Depending on what the players try to do, you could call for saving throws and/or ability checks.
Saving throws are generally made to resist some consequence, and success means avoiding or reducing that consequence. Dexterity saving throws usually imply trying to move away, and Strength saving throws usually imply trying to not move.
Either could come into play here; perhaps the bridge sways, and you tell PCs near the edge to make a Dexterity saving throw to avoid falling off. Or a PC falls in the water, and you call for a Strength saving throw to not be pulled by the river's current.
Ability checks (or skill checks) are generally made when someone tries to accomplish some goal, and success means making progress toward that goal. If you want to give the players some creativity here, then consider that they may want to jump into the water anyway and swim across. Strength (Athletics) would be appropriate, or perhaps a different ability such as Dexterity (Athletics) for more dexterous characters. You could give advantage on the check to PCs who already have swim speeds, to reflect their innate swimming ability.
The skill could also vary depending on what the PCs are trying to do. For example, Stealth may be appropriate if they intend to swim quietly, or Nature or Survival if they try to find some calm path through the rapids.
The DC of the saving throw or ability check should not depend on the party's level, but instead be based on the nature of the task. The Basic Rules provide a table of Typical Difficulty Classes, and most moderately challenging tasks could have a DC between 10-20.
The consequences of failure are up to you. Is there risk of drowning? Do PCs in the current take damage from sharp rocks? Are they pushed back harmlessly? Remember that if the DC is high (20+), failure is more likely, and you should consider the severity of the consequences appropriately.