yes, that frigging water is closest thing to ice water without being in solid form, but... if you fall in your adrenaline is pumping and you will certainly have something on that will keep you from totally freezing before you drag yourself back into someone elses boat, or on shore.
A wetsuit sure is nice if you are actually IN the water, but the darn things make you sweat, and most folks shy away from them. Here is what I just bought (I needed some new gear anyway)--

A new paddle jacket with drawcord at waist, and closure at neck and wrists- THAT item will be your BEST buddy on the whole trip- I will guarantee it. You know how it is always changing gear, as the sun comes out or goes behind a canyon wall, wind picks up, etc. If you do not own one, buy one and you will use it- alot. It also works in the rain when rowing, and is great wind block.

Under that, I bought a "fuzzy rubber" short sleeve top- these come in lots of different trade names, but they are basically thin neoprene on outside (sheds wind and water), and lined with a cozy pile material that does not make you sweat, or stink as bad as a wetsuit. I think they are every bit as good as a wetsuit, and you can pull the top off by itself, w/o having to pull off a whole damn wetsuit down to your underwear. Note that a full farmer john wetsuit is really hard to get all dried out in evening, but all this other gear will dry out on a clothes line in an hour or so.

On the bottom, I wear polypropelene long underwear, with a new pair of heavyduty paddle pants over them. Perfectly waterproof for spray and rain, and not too bulky to deal with if I do fall in. Finish it all off with a pair of wetsuit booties, or some type of river shoes, and you are set. Wool socks with sandals do not work for a spring trip- too cold of water when you are walking in it, getting sprayed, etc.

All of the above I just bought new from NRS River Supply (best quality and prices, I think), for around $400 or so. I needed new gear anyway (old paddle jacket full of small holes), so this gave me chance to gear up, and that is about all I will bring.

A few other things that I think are nice to have that I toss in: some type of paddle gloves to block the cold wind and spray if we are rowing in really cold weather (anything better than nothing, but neoprene does work well), a really good weather-proof balaclava cap that comes over your cheeks and chin (I actually bring my ski fleece one, plus a very thin "paddle skull cap" that straps under my chin and is made of thin neoprene on outside, fleece inside). The skull cap does seal in alot of heat if you swim and can be a true lifesaver - note: you loss 90% of your body heat via your HEAD!! and with the wind we get down there...bring something. It also works great in camp in the AM when it is 29deg in the morning, just to keep you alittle warmer.

I think they will have all of the above at REI or a paddle shop in Boulder if you need to pick any of it up prior to trip. Just remember that the weather down there CHANGES SO FAST, you really have to have gear that you can quickly pull on/off, change as needed, and stuff it away somewhere to keep dry if you do pull it off. That's why I like the items above, as where do you put a big bulky wetsuit if it is not being used?

Check out the Northwest River Supply site, can't remember address, but you will find it...