Here's my thoughts on driving up & down steep inclines, based on my years of experience driving tractors and atv's.
First, do you happen to have an ATV and/or a UTV? If so, have you driven either one down into the river basin? If so, how did that go? If you don't have one, I suggest borrow one or invite a friend over to see if they can drive down it and then back up with no trouble. My point is, if you have trouble driving back up that 45 deg incline slowly with a ATV or UTV, you will almost certainly have trouble on that same incline with the tractor. If the ATV/UTV handles it relatively easy with not much loss of traction, then you *should* be ok to attempt it with the tractor.
If you are planning to drive down the hill with a bucket loaded down with material (gravel, etc), I would probably drive down the steepest part of the hill in reverse. This will put all the weight of the FEL and the bucket full of material at the higher end of the tractor. This should keep the center of gravity somewhere closer to the rear of the tractor, since the bucket with material will weigh more than the rear blade itself will. My thinking here is that if you drove down the hill in normal forward fashion with a bucket loaded with gravel, the CofG will shift to the lower (front) end of the tractor, which may cause the rear end to become lighter, and the rear tires may want to lift up. The rear tires give you the best chance for braking, although in 4x4 mode, the front tires will help with braking. But then, compare their ground contact patch to the size of the rear tires.
Now, whether or not you can get back up is hard to guess with out actually seeing it.... if the traction is good (not a whole lot of loose, sand/dirt or loose rocks, I would think it would make it back up a 45 deg incline in low range going very slow and driving in forward direction like you normally do. The slower you can go while still maintaining traction gives you more time to react, should something go awry. If it were to start losing traction and start sliding, be ready to drop the blade and the bucket quickly, which will hopefully stop it.
A few pictures of the hill from different perspectives would help here a lot. I will say that a 45 deg incline is about as steep as I would ever want to drive up or down on a tractor. It's tricky, but doable, you just have to be extra cautious! If you do attempt it, definitely have someone there to act as a spotter, and take a video of it because I would like to see it.
