historically there have also been negative consequences. the rotating blades or gated runners of water turbines can interrupt the natural ecology of rivers, killing fish, stopping migrations, and disrupting peoples' livelihoods. for example, american indian tribes in the pacific northwest had livelihoods built around salmon fishing, but aggressive dam-building destroyed their way of life. since the late 20th century, it has been possible to construct hydropower systems that divert fish and other organisms away from turbine intakes without significant damage or loss of power; such systems require less cleaning but are substantially more expensive to construct. in the united states, it is now illegal to block the migration of fish so fish ladders must be provided by dam builders.