I regularly ride the Katy Trail (a converted rails to trail) with my DSLR, usually with a couple lenses. My choice is a rear rack mounted bag such as a Topeak. I having padding between the extra lens and camera. Never had a problem. Personally I prefer that over a back pack as my rides are typically 30 - 50 miles and the pack is just not comfortable for that length of ride. The bag slides off the rack so I can carry it when I stop to get something to eat.

We were there a couple weekends ago and to me the biggest challenge was working around the people. There were literally thousands of people shuffling along the sidewalks so you will need to work around that.

Military ammo boxes are my choice. They come in different sizes, waterproof and ding proof. Can put foam padding in them to compartmentalize them so you can take camera and couple lenses, GPS, etc. depending on the size . Attach a rope to box and an anchor point on the kayak/canoe and you are good to go. I've used them for 20+ years.