I have in my kit 65, 75, 125, 150, 210, 300.
I could see my 210 and 300 as very useful to capture details on the river. My 125 and your 135 are useful for moderate wide angles. FYI, Your Nikkor is a plasmat derivative. I'm not sure what the river is like (ie deep canyon, open country, glacial U-valley). In a very deep canyon, a 65-75mm might be more useful if you want to capture the entire canyon. But based on the photos I see on their site I think the 135 and 210 will compliment each other well.
A Sinar P is definitely not the easiest camera to haul around though. If you can get a field camera (eg Tachihara, Shen Hao, Ebony, etc...) for that trip instead, it will be much easier to carry due to the form factor.