The bridge gives you a tailor-made way to shade for depth. Manually make you a gray copy of the whole bridge shape, shift it along the river one way or the other - though down and to the right will give the most "automatic" effect. Copy the shadow. Clip one copy to just the water shape, and slightly ripple-distort that one to match the water's ripples. Clip the other one to everything BUT the water, and warp it so it matches the water-shadow at shorelines, and bridge abutments at cliff's edge.

So far you have an indication the river is below, just not how far. Now, if your bridge uprights might be 12 feet high, to make the water look 100 feet below, give the bridge uprights shadows that are 1/8 the offset you gave the canyon-bottom shadow. If they're twenty feet tall, use 1/5 the offset, and so forth. If the angle of light would put post shadows on cliffside or water, include the length they would cast in the river-bottom or wall shadow layer.

If the angle you're casting shadows at permits, you can darken and brighten sections of canyon wall to put them in shade or full(er) sunshine. As for your canyon-wall pattern, most anything it's made of other than 'devil's postpile' crystals will look from straight above to be stretched vertically - you should probably see some lines from top toward bottom, even if not necessarily all the way.