I think it is a bit more complicated than that, and wasn't it the UK Environment Agency working in Brussels that pioneered the policy!! We would have had this river dredging policy with or without the EU!
Two excellent articles by George Monbiot explain more...
"Maize is being grown in Britain not to feed people, but to feed livestock and, increasingly, the biofuel business. This false solution to climate change will make the impacts of climate change much worse, by reducing the land's capacity to hold water."
"The Labour government in 2005 it published a devastating catalogue of the impacts of changes in land use. As well as the loss of fertility from the land and the poisoning of watercourses, it warned, "increased run-off and sediment deposition can also increase flood hazard in rivers". Maize, it warned, is a particular problem because the soil stays bare before and after the crop is harvested, without the stubble or weeds required to bind it. "Wherever possible," it urged, "avoid growing forage maize on high and very high erosion risk areas."
The Labour government turned this advice into conditions attached to farm subsidies. Ground cover crops should be sown under the maize and the land should be ploughed, then resown with winter cover plants within 10 days of harvesting, to prevent water from sheeting off. So why isn't this happening?
Because the current government dropped the conditions. Sorry, not just dropped them. It issued – wait for it – a specific exemption for maize cultivation from all soil conservation measures."
http://www.theguardian.com/commentisfree/2014/feb/17/farmers-uk-flood-maize-soil-protectionhttp://www.theguardian.com/environment/georgemonbiot/2014/mar/14/uk-ban-maize-biogas