I'm not sure just where you got those figures, but let's assume for now they're correct.
Flows of the Colorado river are in the order of 2,000 to 20,000 cubic feet per second. Let's take an average of 10,000 cfs to make the math simple. The next question is, just how much is a cubic foot?
Turns out to be 7.48 gallons.
So, 10,000 x 7.48 = 74,800 gallons per second on average.
60 x 60 x 24 x 74,800 = 6,462,720,000.
If your billion gallons a day figure is accurate, then, a little less than 1/6 of the water is being used for golf courses. That is assuming, of course, that those golf courses aren't using recycled water.
So, yes, given those conditions, the proportion of the water used for that purpose is significant.