According to professor Brian Cox all winding rivers has a has a ration between the river wave length
and the river with, between 10 and 14.
This river has a ratio near 10.
Creating a smooth profile (mask) with zero values and zero slope outside the limiting values x1,x2:
p = smooth step(x, x1, x2)
profile = 4*(1-p)*p
This profile has a max of one in the mid point of the range.
To create a flat river bottom the constant 4 is replaced with 4.2 and clamped to 0..1
The windings are created by replacing the x above by:
x' = x + 1500*perlin([0,0,z],1500, )
The profile (mask) multiplied by the original displayment (convert disp to scalar)
is the mount od disk to subtract to the original disp.
