Visual bug that distracts and obscures the vision of the player to a minor degree.
Appeared after the most recent core Arma 3 update.
It's a bugged progress bar that I think might be part of the revive/respawn files, if not the fn_net_action_hold_add.sqf files as it appears reviving/respawning once down does not initiate.
A quick way to temporarily fix it is by drinking the river water; I'm assuming it reruns the code and it completes the loop successfully.
As far as I can tell it's this progress indicator: "a3\ui_f\data\igui\holdactions\in\in_1_ca.paa"