Eh, that is a tough one. The Tator Tatshenshini Riveris known to be rough, technical and cold.
The Alpha SL
So layer up, Andrea. Id start with good lightweight wool, as it has good insulation properties even when damp or wet and in my experience is a better "regulator" of temperature than synthetics. The Icebreaker Bodyfit 200 Shakra Zip Top ($75) and matching leggings ($60) are an excellent start.
Then, a good heavy but snug layer, such as Patagonia's Capilene 4 Zip-Neck ($85), an "expedition weight" layer that will be your favorite piece. In many cases, the Icebreaker and Patagonia pieces, worn under a shell such as the Arc'teryx Alpha SL ($300, made with Gore Pac-Lite) will be enough.
But for extra warmth, I like something that has fluffier insulation than fleece. Something like MontBell's UL Thermawrap Jacket ($145) which has a smooth nylon shell for easy layering, and synthetic insulation for water-resistant warmth.
Gloves are the other critical piece. Outdoor Research's Women's Gripper Gloves ($45) are a good start. Find some Mountain Hardwear Woman's Epic Gloves ($40) that fit over the Grippers. They'll give you a shell for extra water and wind protection.
Buy a couple ten-packs of Grabers Hand Warmers ($10 a pack) as well. Just put them in your gloves for extra warmth.
Ill let your guide recommend the best footgear. Probably going to be a combination of neoprene socks and wool socks. Take plenty of dry socks!