Spline Adjust node

Currently my water level is a plane. I would like to add rivers to my world.

I need to set the height of the spline to a specific height so that it cuts through terrain at a pre-determined height. Is there a way I could do this and if so would you be willing to show me an example when you have some time?

It's an interesting use case, I wonder why no one has asked for this before. Currently I don't see a straightforward way to do it, but I'm adding in my to-do list a generator similar to object's Adjust that can change the height of the splines relatively or absolutely. Thanks for an idea!