Would Swimming be a Dex or Str Saving Throw

Im making a small challenge for my party on the way to a Dwarven camp. While travelling they encounter a 20ft wide river that they must cross. I’m just gonna improvise the whole thing to give them some creativity in how to solve this problem.

My question is what happens if they fall in? The river has a strong current as it leads to a section of white-water rapids that is also being fed by an adjacent river making the current more deadly.

If they fall in I am giving the chance to make a Saving throw to try and swim across. What would make more sense: a dexterity or strength saving throw, or even maybe a check instead like athletics.

Also what DC for this check would you recommend for an 8th level party?