Rafters might be able to rejoice with the Vena Microhydro System, a water-powered personal charger designed by Daniel Hull. Using a small turbine, generator, and anchor, the device can harness hydro-energy under a river or stream. With flexible fabric blades the turbine is collapsible and can fit in a backpack along with the rest of the kit.
However, Hull does not specify the required speed of the river or even how much energy the Vena Microhydro System can create. Although he claims it can generate enough hydro-electricity to power your gadgets, you may have to consider if its worth the risk of falling in. Until we know for sure, I probably wouldn’t get my hands wet.