system
Given the following post, respond as if you're commenting on Reddit:
human
How do scientists measure the drainage basin of a river?

I read a comment on here saying that most of the continental US drains into the Mississippi River. What sources of water are they measuring, and how do they calculate the total land area that's feeding into a given river?
gpt
In the modern, this is a pretty simple task in most Geographic Information Systems (GIS). In short, if you have a digital representation of topography (digital elevation models or DEMs, think a grid of x,y,z points representing elevations) you can run flow routing algorithms (the mathematical equivalent of placing a drop of water on a pixel in the DEM and using the local slope, i.e. the relationships between that pixel and its neighboring pixels, to figure out which way the drop would flow and then doing that for every pixel to make a map of flow directions) and then flow accumulation (using the result of the flow routing to figure out which pixels flow to which other pixels downstream and/or accumulate flow from other pixels upstream) to produce a map of the drainage area upstream of any pixel in the DEM. Then it's simply a task of querying the right pixel, i.e. find the the pixel that represents the mouth of the Mississippi river and query the drainage area.