The Colorado River has been carving the Grand Canyon for a few million years now, such that the canyon floor is around 6000 feet lower in elevation than the surrounding area, at about 2400 feet above sea level. If things continue as they have, the Colorado River will continue to make the canyon deeper. If the river eroded away enough rock so that the river's bottom got to sea level, and this process continued to go upstream the river from where it empties into the Pacific Ocean so that the floor of the canyon eventually eroded below sea level, would the canyon gradually be filled in by the ocean? Would this even be possible? If not, why don't we see the mouth of the Colorado (or any river for that matter) gradually move upstream, with ocean filling in where the river once was?
It is unlikely just from erosion. There are a couple reasons for this but because of the time scale over which this process would occur, other phenomenons might change the geologic picture completely.
The reason the canyon formed in the first place is because of tectonics that uplifted the rocks. From this page:
Uplift of the Colorado Plateau was a key step in the eventual formation of Grand Canyon. The action of plate tectonics lifted the rocks high and flat, creating a plateau through which the Colorado River could cut down.
Erosion is much quicker when the slope is steeper so the carving of the canyon slows down as the river flow gets gentler. So the deeper the canyon carves, the more potential force it loses. From the same source as the earlier quote, a stratigraphic profile:
Another reason erosion has slowed down significantly is that the type of rock the water has to carve through is much different the deeper it gets. The river has now reached metamorphised layers that are much harder than the limestones and sandstones from the upper layers. The Vishnu Group is practically bedrock. This page has a nice explanation:
The steeper the slope, the faster the runoff flow and the more force it will have to move material. The less vegetative (plant) cover and leaf material on the ground, the more open the soil will be to the flowing runoff water. Four things cause slope erosion: the amount and rate of rainfall, the steepness or gradient of the slope, the amount and nature of plant cover, and the type of soil and bedrock underneath.
Regarding this question:
why don't we see the mouth of the Colorado (or any river for that matter) gradually move upstream, with ocean filling in where the river once was?
Erosion takes material away but this material has to go somewhere. Sediments are carried downstream and deposited at the mouth of those rivers, becoming what we call river deltas. The deltas usually advance into the sea as the mass of sediments is incredibly huge. Take a look at this picture:
This is the eastern end of Lake Mead where the Colorado river would flow when the reservoir level was higher. The entire flat brownish area is deposited silt from around 50 years of flow. I rafted through that now exposed silt and the walls on either side of the new channel were over 30 feet high. The same thing occurs in the oceans, but over much longer timescales.
So, to summarize:
- There is much less water flowing through now than in the past. If Colorado winters become too mild and dry, then the river's power will be even further diminished.
- The slope gradient of the river can only get gentler, so erosion will slow down.
- The river has reached much harder rock that resists erosion much better.
It would be unlikely for the Grand Canyon/Colorado, simply because the mouth of the river is too far from the ocean, and the outlet is in the relatively shallow end of the Sea of Cortez: https://en.wikipedia.org/wiki/Gulf_of_California That means that sediments will be deposited to form a delta https://en.wikipedia.org/wiki/Colorado_River_Delta and as the upper reaches of the river erode towards sea level, the water will flow slower, eroding less.
However, there are a couple of cases where something similar can happen. The first is where the eroding force is not a river, but a glacier. The weight of the ice means it extends well below sea level, eroding the rock. The classic example of this, of course, are the Norwegian fiords.
A second case is where the sea floor drops fairly quickly to the depths. Then the sediments will flow in turbidity currents, eroding an undersea canyon. Examples are the Monterey Canyon https://en.wikipedia.org/wiki/Monterey_Canyon and the Hudson Canyon: https://en.wikipedia.org/wiki/Hudson_Canyon
Of course this is all complicated by changes in sea level over geologic time scales.