I personally feel it takes a pretty good size river to run a sled safely. take the cowlitz, for instance, there is enough flow that it doesnt really have tight spots at log jams and such. In the summertime my dad and I will float from blue creek down to mission bar with just the oars with no problem. the only time we use the motor is to run back to the top of a hole if we hit a fish or to fight a fish. .
Anyhow, we could run smaller rivers, but it just ISN'T SAFE. You CAN'T manuever a sled with oars (or a kicker for that matter) the same way you can a drift boat. This goes back to the post about running sleds in the nooch above black creek. It can be done... but just not as safely as other rivers. You wont see my dad and I with the sled on any thing but the cowlitz. Just my opinion.