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Rivers and faults
Part of Map Making Tools
Rivers and faults is the last tool in the Shape stage. Rivers and lakes are worked out from the terrain and climate by themselves; use this tool to set where they rise and how they are drawn, to add, move or remove river sources, to draw rivers of your own, and to draw tectonic features (mountain ranges, subduction zones, rifts and hotspot chains) that reshape the terrain around a line.
- Rivers and lakes: rivers rise wherever the land is high enough and moist enough, and run downhill to the sea or into another river (see Rivers and lakes).
- Custom rivers: a river you draw runs exactly along your line, and the ground is shaped into a valley around it (see Custom rivers).
- Tectonic features: drawn as lines and laid over your terrain (see Tectonic features). Maps made before version 3.0.0 of the module use the original fault lines instead (see Fault lines on older maps).
Rivers and faults with the tutorial's custom river and rift listed, and the bar in Fault mode.
The rivers you have drawn, with a count in the section's title. Click a row to open the river in the inspector. Each row has Focus view (zooms the map to the river) and Delete. An empty list says what to do: choose River on the bar, then click the map.
The tectonic features (and, on older maps, fault lines) you have drawn, with a count in the section's title. Each row's swatch is the colour of its type. Click a row to open it in the inspector. Each row has Focus view and Delete. An empty list says what to do: choose Fault on the bar, then click the map.
The badge says rebuilds rivers. The section starts closed. These settings change where rivers rise and how they are drawn, and are saved with each map.
- Spring altitude (min): how far above sea level the land must be for a river to rise there. Higher values keep river sources to hills and mountains, away from low plains and the coast.
- Spring moisture (min): how moist the land must be for a river to rise there. Higher values keep river sources out of drier land, such as deserts.
- Width of all rivers: scales how wide every river is drawn. Rivers still start narrow and widen as tributaries join them.
- River meander: how much rivers meander where the land is flat. At 0 they never meander; above 1 they also meander on gentler slopes.
River sources are placed afresh every time the rivers are worked out, so changing Spring altitude (min) or Spring moisture (min) moves them straight away. River sources you have added, removed or moved yourself are kept.
The Draw modes:
- Select: click a river or fault to open it in the inspector (a fault can be picked anywhere in the area it reshapes). Nothing is drawn or moved, and rivers and faults are shown without their points. Escape closes the inspector.
- Source: add, move and remove river sources (the tooltip says Add new river source). See River sources.
- River: draw a custom river (the tooltip says Draw custom river).
- Fault: draw a tectonic feature, or a fault line on an older map (the tooltip says Draw fault line).
Choosing Source, River or Fault starts editing; the panel's state block then shows Stop placing (for Source) or Stop drawing. Start drawing in the state block starts in River. Choosing Select, or clicking the stop button, ends editing. When the bar is short of room, Source moves into More.
In River mode the bar adds:
- River width: how wide the river is where it starts: Small, Medium, Large or Extra large. It widens further as other rivers join it.
In Fault mode the bar adds:
- Tectonic fault type: Mountain range, Subduction zone, Rift or Hotspot chain (on an older map: Convergent, Divergent, Slip or Hotspot).
- Range style: shown when Mountain range is chosen: Simple, Fold belt or Rugged (see Mountain range).
- Fault line thickness: how far the feature spreads from its line, from 10 to 400 pixels (up to 1,000 in the inspector). For a hotspot chain, this is the width of the band its islands lie in.
- Fault line strength: how pronounced the feature is: the height of a mountain range or of a subduction zone's volcanoes, the depth of a rift, or the height of a hotspot chain's youngest volcanoes.
Every other setting for a feature is in the inspector. When the river or fault you are drawing is open in the inspector, the bar's settings follow it, so the rest of the line is drawn the same way.
At the end of the bar:
- Pause regeneration (the pause toggle) turns automatic terrain generation off and on. While automatic generation is on, the terrain is updated shortly after your last edit, so a quick run of edits results in a single update, and only the part of the map that changed is recalculated, so most edits apply in a fraction of a second even on large maps.
- Apply changes: while generation is paused, changes that would affect the terrain are collected; click this to apply them. This lets you draw or edit rivers and faults without the terrain being generated again after each point.
Pausing only lasts for the current tool and editing session. It turns itself off when you change tool or stop editing, and any changes collected are applied then. A Guided map's land masks have the same toggle (see New map).
Source mode: the river sources as rings, the tutorial's own source in the desert (right), and the source it removed (left).
While you edit, every river source placed automatically is shown as a ring.
- Add a river source: in Source mode, click the map. A river rises there whatever the land and climate are like.
- Move a river source: drag it. A ring dragged to a new spot becomes a river source of your own there, and a removed river source marker is left where it was.
- Remove a river source: Ctrl+Click it (Cmd+Click on macOS). Removing an automatic one leaves a removed river source marker, and no river rises there any more; Ctrl+Click the marker to bring the river source back. A river source you added yourself is simply deleted.
- Choose River or Fault on the bar, and set the bar's settings for it.
- Click to add points along the line. A new river or fault opens in the inspector as soon as you place its first point.
- Right-click to finish the line. Choosing another mode also finishes it.
While editing a line:
- Drag a point to move it.
- Shift+Click a line to add a point.
- Ctrl+Click a point to delete it.
- Double-click a point to open the line in the inspector.
Fault lines may run a little beyond the edge of the map; rivers may not.
Opened by clicking a river in Select mode, clicking its row in Custom rivers, double-clicking one of its points, or starting a new one. Its header reads "river".
- Name: the river's name.
- River width: Small, Medium, Large or Extra large. Changing it shapes the river's valley again.
- Focus view and Delete.
Opened in the same ways from a fault. Its header reads "fault" with its type, and its swatch is the type's colour.
- Name and Description: for your own reference; changing them does not change the terrain.
- Tectonic fault type: the feature's type. Giving a feature a new type shows that type's settings at their defaults and drops the settings of the type it was.
- Range style: for a Mountain range only.
- Fault line thickness: 10 to 1,000 pixels. For a mountain range, rift or subduction zone this is its width either side of the line; for a hotspot chain it is the width of the band its islands lie in.
- Fault line strength.
- The settings of its type: Arc distance and Trench depth for a subduction zone; Floor texture and Volcanoes for a rift; Spacing, Spacing change, Scatter, Variation, Pulses, Composite islands and Drowned share for a hotspot chain (see Tectonic features).
- Reversed: for tectonic features only (see Which way a line runs).
- Focus view and Delete.
In both inspectors, changes apply at once and each is one undo step. Every change except the name and description shapes the terrain again.
Rivers are worked out from the terrain and climate every time the map is refreshed, so they always match the land as it is now.
- Where rivers rise: river sources are placed from the map's seed wherever the land is at least Spring altitude (min) above sea level and at least as moist as Spring moisture (min) (see Hydrology). They are not saved with the map: only the river sources you add, remove or move yourself are saved. So when you edit the terrain or change these settings, the river sources move with the land straight away, and none rise in the sea.
- How rivers run: a river follows the lie of the land downhill until it reaches the sea or joins another river. A river that runs beside another joins it rather than running alongside it.
- Lakes: where a river runs into a hollow, water pools into a lake. Once the lake is full, the river breaks out through the lowest ground around it and flows on. A river only ends in a lake in a wide hollow with no outlet for a long way, or leaves the map where its only way out is off the edge. A small hollow below sea level inland, cut off from the sea, is treated as a lake rather than the sea, so a river runs through it. The largest lake is the same share of the world at any map size or zoom, so a regional map allows the same lakes as its parent.
- How rivers look: a river starts narrow at its source and widens as it flows and as tributaries join it. It narrows where it falls steeply, widens where the land is flat, and meanders across plains. A large river widens at its mouth, and can split into a delta where it reaches the sea across flat land. A river is drawn straight across a small pool on its course, and stops at a larger lake.
- Colour: rivers are drawn as water, in the same tint as the sea and lakes (see the Water section of Terrain), and frozen rivers are drawn as ice (see Freezing threshold in Biomes). They belong to the Water and rivers layer, which the Terrain layers list in the map controls shows and hides (see Map Canvas).
Note: a regional map has the same river sources as its parent within its area, and rivers flowing in from outside enter at its edge as wide as they were on the parent. The rivers flowing in are worked out when the regional map is created. If you later change the parent's rivers, create the regional map again to bring them up to date.
A custom river runs exactly along the line you draw, from its higher end to its lower end. It stops where the line reaches the sea or runs into another river. If your line ends on land, the river carries on downhill from the end of the line until it reaches the sea or another river, as any other river does.
A custom river is drawn in the same way as any other river. Rivers that run into it join it, and it widens as they do. It keeps to your line without meandering, and meanders as usual once it carries on beyond the end of your line.
The ground along your line is lowered into a gentle valley, so the river sits below its banks. The valley keeps the texture of the land around it and fades out into the untouched land at its edges. Where your line crosses higher ground, the valley is cut deeper, so the river never has to flow uphill (see the FAQ).
Maps made before version 3.0.0 of the module keep their original rivers until you update them (see Updating older maps). Their river sources were saved as pins when the map was made, so the hydrology settings no longer moved them, and a custom river carved a channel down its line with a river source at its highest point.
Updating such a map removes the river source pins it saved and places river sources afresh from the land and climate, as on a new map. River sources you added yourself are kept. Custom rivers then run along their lines and are shaped into valleys, as described above. Many rivers take a different course after updating, so check them afterwards.
A tectonic feature reshapes the terrain around a line you draw, in the way the real process would: raising a mountain range, cutting a trench beside a chain of volcanoes, opening a rift valley, or building a chain of volcanic islands. Each feature is worked out from the terrain beneath it, so it takes on the character of the land or seabed it crosses.
The four tectonic features: a mountain range, a subduction zone, a rift and a hotspot chain, with the hotspot chain in the inspector.
The direction you draw a line in matters for a subduction zone and a hotspot chain. Symbols along the line show the direction, using the standard symbols of geological maps:
| Feature | Symbol |
|---|---|
| Mountain range | Triangles on both sides |
| Subduction zone | Triangles on the side where the volcanoes rise |
| Rift | Ticks on both sides |
| Hotspot chain | Arrowheads pointing towards the youngest volcano |
- Subduction zone: the volcanoes rise on the left as you walk along the line from its first point, and the trench lies on the other side.
- Hotspot chain: the first point is the oldest end and the last point is the youngest.
To turn a feature round without redrawing it, tick Reversed in the inspector: a subduction zone's volcanoes then rise on the right of the line as drawn, and a hotspot chain's youngest volcano is at its first point. Untick it to return the feature to the direction it was drawn in.
A mountain range where two plates collide. Its height is set by Fault line strength and its width by Fault line thickness, and it fades out smoothly towards its ends and edges. Range style (on the bar and in the inspector) chooses its character:
- Simple: the land's own hills and valleys are raised and enlarged, so the range grows out of the terrain around it. On smooth ground it adds hills of its own.
- Fold belt: long parallel ridges and valleys running along the range, like the Zagros or the Appalachians.
- Rugged: sharp ridges and deep valleys running down from a crest, like the Alps or the Himalaya.
One plate sliding beneath another. On the side where the plate sinks, a deep trench runs along the line with a low rise beyond it. On the other side, the ground climbs over a forearc to a chain of stratovolcanoes (volcanic islands at sea, or a line of volcanoes along a coastal range on land). Its width varies a little along its length, never beyond the Fault line thickness you set.
- Arc distance: how far from the trench the volcanoes rise, as a share of the width.
- Trench depth: how deep the trench cuts into the seabed.
Land being pulled apart. The rift is a chain of tilted blocks: along each stretch, one side drops at a single steep scarp and the other falls in smaller steps, and the steep side swaps from one side to the other along the rift. The floor is partly filled with sediment and its shoulders are raised. Its width varies a little along its length, never beyond the Fault line thickness you set. Where the floor drops below sea level it floods.
- Floor texture: how rough the rift floor is, from smooth sediment (0) to young lava fields and small fault blocks (1).
- Volcanoes: how many volcanoes rise on the floor and shoulders. Set it to 0 for none.
A chain of volcanoes left behind as a plate moves over a hotspot, modelled on Hawaii. The youngest volcanoes are tall, active islands with a summit crater. Towards the oldest end they sink, round off and erode into smaller islands, then become atolls at sea level and finally seamounts below it. The hotspot's output rises and falls over time, so busy periods leave clusters of large islands and quiet periods leave long empty stretches.
- Fault line thickness: the width of the band the chain's volcanoes lie in. A young island is about a third of it across its radius, so set the thickness to about three times the island size you want.
- Spacing: the typical distance between volcanoes, in island sizes. Low values merge neighbouring volcanoes into larger islands.
- Spacing change: negative values crowd the volcanoes together towards the young end; positive values spread them out, as if the plate sped up.
- Scatter: how far volcanoes stray either side of the line.
- Variation: how much island sizes and the gaps between them vary. At 0 every island is the typical size and evenly spaced.
- Pulses: how much the hotspot's output rises and falls along the chain.
- Composite islands: how often extra volcanoes grow on an island's flanks and merge with it into one larger island.
- Drowned share: the share of the chain, from its oldest end, that has sunk below the sea as atolls and seamounts.
Tip: seamounts and drowned atolls are under water, so they are easiest to see with only the Elevation layer shown.
Tectonic features add to the terrain that is already there rather than replacing it. They look their best where the terrain already suits the process, and out of place where it does not. Before drawing one, look at the land or seabed it will cross and ask whether that feature could have formed there.
Good places for each feature:
- Mountain range: along ground that is already high or hilly (a Simple range enlarges the hills it finds), along the line where two landmasses would have met, or to make more of a range the tectonic engine has already raised.
- Subduction zone: along a coast that faces the open ocean, drawn so that the ocean is on the trench side. The trench forms offshore and the volcanoes rise along the coast, as in the Andes or Japan. Drawn across open ocean, it makes an island arc such as the Aleutians or the Lesser Antilles.
- Rift: across the interior of a continent or a high plateau, as in East Africa. If the rift should open into the sea, draw it on past the coast, since a rift rises back to the ground level at its ends.
- Hotspot chain: across open ocean, where its islands build up from the seabed, as Hawaii's do. On land it builds broad volcanic domes and cones over the ground.
Places that look wrong:
- A subduction zone or hotspot chain drawn through an existing mountain range. The trench cuts a valley through the peaks, and the volcanoes sit on the range instead of rising from it.
- A subduction zone across the middle of a continent, which carves a long trench through the land with nothing on either side to explain it.
- A rift across deep ocean, which only digs a ditch in the seabed. On Guided v2 and Advanced (tectonics) v2 maps the engine already builds mid-ocean ridges where the seabed pulls apart.
- Two features laid over each other. Each feature is worked out from the terrain beneath all the fault lines, so where two overlap their effects are added together rather than one building on the other (a hotspot chain across a rift does not fill the rift). Keep overlaps to their edges.
Note: a tectonic feature can raise terrain above 100% elevation or lower it below 0% where the process calls for it, such as a high range or a deep trench.
Note: the first time a feature is worked out, it can take a few seconds on a large map. After that, editing one feature only recalculates that feature, and edits elsewhere on the map only recalculate a feature if they change the terrain beneath it.
When you create a regional map, fault lines and tectonic features are widened to match the zoom, so they look as they do on the parent map: the same width in the world, the same texture and the same volcanoes. A feature is kept if it reaches into the cropped area, even when its line lies outside it, and it keeps its whole line so it is laid out as on the parent. Features cut by the edge of a regional map can differ slightly from the parent.
Maps made before version 3.0.0 of the module keep the original fault lines, and look exactly as they always have. On these maps, Tectonic fault type offers the original types:
- Convergent: raises a ridge along the line. It fades out smoothly to nothing at its edge.
- Divergent: lowers a valley along the line. It fades out smoothly to nothing at its edge.
- Slip: offsets the terrain sideways along the line.
- Hotspot: a chain of volcanic domes that shrink from the first point to the last.
These have only Fault line thickness and Fault line strength, and no Reversed.
Updating an older map to the latest terrain engines (see Updating older maps) converts its fault lines into tectonic features: convergent faults become simple mountain ranges, divergent faults become rifts, and hotspots become hotspot chains running from their oldest volcano to their youngest. Slip faults are removed, since there is no equivalent tectonic feature. The converted features look different from the originals, so check them after updating and adjust their settings as needed.
- No river or fault gets a label by itself. To name one on the map, add a label with the Labels tool.
- See the FAQ for why custom rivers can sometimes cut unexpectedly deep valleys, why rivers can change course after a terrain edit, and which edits are applied quickly and which regenerate the whole map.
- Press ? to see every shortcut for river sources and lines.
Next: Regions