Thanks to everybody who weighed in on this. I believe I have found the culprit and it is me. Yes, a higher ISO and/or IS would have eliminated this motion blur. However, it wouldn't have happened in the first place had I taken the time to make sure everything was solid before I snapped the photo. That's what happens when you get in a hurry; you skip over some of the basics that you've been doing for a long time! All of the Canon 70-200 L's, regardless of aperture or IS, are really sharp, so that's why I was surprised to see this at first. I've used this same set up in creeks with exposure settings of .5 to a full second without any problems before.
In any event, I'll take care to keep everything still next time. Thanks again for the feedback.