The whole population of Montana is less than that of a moderately sized city. By the way.

I think the estimate was around 80,000 without college students.

All of the towns around the place I grew up probably add to 15,000 people.

the most closest smallest watershed about 10000

I live 30 east of the continental divide in Colorado. I see the snow fall above me. I drink it when it melts. It flows from me toward the Mississippi and the Atlantic. My watershed is shared by everyone east of the divide in the Rockies, parts of southern Canada and all of the midwest and eastern US that drains to the Mississippi, no? Maybe 100-150 million people. I try to be nice to the people downstream.

Pushing 3 million residents. Although many more are here on any given day. There are Many hotel rooms in Las Vegas.

Approximately 15 million on the San Francisco Bay, San Joaquin and Sacramento river watershed. I can only guess since population information I use in my work is linked to government jurisdictions not watershed boundaries.

1,5 milion persons

I have water rights on the prickly pear creek, along with approximately 750 other people. The prickly pear creek is a tributary of the Missouri River, and the Missouri River flows through many states. I would estimate the entire watershed of the Missouri River includes in excess of 1,000,000 people.

Well, everyone in the Helena Valley--about 45,000. We are surrounded by mountains that create boundaries for the water courses. Check the maps and books at the local library.

About 500,000.

It was half that 25 years ago.

I know the population because it's in the newspaper from time to time.

I bet this map (population per watershed) must exist somewhere.

This is another "depends" question. The valley drained
by the stream that drains my house lot is about 15,000
but the Charles River watershed is more like 3 million.

If you mean the immediate watershed, two people.

If you mean to the river, probalby 40,000 or more.

If you mean to the ocean, or at least the Gulf of Mexico, probably 1.5 million.

Method: go to the library and look at an atlas of economic districts.
Pick up census data from the net and collate with topozone.

Which watershed? We've changed the natural one with dams and canals.

Though, if we look at the entire central valley, which is the bigger watershed, even with just natural routes, the population has to be in the millions.

1. wikipedia
2. Local council offices
3. Guesstimate

Roughly 250k. Half a mil if you expand it based on DEP measurements of our watershed. I consider where our water supply originates directly as our watershed, although I readily admit the greater watershed of north Jersey is much larger and affects 5 million of NJ's eight million people.

500,000