Some sites have access to a stream with a change in elevation. These sites can use small hydropower systems to generate electricity. If the difference in elevation is above 30 metres (100 feet), and the stream runs in all seasons, this can provide continuous power with a small, inexpensive installation. Lower changes of elevation require larger installations or dams, and can be less efficient. Clogging at the turbine intake can be a practical problem. The usual solution is a small pool and waterfall (a penstock) to carry away floating debris. Another solution is to utilize a turbine that resists debris, such as a Gorlov helical turbine or Ossberger turbine.