You don't need a waterfalls.
I didn't say anything about water falls, but about water falling, or going from a higher place to a lower place, like the bowling ball I mentioned. Water flows because it is falling downhill. The bigger the hill, the more energy it releases on its trip from top to bottom. The problem in Poland is that there are very few big hills for water to run down, so the potential energy is very low. Hydroelectric power depends on two things: the volume of water moving through the turbines, and the difference in height that it drops. Poland has the first, but it doesn't have the second.
A hydroelectric dam is, though, essentially a large artificial waterfall. When the water flows over the edge, it move the turbines, like a pinwheel in the wind. The higher the dam, the faster the water falls, and the faster the turbines can turn, producing more electricity. Just like the pinwheel spins faster when the air blows faster. And the higher the difference in height the water travels downwards, the more energy it releases, and the cheaper and easier it gets to get energy from.
All you need is a swift current
A swift current happens when water moves down a big hill, that is, when there is a large height distance. The steeper the hill, the faster the current, and the more energy you can get from it. On the plains of Poland, there are no big hills, so the rivers are not very swift. So the energy you can get from them is generally is worth less than the cost of building a hydroelectric plant that could harness it.
A simple example is it doesn't take much current from a river to turn a big gear which turns a smaller gear which turns a smaller gear until the smallest gear is spinning faster then lightening generating electricity.
That doesn't make any difference (in terms of energy production). The energy produced by the flow of the water remains the same. Gears can't create energy, only transmit it. So the output of the turbine remains the same. What would happen is that you can turn only a much smaller turbine, which produces much less electricity.
You seem to be thinking about waterwheels, which were once used to power mills. Yes, those are possible even on slow rivers. But even in the old days, the lazier the river, the bigger the waterwheel you had to build. It was more economical to either build them on swift streams, or to build a dam. Waterwheels like that were not very efficient, which is why you rarely see them nowadays. And their power output was minuscule compared to a hydroelectric dam. Like a gazillionth of what a good dam produces.
