High Plains

I'm postponing writing about the astral monk for now since I need to look into it more.
Continuing upstream, the journey has become less arduous for a change. Bypassing the waterfall, the party finds themselves in an upper mountain valley. The canyon walls recede to a much broader plateau, with tough scrub grassland covering the slopes on either side of the river. There are scattered low pines, but no extended forest areas. The hills rising from the river are steep, but not impossibly so.
The river widens by about a third, and has less eddying, but it is still flowing swiftly. Before long, you actually begin to notice signs of habitation, mostly in the form of cut timber. With the gentler grade and slower pace, the river has been drifting one direction, then another as it flows, in broad sweeping curves that break visibility upstream. There are also many places where spines of hard rock run down from the larger peaks above, and the river has cut its way around them.
Making one of these turns, you notice a lone cabin with two levels, one at the river's edge, partly overhanging the river. The upper level is offset, following the contour of the rising hill. At the back of the cabin, there appears to be an enclosed hallway leading directly into the hill itself. On the far side of the stream, you see a cribbed stone wall reaching out into the river. It has been constructed in such a way that the water flows through it with little constraint, but with it rising two feet above the waterline, there is no way to get the boat past without running it directly next to the cabin's dock.
The party need to make Perception checks (DC19) at this point. If successful, they notice chains anchored to the end of the wall running into the water. There are also chains rising from the river's edge to large eyelets protruding from the bank about 10 yards downstream from the cabin. Anyone with a sailor background will guess there is some underwater mechanism to trap the boat between the cabin and the cribbed wall. Everyone will understand this if they all fail their checks, or fail to halt their progress upstream. A grinding is heard, and the chains go taut with a wire mesh netting hanging from it preventing passage up or downstream from the cabin. A booming voice calls out, "Tie off at the dock, and await instruction."
Although very ominous sounding, the simple situation is that this is a toll gate on the river. The upper valley, into the vale of Aron's Bowl, is reasonably well populated, and the government, such as it is, has set up this toll station to make certain none of the miners sneak their goods downstream without proper taxation. Needless to say, the organization of "government" is an extremely loose one, so not everyone intends to follow their decrees. With a sparsely manned station, the setup is elaborate, and precautions taken to make people follow the process.
Unfortunately, there is only one individual manning the toll gate. Fortunately, she is a very capable wizard of fifth level.