Yup, you can use a powerhead - that's what I have read you use to make a current in your river bed tank. Or, if you have a powerful canister filter, you could raise the filter's outflow tube above the water (and tilt it more horizontally if it's possible) so that it would make a current.
An undergravel filter (UG) is - as it's name suggests - a filter that is designed to be placed under the gravel. Not many people like it (myself included) because it gets clogged up a lot and, this way, it's not effective at all. Power filters, canister filters, or sponge filters (depending for what tanks) are the best. In my opinion, a UG filter is too much of a hassle.
That's the problem ... lol ... I guess never? Or until it gets so clogged up that it doesn't filter anymore, and you have to tear down the entire tank in order to clean the filter. That's why I would never want to have an undergravel filter. Yuck!