What divides Hydrological Unit Code areas of the same level?

Question

I'm trying to better understand the Hydrological Unit Code (HUC) system of the United States. I understand the basic hierarchy of it (HUC4, 6, 8, 12) and watersheds/drainage areas. Here's what I want to clarify:

A group of HUC8s can be within a single HUC6, because each HUC8 ultimately drains to the pour point of their parent HUC6. What separates one HUC8 from another HUC8 downstream of it, both within the same parent Hydrologic Unit? I suspect the answer isn't the natural drainage area, because the downstream HUC8 would contain the upstream HUC8 in its natural drainage area - that's based on me thinking that as you shift the discharge point you're examining further downstream, the drainage area you're examining increases.

The most concise, relevant explanation I've come across is from the USEPA's EnviroAtlas Data Fact Sheet on the subject:

A watershed is defined as the geographic a rea within the boundary of a drainage divide. Watershed boundaries follow the highest ridgeline around the stream drainage area; the bottom of the watershed or the pour point is the lowest point of the land area where water flows out of the watershed. Hydrologic unit boundaries do not always surround a complete watershed but may delineate truncated portions of a larger watershed — for example, the mid-stem of a larger stream or river along with the tributaries in that area. Hydrologic units are generally synonymous with watersheds when their boundaries include all the source area contributing surface water to a single defined outlet point. This distinction between watersheds and HUCs is important in the context of water resources data analysis and water quality monitoring, because the area contributing to the downstream outlet point in a single HUC may extend beyond its boundaries in an upstream direction to include a number of other sub-basin HUCs.

This still does not clarify exactly what separates one HUCx from another HUCx downstream of it, both within the same parent HUC?

Answer 1

I'm confused about your misunderstanding as the answer to your question is in the Wikipedia link you posted:

Regions receive a two-digit code. The following levels are designated by the addition of another two digits.[8] The hierarchy was designed and the units subdivided so that almost all the subbasins (formerly called cataloging units) are larger than 700 square miles (1,800 km2). Larger closed basins were subdivided until their subunits were less than 700 square miles.[7] The 10-digit watersheds were delineated to be between 40,000 and 250,000 acres in size, and the 12-digit subwatersheds between 10,000 and 40,000 acres.[6] In addition to the hydrologic unit codes, each hydrologic unit was assigned a name corresponding to the unit's principal hydrologic feature or to a cultural or political feature within the unit.[7]

So, once you are outside of the 2 digit regions, HUC boundaries (or dividing/outlet points along a continuous stream) that separate an upstream watershed from one a downstream one (that should incorporate the upstream HUC using the classic definition of a watershed) are set to evenly divide up the larger (or containing) HUC.