Landscape slescted region convert to staic mesh?

Hi guys, Is it possible to transform a selected area, selected with the selection tool and convert that selection into a static mesh.
If not it should be a great feature to add to the engine.
I need to do this because I sculpted a river in my landscape that have cascade and waterfall. And I need to have the flat and some of the slope area to make a Static mesh of the river.
I try to do it with bsp brush, but is a nightmare.
thank-you

hey Sunset, did you every figure a way around this?