I will answer in reverse order:
(2) the least impact to the stream ecosystem, presuming you intend to cross, is to use the preexisting track. First, this restricts disturbance to an already disturbed place (kind of like walking on the trail, not next to it). Second, you aren't going to need to break ground to another part of the stream, again limiting new disturbance. (But, did that track heading off just a bit back there lead to a bridge or safer place? Check the map.)
(1) Safety. Since somebody else managed to get across, it is conceivable that you can too (avoid places where tracks go in but none come out!). But, conceivable is not a clear yes. You need to determine for yourself if it is safe for you to cross now, as opposed to them back when. So:
- What is the current water depth, do you need a snorkel (and have one)?
- What is the bottom like (rock, gravel, sand, mud, big holes), and is it possible that it has changed since the last crossing, such as being washed out in a flash flood?
- What do the entry and exit look like, and do you have approach and takeoff angles that can handle it (and don't forget how deep the hood might go in the water on a steep entry)?
- How do you rate your abilities (previous experience), your group's abilities (if multiple vehicles), and your rescue gear (winch, traction devices, mechanical knowledge and tools if you stall out the engine with water)?
- Did you actually walk the proposed crossing to check it out?
- If all else fails, how close to civilization are you, and how good are your communications (cell, radio, PLB)? You did leave a proposed itinerary and return time with someone back in civilization, didn't you?
"Is it safe?" is ultimately about risk assessment giving the whole system a good look. If you are alone, and it was deep when you walked it, and has tree snags, and you are out in a vehicle you borrowed from a friend without any equipment, there is no shame in turning back and leaving it for another day. There is no shame in turning back in a well-equipped group if it just doesn't seem like a good risk. Even if you just feel uneasy, reconsider going - our brains have millions of years of evolution trying to keep us alive with vaguely uneasy feelings so listen to them! A little healthy paranoia goes a long way at times.
Have fun, tread lightly, and be safe.