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.