A diffuse pollution game
Walk a procedurally generated river catchment and repair it: plant riparian trees, dig ponds and build leaky wooden dams. The risk map you work from isn’t decoration. It’s SCIMAP, a real diffuse-pollution model, re-solving on the terrain every time you build.
Free, in the browser, nothing to install. Works with a keyboard and mouse or any game controller, and in VR on a Meta Quest.
Trailer
How it plays
Each catchment is generated from a seed: ridges, valleys, a river network, fields, woods, a village and a fishery downstream. It’s losing soil to its river, and you have three tools to fix that.
1 · The map
Press M for the risk map. Bright ground is both erodible and connected to a watercourse, and that’s where sediment reaches the river. Cycle through source risk, connectivity, erosion and in-channel risk to see why.
2 · Build
Leaky dams go in small headwater streams, ponds in hollows off the channel, and trees on the bright ground beside the water. Before you commit, the readout tells you how much land drains through the spot. If the answer is “almost nothing”, it says so.
Gather wood and stone as you go, and find the spade before you can dig.
3 · Watch it respond
Every tree, pond and dam re-runs the model, so the map and the colour of the river change the moment you build. It doesn’t flatter you, either: under the model’s land-cover weights, planting trees on rough grazing makes erosion slightly worse. The game lets you do it, and tells you.
4 · Test it
Press R to run a 1-in-30 design storm. The hydrograph plots the flood peak with your work against the same storm without it, and the village and the fishery respond to the difference. Smaller storms arrive on their own, too.
5 · Plan
Tab opens the overview: the drainage network from the ridges to the outlet, the village and the fishery, and where to find wood, stone and the spade. The risk layers show here too.
The model
SCIMAP maps where diffuse pollution is likely to come from by combining two things: how erodible the land is, and how well each point is connected to the river when the ground is wet. Downstream runs it live, in a Web Worker, on the terrain you’re walking over.
The wetness index, the Network Index, FD8 flow routing and the percentile stretch are transcribed from the reference implementation, with the formulas kept verbatim.
There are three deliberate changes: a single-sweep Network Index, in-channel risk accumulated along the flow path, and stretch bounds frozen at the start so that improvement shows as improvement. Each one is argued in the code.
Generation is exactly reproducible, so a save is just a seed and a list of what you built. That’s a few hundred bytes. Press K to copy a link that rebuilds your catchment for anyone.
The reasoning, and the bugs that looked plausible while being wrong, are written up in DESIGN.md. More on SCIMAP itself is on the SCIMAP project page.
Controls
| Keys | Gamepad | Does |
|---|---|---|
| WASD | Left stick | Walk (Shift to run); drag or right stick to look |
| 1 2 3 | D-pad | Plant, leaky dam, pond |
| F | A | Build where you’re facing |
| E | B | Gather wood, stone or the spade |
| M / N | X / Back | Next risk layer / overlay off |
| Tab | Y | Catchment overview map |
| R | RT | Run a 1-in-30 design storm |
| Z | D-pad left | Undo the last feature |
| K | Start | Save, and copy a share link |
Bluetooth or USB, Xbox or PlayStation: the browser maps them all onto the same layout. Connect it and press a button once to wake it up.
The same seed always builds the same catchment, so a class can all work on one
landscape. Add ?seed=132 to the play link,
?size=small or large to change how big it is, and
?lang=en, es, de or fr to pick its
language.
Open catchment 132, the one in the trailer.
Any recent desktop browser with WebGL. It’s built for a laptop or desktop: on a phone you can look around, but you’ll need a controller to walk. On a Meta Quest it runs in VR.
In VR
On a Meta Quest, Downstream becomes the diorama it’s drawn to look like. Open the game in the headset’s browser and press Tabletop under Enter VR: the catchment shrinks onto a table in front of you, your character stands on it, and you look around by moving your head. Or press First person to walk it at life size, through your character’s eyes.
At the table
Your character is about the size of a chess piece. As they walk, the landscape glides along beneath you, and it turns in snaps rather than swinging round, so the horizon stays level. The readout floats over their head, telling you what drains through a spot before you build.
Everything else is on the panel at the edge of the table: point the right controller at it and pull the trigger.
Zoomed out
Push the right stick and the catchment comes to rest on the table as a model about 1.4 m across, on its own plinth, with the risk map laid over it. Lean in to follow a stream from the ridge to the outlet, or crouch to see the valley’s profile in the side of the board.
First person
Press First person instead and the catchment is life-size: you see it through your character’s eyes, at their height whether you sit or stand. Look down and the build spot is a few steps ahead, with the readout floating over it; the panel waits low on your left.
Walking moves your view, so it asks more of your stomach than the table does. Turning stays in snaps, and the horizon never tilts.
| Touch controllers | Does |
|---|---|
| Left stick | Walk (left trigger to run) |
| Right stick | Left and right turn the table; up and down zoom |
| Right stick click | Jump between close up, halfway and the whole catchment |
| A or right trigger | Build where you’re facing |
| B | Gather wood, stone or the spade |
| X / Y | Next risk layer / overlay off |
| Grips | Previous and next tool |
| Point at the panel | Storm, undo, save, pick a tool or layer, and leave VR |
A Meta Quest and its own browser: no app and no install, and the same link as everywhere else. The Enter VR button only appears in a browser that can start a VR session, so you won’t see it on a laptop. Other headsets that support WebXR may work too.
The table is set from your eye height when you start, so either works. If it ends up in the wrong place, hold the Meta button on the right controller to recentre.
A headset draws every frame twice, once for each eye, so it gets a lighter scene than a desktop does: fewer trees, and shadows updated less often. The model underneath is exactly the same.
No install and no account. Your catchment is generated in a second or two.