What will happen if you throw your old smartphone in a river? How fast will its materials degrade? What is the timeline?

The plastic won't degrade fast.The metal components will rust very quickly.

throw it in the river and it will sink , usually if the phone isnt water proof then its buggered , water and electrical parts dont mix .

IT will take long time.

When I was in Sea Scouts one of our adult leaders was known for loosing phones in the Columbia. I think once someone tried calling it a minute or two after he dropped it and it rang as if it was still on. Obviously it stopped working and turned off after subsequent dials.

Some of the metals in the chips of the phone are toxic and take very long time to disintegrate or even will not disintegrate.

That of course depends on the water. Fresh or salt water river? Polluted or clean water? Cold or warn water? Silty or Rocky bottom? Fast moving water slow or fairly stagnant? 

Cellphones can be recycled (at Best Buy or wherever).