You might think that this question would be hard to answer. But thanks to the folks at the United States Geological Survey (USGS), we have an answer. And it's a relatively small amount compared to the total volume of rock that our planet contains. (Thanks to Lee Allison at the Arizona Geological Survey for bringing this interesting link to my attention).
See the full USGS link here.
Some highlights from this site is that it explains the various forms
that water takes on our planet - water vapor,
oceans, rivers, lakes, and groundwater. The graphic below shows the
Earth relative to the volume of all of it's water (large
bubble). The diameter of this water sphere is about 860 miles and has a
volume of about 332.5 million cubic miles. That's a lot of water but as
you can see the vast majority of the earth's volume is rock. Note the small bubble of water to the right of the larger sphere. That medium size bubble represents all of the liquid freshwater on earth and represents a volume of 2.5 million cubic miles, with a diameter of about 170 miles.
Amazingly, 99% of this freshwater is groundwater! Finally, you can see a
tiny bubble dot just north of Florida and this represent the total volume of earth's lake and river water. This volume is just over
22,000 cubic miles with a diameter of 35 miles. That's a lot of water but still quite small compared to just about anything else depicted here.
A graphic at the bottom of the USGS link shows that all of the river water in the world totals a volume of only 509 cubic miles. In my book, "Carving Grand Canyon", I mention that the amount of rock removed from the canyon equals about 1,000 cubic miles, meaning that all the river water in the world would only fill the canyon a little more than half of that volume. Of course, the canyon is much wider at the top than at the bottom, so the majority of the canyon's depth would be filled. But still, the Grand Canyon is a very big place.
Amazing information is now at our disposal! Thanks to studies like these.