How far upstream would severe sea level rise (1m) noticably affect a large river?

Question

The projection for sea level rise due to global warming seem to hover about 1m by 2100, let's use this figure. How far upstream would this noticably affect a large river, e.g. the Rhine?

I think it is obvious that the Delta would be changed severely, while it is nigh impossible to say accuratly how exactly (I assume). Upstream, I think the following effects com into play: Less hydraulic head means less flow velocity, more sedimentation and a higher water level in the river bed (the river needs more cross section to achieve the same volume flow with less head). Are these assumptions correct so far?

Now the main question is how far upstream we would notice these changes in river behavior: Only at 1m above new sea level, or 10m, or 100m? Is this stronger determined by linear distance along the river, or by height above (new) sea level?

I prefer the Rhine as an example because it's the closest large river. If someone has a good answer for another large river I'm happy too.

Answer 1

This is both a simple and complex answer.

The simple bit is that a 1m rise in sea level would impact the river to a point 1m higher than the current point (with a slight error margin dependent on flow rates etc)

The rise itself over the next 85 years will not push back along the river (as Michael mentioned) as it's not like a tidal bore such as the Severn Bore.

The complex bit is that the 1m rise could be along an estuary, or across winding wetlands, or a wide delta or other feature, so the change to river flow could actually be quite catastrophic. And these effects could impact very far upstream, especially in the case of a river such as the Rhine, with a heavily populated floodplain. Wikipedia notes the last 50km of the Rhine are at 0m elevation, so that's an incredibly large possible impact area.

Answer 2

It seems that your indications regarding changing conditions following sea level rise are valid. Rising base level will move the current depositional processes inland some distance as you have indicated.

The process will apply basically in the same way to all rivers that terminate in a body of water that will be affected by sea level rise, for example rivers that terminate in a lake that will not rise due to the changing climate will not be affected. There are some circumstances where lakes in steep terrain could be affected by increased meltwater from surrounding glaciers, and that increase could be significant, but the models predicting sea level rise are not referring to particular local conditions, which are on a case by case basis, but possibly very important to the local populations.

Where the process becomes interesting is what happens to a particular river, because they will all be different. In order to determine what will likely happen, one must consider the physical aspects of the river, what is the change in elevation near the terminus, what is the topography of the floodplain, how much does the river meander as it approaches the terminus? These and other similar factors will determine what happens as the base level rises.

If you consider a river that has very little change in elevation, and meanders across a wide floodplain, the amount of inundation caused by a one meter rise in base level will cover a large area of what is now floodplain and likely intermittently wet or dry based on seasonal fluctuations. Compare this with a river that occupies a canyon, or is deeply incised and has a steep gradient to the base level, the base level will rise within those constraining features and little will likely happen to the narrow floodplain that has been present.

First step for your inquiry about the Rhine River is to look at the topography surrounding the river, and mark out a one meter change in the area surrounding the river beginning from the terminus, as the terrain becomes steeper toward the head water the change from current condition will diminish.

Direct answer to your question is that the inland affect of one meter of base level rise could reach much farther than one meter upstream depending on the topographical features near the river terminus, and each river will be different.