Who do I even contact about OHV damage on a forest service road?

It's likely that reporting the specific, repeatable damage will actually be more effective than just saying 'SxS are ruining the road.' When you call the ranger district, they don't typically respond to a complaint about general traffic flow; they respond to violations of regulation or specific threats to public resources. You need to frame this as a resource degradation issue. For example, instead of saying 'they are destroying the road,' report the specific, measurable impact. 'There is a low-lying crossing at coordinates X, Y, where multiple vehicles are consistently creating deep rutting that is currently unable to support reasonable weight, leading to drainage issues that are rapidly accelerating.'

That changes the conversation from a noise/traffic nuisance to an environmental damage incident, which is exactly what the FS cares about. Documentation is key. Since you are alredy worried about being the guy who gets things shut down, take detailed photos of the damage, focusing on the ruts and the erosion. Use a ruler or a known object for scale. Time-stamped photos showing the deterioration over a few weeks will make the case undeniable. Mention specific times of day or days of the week when the worst abuse occurs. This helps them pinpoint when to send patrols.

Regarding the outcome, a complete gate installation is one possibility, but it's not the only one. If the damage is severe enough, they may just implement a weight limit or restrict access to that specific low-water crossing. If the problem is concentrated in one area, they might be able to restrict access to that specific mile of road while allowing travel on the rest of the corridor. The point is that the data you send helps them choose the least restrictive intervention posible while protecting the resource. Focus on the evidence and the specific location of the degradation. That's the only way to get a response that actually lasts.

Yeah, framing it as resource damage is what worked for me. We had a similar thing on a forest road by my cousin's place, and he emailed the district office with photos of a washed-out section from three diffrent weekends. Got a call back within a week and they ended up putting a couple of water bars in and regraded the crossing, never gated it. The ranger even told him the photos with visible landmarks made it easy to find the spot without GPS. So add a note about which pullout or tree the damage starts near, it genuinely helps them get a crew out there fast.

One thing I'd add is to check wheather that road has an actual open road designation or if it's a designated route on the Motor Vehicle Use Map. If it's open to all vehicles, SxS running it isn't a violation in itself, wich means the district has to treat it as a maintenace issue rather than an enforcement one. My local forest publishes the MVUM online and I've cross-referenced complaints against it more than once, it changes the whole tone of the conversation when you can say 'this is route 43.1, seasonally open, currently deteriorating beyond drivable condition.' Also worth logging dates rather than weekends in general, since a pattern like every Saturday afternoon reads as recurring use, not random bad luck.

:I'm not buying the part about them picking the least restrictive option automatically. That's only true if the district has money for maintenance, and most don't. Around here the default fix for a bogged-out crossing on a low-use road is to just close the segment because nobody can justify spending on repairs. The data helps them justify a closure as much as it helps justify water bars, depends entirely on what's already in thier budget that year. Worth asking whoever you talk to what the actual fix options are before you hand over a stack of photos.