Hi there,
The key part is the ‘erosion’ part in river erosion model… World Machine’s erosion device functions by running water over your terrain and eroding or depositing sediment as the water flows and forms channels and then rivers from that process. World Machine’s erosion rivers are an emergent natural property of the terrain and cannot be guided in any way.
Thus, you are not wrong in not finding a “river device”! World Machine doesn’t have one directly that you can use to, for example, place a spring, or force a river in a certain location. World Machine is more concerned with the actions the rivers have on the terrain rather than the rivers themselves. This is definitely a frequently-requested area to enhance, as it would be useful to guide rivers more directly!
There are two ways that I tend to work with rivers in WM:
[ol][li]If I don’t have a specific location I want to have a river, I use the flow output from the erosion device. The flow output contains a map that contains the intensity of the water flow across the terrain; this corresponds to river locations. I usually massage this output a bit to cut off the tiny tributaries and only output larger streams when I am using it to place river texturing on the terrain or output a river location mask.
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[li]Place large rivers manually. If you are creating, for example, a Grand Canyon-like terrain, I would draw the location of the river manually in a Layout Generator using the path tool. This will both create the terrain and also output a mask that indicates the water surface.
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I will try to reply again to this post with a simple example showing very clearly what I mean… but you can also open up the example file “Tutorial 5 - Rivers and Roads” in your World Machine Documents\Examples\Tutorials folder. It shows an example of using the path tool to create a road (the same technique works well for rivers…)