Just be aware that this will not be a particularly comfortable seat. By nature the river rock is irregularly shaped so the bench surface will be highly textured. It's fine for standing on in our steam shower but as for seating....OUCH!

There are many 'mosaic' companies that offer the rock look as tiles in sheets that line up together. You could use that directly over a concrete base.
Before you commit totally, please consider what you may want to do using custom art fired on tile. For creative ideas see my Houzz page

Thanks for your comments: Michigammemom - it was actually comfortable because it was laid evenly and flat, so when you sat on it it felt like a smooth concrete seat. Aesthetic Tile - I will take a look at your website do you have any contemporary styles?

My work is entirely custom, so the style is all determined by the art selection and the tile sizes and shapes. I can reproduce just about any art - whether it's a painting, photograph, archival documents or maps, drawings, something abstract, whatever. A high resolution digital image will produce the best final image quality. Using your own art or that purchased without restrictions avoids licensing / copyright issues. With the right tile shapes (i.e. vertical strips) will allow you to conform to a curved shape such as the front and back of the seating area. The art tile doesn't need to cover all the area; in fact it might look best if a complimentary material (i.e. stone) is used with it. For all outdoor installations I would use porcelain tile since it is frost resistant. The finished tiles are completely weatherproof and will never fade.