definitely not 100% player on this one.  though I do believe the freezing/thawing issue impacts it strongly.  My untouched brook is stagnant after the first freeze, whereas my aquifer source and all it's attendant plumbing is still pure, even with the multi-z-level drops works in to put the wells by the bedrooms about 10 z-levels below....

[Sphalerite]

Can a site have both stagnant and salty water?  My current oceanside fortress has several murky pools, none of which seem to be stagnant.  They're probably all salty instead.

[Loud Whispers]

player changes can still cause stagnation.

And this is what I'm looking at. Bit odd, and I don't know how it works exactly, but doesn't water have to pass a tile/come into contact with stagnant water to become stagnant itself?

[NW_Kohaku]

And this is what I'm looking at. Bit odd, and I don't know how it works exactly, but doesn't water have to pass a tile/come into contact with stagnant water to become stagnant itself?

That's the problem - stagnation can "leak" into a freshwater source simply by having a stagnant pool randomly happen to be in the same area because "damp" soil carries stagnation.

Or, in other words, if a single puddle of stagnant water is ever adjacent to a freshwater stream to within a few tiles, the entire stream will be tainted forever. 

[Loud Whispers]

My brook was perfectly fine before, and it was only until I mined out the ice that this change occurred. And it's isolated to those squares, so river tiles are supposed to apparently protect against this stagnation. It wasn't a sudden leap of water from a murky pool to the brook - it was an immediate change the moment the river thawed, affecting only the areas where the river was dug.

[i2amroy]

My brook was perfectly fine before, and it was only until I mined out the ice that this change occurred. And it's isolated to those squares, so river tiles are supposed to apparently protect against this stagnation. It wasn't a sudden leap of water from a murky pool to the brook - it was an immediate change the moment the river thawed, affecting only the areas where the river was dug.

It' because when you dug out those areas you were basically creating "pools" in the middle of your brook. These pools then when stagnant, spreading to the nearby area. If you had done it to a river instead of a brook then nothing would have happened.

[CodexDraco]

This happened to me when I pumped water out of my river but I didn't properly wall off the output. The whole river became stagnant the second when the pumped water touched the river.

[wierd]

Again, these would nearly all be solved by a simple check for the "flow" tag.

*shrug