The total quantity of water being used per year is approximately right. As you say, it is much more than the city of Calgary uses, to give some comparison.
The question of impacts downstream is rather tricky. There is no doubt that the withdrawal of water and the impact of the groundwater taken will likely have some impact on the flow of the rivers downstream in the Mackenzie system. That is ameliorated somewhat by the fact that they go through big lakes in the Peace-Athabasca delta before it gets into the Mackenzie River, and there are other big tributaries.
One of the things we found is that the Liard River, which flows into the west side of the Mackenzie from Yukon, has actually been going up in flow. That is a typical pattern with climate change. You see the northern rivers getting more snow, mainly off the more open sea. There's more flow there, and in the southern rivers you're getting much less flow. The Athabasca River is typical of the southern rivers.
The total flow of the Mackenzie River may not change a lot, but the inflow from the Athabaska River into the Peace-Athabaska delta and into the lakes that feed into the Mackenzie system is certainly going to have an effect.