That is true depending on who you ask (environmentalists vs. oil companies)
They do release aprox. 40% of all the greenhouse gasses in Alberta.
As well the operations there currently divert 359 million m3 of water from the Athabasca River, or more than twice the volume of water required to meet the annual municipal needs of the City of Calgary (current pop. 1.3million people)
More info
here.