To be clear, I'm not saying that the draining of Lake Erie necessarily means that Lake Huron/Michigan or Superior would drain too. If that were to happen, the two events would be quite separate and likely separated by millennia. Lake Erie's water level will lower when/if the Niagara river erodes all the way back to to the outlet of Lake Erie, and Lake Erie will become a river, when it erodes all the way to the middle of the lake (actually the deepest part). Obviously, this event, should it happen at all, would be thousands if not hundreds of thousands of years from now, assuming that climate doesn't change such that the Great Lakes dry up, or that there isn't further geological activity/uplift. Would this draining of Erie result in a catastrophic flood when it does happen? I don't know, I suppose it depends on the erodability and topographic character of the outlet area at the time. But it is surely the case that the draining of Erie (i.e. the reduction in its capacity to store water) would result in an immediate but temporary increase in the flow through the Niagara river, as it conveys the regular flow from the upper Great Lakes as well as the additional storage loss of Erie. Once the river eroded to the deepest part of Erie, however, flow would resume to simply be controlled by the water delivered to the river by Huron. Would the river continue to erode to the outlet of Huron and result in a similar draining of the upper Great Lakes? I don't know, but I do know that would be over extensive geological time scales and that there is plenty of opportunity for climate change to have a great impact on this possible scenario--Erie draining is much more immediate by comparison. However, something that many Canadians like myself don't often realize, because we live in the country with the greatest number of lakes, is that lakes are generally not that common worldwide. They represent temporary landscape features that are reflective of a relatively recent (in geological terms) disturbance...in the case of most of Canada's lakes, a recent glaciation. Lakes eventually drain as their outlets are eroded down and fill in by sediment delivery from upstream. And the same will likely be true of all of the Great Lakes over a long enough time. There is nothing permanent about our landscapes and that's exactly what makes Earth so awesome as a planet. It's dynamic and varied and today's 'great' lakes can be tomorrow's dry desert plains. We may well be devastating this planet, and its ecosystems specifically, but in the long-view, we'll likely be nothing more than a blip in the history of Earth, and one that it will erase most evidence of after enough time. I don't know why, but there is something reassuring about that thought to me.