lower basin states every year. The reason I say "on average" is that technically the compact/law requires 75 million acre feet over a 10 year period. Powell is the dividing line between the upper and lower basins. So in really bad years like last year, less than 7.5 million acre feet might flow out of Powell and in very wet years like this year, more will flow (9.5 million acre feet is what is slated to flow out this year).

The whole thing is generally a mess because the compact was founded on the belief that the average flow on the Colorado was 16.4 million acre feet. The upper states got 7.5 million and the lower states got 7.5 million totaling up to 15 million and leaving 1.4 million for Mexico. The problem is that historically the actual average appears to be closer to 13.5 million acre feet per year (leaving a deficit of .5 million acre feet every year just for the US States, not counting Mexico at all).

In a prolonged drought like we have had for the last 20 years, the reservoirs just get lower and lower because no state is willing to cut back.

The wiki article is actually pretty good to get a general understanding of it: https://en.wikipedia.org/wiki/Colorado_River_Compact