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"Remnant areas" occur along coasts where individual streams are too small for the given subdivision type. Such remnants were combined into a single unit if they were adjacent and could be combined. These "composite" units are called "frontal units". They are non-classic because they have more than one outlet.<ref name=WBDstandard/>
For example, the coastal area along [[Puget Sound]] between [[Seattle]] and [[Mukilteo, Washington|Mukilteo]], is delineated at the finest "subwatershed" level as "Shell Creek-Frontal Puget Sound", HUC 171100190203. This hydrologic unit includes numerous small streams that drain directly to Puget Sound, including [[Pipers Creek (Seattle)|Pipers Creek]] and [[Boeing Creek]]. As a consequence of the smallest "subwatershed" being non-classic, every higher level unit containing it are also non-classic "frontal" units—"Lunds Gulch-Frontal Puget Sound" (HUC 17110019), "Puget Sound" (HUC 171100 and 1711), and "Pacific Northwest Region" (HUC 17).<ref name=WBD>{{cite web|title=Watershed Boundary Dataset|publisher=[[United States Department of Agriculture|USDA]], [[Natural Resources Conservation Service|NRCS]], National Cartography and Geospatial Center|url=http://www.ncgc.nrcs.usda.gov/products/datasets/watershed/|accessdate=September 4, 2010}} [[ArcExplorer]] [[Geographic information system|GIS]] data viewer.</ref>
"Noncontributing areas" are drainage areas within a hydrologic unit that do not drain to the unit's outlet. They can be caused by such things as potholes and [[Kettle (landform)|kettles]], closed basins, [[Dry lake|playas]], and [[cirque]]s. If a noncontributing area is large enough, it was designated as a hydrologic unit of its own. The largest such example is the [[Great Basin]], designated a hydrologic unit the Region level. When a noncontributing area was not large enough to be designated a hydrologic unit, it was merged into the surrounding or bordering larger hydrologic unit.<ref name=WBDstandard/> Special decisions were required for "semiconfined basins" that contribute flow to another area in wet years but become noncontributing in dry years—[[Goose Lake (Oregon-California)|Goose Lake]], for example. The USGS instructed the people doing the delineation work to take extra care in the case of semiconfined basins and to seek assistance from others, but to ultimately make their own decision on whether the semiconfined basin should be designated a noncontributing area or not. Another special case occurs when noncontributing areas very small and dispersed, or scattered throughout a drainage area. These were considered part of the encompassing hydrologic unit. In short, noncontributing areas cannot be subject to strict criteria for delineating, and methods vary from state to state, landform type to type, and special cases. The effect of noncontributing areas on specific hydrologic units is explained in [[metadata]] as best it can.<ref name=WBDstandard/>
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