The use of renewable sources must be balanced with environmental aspects. Especially it relates to the use of river water flow. Environmental requirements are hard to achieve on large hydropower. Much hydro is built and yet can be built on small rivers, usually building the dams which create small lakes and prevent movement of fauna along the river. In places of large inclination of river channel the dam can be substituted by water ducts, such as pipes. The flow of water in the pipe meets resistance to its movement and some part of water pressure is lost. There is little resistance to movement of water by laminar flow but this flow exists by small velocities. So it can be counted on turbulent flow. By a given diameter of pipe the maximum power can be get when 1/3 of the water pressure is used on water movement in the pipe. The cost of pipe by a given flow is proportional to the pipe length multiplied by the root of fifth degree of the pipe length. Artificial rapids can be made which reduce pipe length. Such a hydro must be connected to a grid. It operates in the rhythm of the river.