[bregma]

How would that work. Would the Niagara gorge suddenly be able to increase its capacity orders of magnitude beyond its physical limits? Would the Detroit and St. Clair rivers suddenly be able to deliver an increased flow into Eerie?

[jblindsay]

Yes, that's correct, it would increase its capacity by orders of magnitude, if not temporarily. The capacity of Lake Erie to hold water is set by the elevation of its outlet, which is currently about 174 m above sea level. Niagara Falls is a knickpoint in the Niagara river, and over time knickpoints in rivers are always smoothed out. This is why Niagara Falls is eroding upstream, and we spend money each year trying to minimize this. As it works it way upstream, it will eventually intersect with the outlet of Lake Erie (over geological time scales), and will start to erode the outlet elevation downwards to achieve that nice concave-upwards longitudinal profile indicative of a river in dynamic equilibrium. In doing so, it will reduce the capacity of Lake Erie quite significantly. Someone above stated that the Great Lakes are below sea-level, and that's true for parts of most of the lakes, but not Erie, which has a maximum depth of about 64 m. It will eventually, over the long term become a wide part of the long river connecting Huron and Ontario. By the way, there are plenty of precedents for rivers suddenly and temporarily increasing their capacity to handle a sudden increase in flow; Google 'channelled scablands' and you might be surprised. I often joke with students that I won't want to live in Missisauga on the shores of Lake Ontario, which I do currently, when this event happens. Of course, it's only a joke because we're talking about a very, very long time away. As for the Detroit and St. Claire rivers increasing their capacity to Erie, I'm not sure why that would be necessary. Flow through this newly created 'Erie River' would simply adapt to carry the capacity of water supplied by Huron all the way to Lake Ontario, just as the Niagara river does now. There's no need for upstream change. Just think, the St. Lawrence River has the ability to convey the entire water supplied by all of the Great Lakes to the Atlantic.

[Kon-Peki]

Perhaps the missing component in understanding this is the time frame for draining Huron/Michigan into "River Erie".

So Niagara is slowly eroding and will eventually reach a tipping point where further erosion will increase the rate of outflow from Erie. The increased outflow will increase the rate of erosion which will increase the rate of outflow, and so on. Correct?

When this is happening, the rate of outflow from Michigan/Huron via Detroit and St Claire will increase, but erosion is not necessary for this to occur?

Say we built a dam on the Detroit River that keeps Michigan/Huron at its current water level. And then Erie turns into a river. Once that transition is complete, we destroy the dam. How much time is necessary for Michigan/Huron to drain? Is the timescale days, weeks, months, years, etc?

[jblindsay]

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.

[Kon-Peki]

Thank you, that helps out quite a bit.