I originally thought that the Mid player should place two wards in the bushes closest to the stumpy bush in both rivers.
Help please :P
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Couple of good reasons to ward here.
1) Provides a lot of information on where the enemy jungler is and what buffs he currently has. This means your one ward helps all 3 lanes and the jungler to gain helpful information.
2) Allows you to safely steal wraiths on champions that push (Morg, Karthus, Galio, Soraka) which increases your farm and lowers the enemies.
3) When combined with a ward on tri brush, you will create an extremely safe situation for you and your top or bottom lane depending on which map side you are on. By warding both exits to the jungle you will not only see him much sooner then with a river ward, you make it so his only access to the river is through lane which probably won't go unnoticed during laning phase. An additional benefit to these two wards is you can save a ward on drag (assuming they can't solo it over the wall ie Tyrn, Shaco, Lee Sin) which means more gold for bot lane or an additional ward for their brush control.
Last couple notes, your link doesn't help to explain the position at all. Secondly you shouldn't need to ward both sides of the river unless you think are in a really dangerous match up, or the side lanes are constantly roaming in which case ask for help from support so you don't end up spending 600 gold on wards in the first 25 mins. Also if you ward one side of the lane feel free to position yourself closer to that side as it is going to be much safer. Every half second can count in escaping so if your 95% sure the gank will come from bottom side, create distance from it to make your escape easier.