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)?