Oops - Hundreds Of Endangered Sacramento River Salmon End Up In Irrigation Ditches
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The web of ditches eventually drains into the Yolo Bypass through a canal called the Knights Landing Ridge Cut. Officials suspect this is one way the salmon swam astray, probably when the Yolo Bypass flooded after heavy rains in December. The balance of winter was odd. That wet December was followed by the driest January-through-June in recorded history. This led to unusually low flows on the Sacramento River for most of winter.
As a result, winter-run salmon may have gotten their primary migratory cue from the agricultural canals. That water is diverted far upstream, and because natural runoff in the region was low, the canals may have held a stronger Sacramento River "scent" than the main stem river, which also includes a mix of waters from the Feather River, the American River and other sources. "The big topic that comes up with this water year is if that attracting flow of Sacramento water is one thing that brought the fish into the canals," said Shaffer. "In a normal water year, the fish do not get off course."
Another possible entry is the Colusa Basin Drain, a large concrete structure with large steel flapper gates, near Knights Landing along the Sacramento River. Under certain flow conditions, drainage from the basin empties here instead of from the Ridge Cut, which is at the north end of the Yolo Bypass.
The California Department of Fish and Wildlife determined there were 23 days in January and February when flow conditions could have allowed salmon to migrate through the drain. The large number of rescued fish caused officials to worry initially that the strandings could jeopardize the entire winter-run population.
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http://www.sanluisobispo.com/2013/08/11/2626317/hundreds-of-salmon-stranded-in.html