Good background article on the amount and availability of water needed to frack all those wells in North Dakota from a post by Bruce Oksol at Million Dollar Way. He converts the amounts into acre-feet of water in his post, Update on Availability of Water Necessary to Frack Wells in the Bakken.
For future reference, here is the conversion calculation:
In a more recent article the estimate was 1 million to 3.5 million gallons of water is used to frack a Bakken well (see paragraph 3 below). The conversion factor: a acre-foot = 325,851 gallons. Therefore 1 million to 3.5 million gallons converts to 3 acre-feet to 10 acre-feet.
Currently, it is estimated that about 2,000 wells will be fracked each year in the Bakken. That equates to somewhere between 6,000 acre-feet to 20,000 acre-feet of water being required to frack Bakken wells on an annual basis.
One acre-foot is the amount of water to cover an area one acre in size to a depth of one foot. Thus, one acre-foot.
He then mentions Lake Sakakawea can hold 23.8M acre feet of water. The amount needed to frack all those wells is slightly less than one-tenth of one percent of the lake.
The Army Corps of Engineers is expected to release 30,000 acre feet from the Missouri that is considered excess, according to an article in The Dickinson Press, linked by Mr. Oksol.
That is far more than enough to frack all the wells annually. Availability of water is not an issue holding back getting all that oil out of the ground.