Isn't there a lot of land that is only a few meters above sea level (where a lot of humans like to live)? Say you have a valley and a river has been transportin

Isn't there a lot of land that is only a few meters above sea level (where a lot of humans like to live)? Say you have a valley and a river has been transporting down matter for a long time, forming a flat landscape at the bottom of the valley, only a few meters above sea level. If the sea rises only a few meters, this valley will be flooded easily, and there will be a lot of new surface area per cubic unit of water, in other words a lot of surface area for new plankton. A pity about the humans that probably lived in that valley, though. ;)