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?