The stretch from US-15 to Lee State Natural Area (formerly Lee State Park)was basically clear. There are some logs that you have to skip over (we were using plastic 10 foot yaks, so we just developed speed and hopped over them). The river started to really get messy below the I-20 bridge. The last two hours probably had five portages. They were all short and relatively easy, but some of them had goopy mud banks -you will get dirty.
The current was mild, forgiving actually. I think that with longer boats, some of the turns to navigate through the pile-ups will be a little dicey. But the good news is that, even if you end up sideways against a jam, the current is not strong enough to cause any immediate danger as long as you stay calm and do not roll your edge over to the waterline. And, if you are in the piles, there are plenty of hand-holds to help you get out of them.
There were some very shallow spots, just follow the darker water. There were plenty of good haul out spots for lunch/rests/swims. We saw a very large king snake, a brown water snake, an otter, a kingfisher, about ten GB herons, several groups of ducks, and many turtles. I counted seven wasp/hornets nests, but they are really out of the way as long as you stay away from the edges. None of the nests posed any threat to navigation of the river.
The river temperature was refreshing. There is a major logging operation underway just beyond the I-20 bridge. Just prior to the I-20 bridge there were two homes on the right bank. Other than those, there was no development or encroachment on the river. It is definitely a beautiful and natural stretch of river and is (opinion) worth the portages.
In my guesstimation, as long as the level doesn't drop more that three inches or rise more than six inches, conditions should stay pretty much the same. Too much drop and you will spend more time walking than paddling; too much rise and I think the current around the strainers will start to increase the danger level.
Bridges: US-15 landing, a railroad trestle at about 30 minutes, I-20 (no landing) at about 3 hours, SC-401 landing at end (about 6.5 hours).