This article is rated Start-class on Wikipedia's content assessment scale. It is of interest to the following WikiProjects:

Rivers High‑importance This article is within the scope of WikiProject Rivers, a collaborative effort to improve the coverage of Rivers on Wikipedia. If you would like to participate, please visit the project page, where you can join the discussion and see a list of open tasks.RiversWikipedia:WikiProject RiversTemplate:WikiProject RiversRiver High This article has been rated as High-importance on the project's importance scale. Geology Mid‑importance This article is within the scope of WikiProject Geology, an attempt at creating a standardized, informative, comprehensive and easy-to-use geology resource. If you would like to participate, you can choose to edit this article, or visit the project page for more information.GeologyWikipedia:WikiProject GeologyTemplate:WikiProject GeologyGeology Mid This article has been rated as Mid-importance on the project's importance scale.

I added this to the definition: "The mainstem extends all the way from one specific headwater to the outlet of the river, although there are multiple ways to determine which headwater (or first-order tributary) is the source of the mainstem." Then I added a "Clarification needed" tag.

I added this to emphasize that the mainstem is not just the segment of the river with the highest order (near the outlet), it stretches all the way to one specific first-order tributary that must be chosen by some method.

Unfortunately, I can't find a concise and specific definition. The USGS provides maps of many river mainstems, each with one special headwater chosen as the "source", but they don't say how the source is chosen. Some other sources claim the mainstem is whatever river path is the longest distance from headwater to outlet, but simply looking at the Mississippi according to the USGS seems to indicate they are using a different definition. I found no sources that explained the USGS data, and they have lots of data on headwaters, tributaries, and mainstems. So I left it completely uncited. 209.6.225.254 (talk) 07:04, 17 January 2024 (UTC)Reply

This expression is used at Amur to describe the Argun. Based on the definition in this article, it seems to be a contradiction in terms: according to the definition in this article, either a river is the main stem or it is a tributary. I presume the definition is a portion of the "main stem" (according to some other definition) upstream of some confluence that is arbitrarily given a different name. But either "main stem tributary" is nonsense, or the definition of "main stem" given in this article is wrong and is not in fact contrasted with "tributary". Hairy Dude (talk) 22:05, 18 June 2025 (UTC)Reply