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

<blockquote>
  <p>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]</p>
</blockquote>

<p>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. </p>

CC BY-SA 3.0
2016-06-13T15:19:34.953Z
My uncertainty was about how the HUC boundaries are divided up. 'Evenly' is made clear in the Wikipedia page and in your answer - I was wondering if there is something more to it, to give HUCs more meaning as naturally or politically distinct areas.
cr0
CC BY-SA 3.0
2016-06-13T15:32:27.043Z
I don't believe so, but could be wrong. Since political boundaries don't align with watersheds (and even span one to several at a time) I can't imagine that political boundaries had a driving force on the division....but maybe a minor one? Since if a division included a certain political feature it might be named for that political feature. As for other naturally distinct area (a change in the level 3 ecoregions?), I suppose some could drive the naming certainly, but would probably only affect the division if the impact of their consideration didn't greatly alter the area of resulting HUCS.
traggatmot
CC BY-SA 3.0
2016-06-11T21:20:37.817Z
2016-06-11T21:20:37.817Z
traggatmot
<p>I'm trying to better understand <a href="https://en.wikipedia.org/wiki/Hydrological_code#United_States" rel="nofollow">the Hydrological Unit Code (HUC) system of the United States</a>. I understand the basic hierarchy of it (HUC4, 6, 8, 12) and watersheds/drainage areas. Here's what I want to clarify:</p>

<p>A group of HUC8s can be within a single HUC6, because each HUC8 ultimately drains to the pour point of their parent HUC6. <strong>What separates one HUC8 from another HUC8 downstream of it, both within the same parent Hydrologic Unit?</strong> I suspect the answer isn't the natural drainage area, because <em>the downstream HUC8 would contain the upstream HUC8 in its natural drainage area</em> - 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.</p>

<p>The most concise, relevant explanation I've come across is from <a href="https://enviroatlas.epa.gov/enviroatlas/DataFactSheets/pdf/Supplemental/Watershed.pdf" rel="nofollow">the USEPA's EnviroAtlas Data Fact Sheet on the subject</a>:</p>

<blockquote>
  <p>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.</p>
</blockquote>

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

<hydrology><gis>
What divides Hydrological Unit Code areas of the same level?
cr0
# What divides Hydrological Unit Code areas of the same level?

(Asked by: username_0 on 2016-05-23)

I'm trying to better understand [the Hydrological Unit Code (HUC) system of the United States](https://en.wikipedia.org/wiki/Hydrological_code#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](https://enviroatlas.epa.gov/enviroatlas/DataFactSheets/pdf/Supplemental/Watershed.pdf):

> 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?**

**Tags:** hydrology, gis


---

## 1 answers

### Answer by username_1 on 2016-06-11. Score: 1 | Accepted answer

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.

> **Comments:**
> **username_0**: My uncertainty was about how the HUC boundaries are divided up. 'Evenly' is made clear in the Wikipedia page and in your answer - I was wondering if there is something more to it, to give HUCs more meaning as naturally or politically distinct areas.  
> **username_1**: I don't believe so, but could be wrong. Since political boundaries don't align with watersheds (and even span one to several at a time) I can't imagine that political boundaries had a driving force on the division....but maybe a minor one? Since if a division included a certain political feature it might be named for that political feature. As for other naturally distinct area (a change in the level 3 ecoregions?), I suppose some could drive the naming certainly, but would probably only affect the division if the impact of their consideration didn't greatly alter the area of resulting HUCS.  

---

