Waterwalking is better than Gathering Storm

In Season 16 games barely ever reach minute 30.

Everyone hits 18 way before minute 30.

Gathering Storm 20 min.: + 24 AP ; 30 min.: + 48 AP.

Waterwalking grants 10 ms and 13-30 adaptive force (based on level) when in the river

A lot of things are in the river during every stage of the game:

Epic monsters 

Teamfights

Roaming

So river rune assumed to be relevant for anyone who is interesed in teamfighting and bonus adaptive stat.

WRs of midlane mages reflect that.