Deschutes River, Ore.
Given that the Deschutes River originates in a mountain range named after water falling downhill -- the Cascades -- you'd be correct to expect a flow. Eventually this water flow leads to the city of Bend, Ore., where the river drops 100 feet (30.5 meters) in one linear mile (1.6 km) [source: WaterfallsNorthwest.com].
The reason for this drop is the split around Lava Island. Think you'd rather portage (carry your watercraft around) this class V to VI mile? Think again. The Web site GORP writes that "the portage over the falls is over one mile of lava, brush and hills. It is likely this portage would result in personal injury" [source: GORP]. Basically the section of river around Lava Island is so tricky you don't even want to walk around it.
Even more hardcore than walking around it is, of course, running it. GORP describes this section like this: "Class VI. Lava Island Falls is a continuous Class IV-VI rapids for about a half mile, followed by mostly Class VI interspersed by lesser rapids. Not recommended for floating" [source: GORP]. Still, while you'd be hard pressed to find a guided float trip that runs this section, there's no surfeit of kayakers and other private paddlers who run it every season. If you plan to be one of them, listen closely to these instructions from Allaboutrivers.com: "To run the second drop you have to make a hard right to left move in order to avoid a nasty hydraulic called 'Brad's Butthole'" [source: Allaboutrivers.com]. Nobody wants to get stuck in Brad's Butthole.