"The concern is if enough volume and order flow gets diverted into dark markets, at some point we would ask whether you would have reached a tipping point at which the public price discovery from displayed markets is being hurt, " said Jamie Brigagliano, co-director of the SEC's trading and markets division.

WSJ: Ban on Flash Orders Is Considered by SEC

Over time, the channel has been altered, wetlands drained and the flow of the water controlled and diverted for use in nearby cities and farms.

BBC: Nebraska��s awe-inspiring crane migration

The flow of the mighty river has been diverted, billions of dollars have already been spent, and the work of moving more than 1m residents from the area is well advanced.

ECONOMIST: Zhu and the dam-busters