USGS stream gauges have an associated drainage area, but the associated hydrologic unit listed by USGS is often different than the actual drainage area. How can one determine what hydrologic unit corresponds exactly with the drainage area of a stream gauge, and where applicable, vice versa (...what stream gauge corresponds exactly with a hydrologic unit that is a watershed)? I expect that this cannot be done for all HUC levels, but that it should be doable for certain HUC levels which are almost always actual drainage basins (e.g. HUC12s are almost always true watersheds with an average area of 40 square miles, so won't some of those line up with USGS gages of similar sized basins?)

To demonstrate the problem: let's say I'm trying to examine the properties of a watershed - a HUC12 - and want to see stream gauge data for it. To find stream gauge data, I'd go to the USGS Water Resources site and hone in on the area of the watershed of interest - the issue is that many gauges have a Hydrologic Unit 8 listed on the gauge's Summary Information page that is different from the drainage area listed on that same page and from the watershed delineation one would get if starting with a DEM and the gauge's coordinates in GIS.
That brings me back to my question in bold above - any guidance or clarification on finding paired USGS stream gauges and HUC's that are drainage areas (since not all hydrologic units represent actual watersheds that can be gauged)?

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    $\begingroup$ Do you still want to know how to do this? $\endgroup$ – traggatmot Jun 10 '16 at 19:22
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    $\begingroup$ If there is a way, yes @traggatmot ! I'm increasingly convinced there is not a way to do this - or at least not across the board - because from what I've read only the 6th out of 6 HUC classes consistently represents physical drainage basins. I.e. if a particular HUC8 boundary doesn't represent a physical drainage basin, how can it be gauged? If there is a gauge somewhere in or at the lowest point of such a HUC8, how accurate is it to say the gauge data corresponds with that non-drainage-basin HUC8? $\endgroup$ – cr0 Jun 11 '16 at 4:08
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    $\begingroup$ That being said, to answer this question, I also may need help understanding this somewhere along the line: earthscience.stackexchange.com/questions/8054/… $\endgroup$ – cr0 Jun 11 '16 at 4:08
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    $\begingroup$ I should've been more specific in my comment, because my answer to is going to be that there is no way to specifically tie stream gages to an exact predefined HUC area. The HUC that is listed on the stream gage website is the HUC (or HUCS) that overlap or contain the stream gauges catchment area. There may be some stream gage catchment areas that overlap perfectly with a HUC, but this is probably the exception and occurs by chance (or maybe design, but most dream gauge placements aren't chosen to align their catchment areas with the HUC's boundary). $\endgroup$ – traggatmot Jun 11 '16 at 21:26
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    $\begingroup$ There is a specific dataset the contains information and water should boundaries for all (or almost all) active stream gages, which also contains various attribute information for the stream gage catchment area. This dataset is call GAGES II and the NLCD classes contained in each catchment area was recently updated with NLCD 2011 results. $\endgroup$ – traggatmot Jun 11 '16 at 21:27

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