In point of fact virtually all of the data we publish on WildTroutStreams.com owes its existence (more or less) to the management of water quality.  Since the first decade of this century, the link has been quite explicit, since the US EPA shifted all states to a "Designated Use" paradigm for managing water quality.  Most states now classify coldwater surface water in some fashion (this is where we look first for wild trout).  Often, it is explicitly described as "trout" or "salmonid" water (terminology differs state by state).  Some states, like Oregon, break it down further: classifying water by whether it supports Char (like bull trout, which require even colder water than most salmon and true trout) or other Salmonids, etc, and whether it's used for spawning, rearing, migration, etc.  But that's the exception.  Most states will at least distinguish cold water habitat from warm water, and will classify water where trout can actively reproduce.

Much of this information, where it exists, is accessible through this little networked KMZ that links you to the EPA "WATERS" database.  WATERS was a massive project to provide a single, reasonably seamless database for all of the state by state water quality data.  It lets you query Designated Uses, which is probably the most important single piece of information.  But it also will show you threatened and impaired waters, and fish consumption advisories, which might also be of interest.  A sample is shown below.

We've loaded the KMZ, and checked only one layer:  within the "EPA Water Program Features", the "Water Quality Standards" check box.  Note, you'll see nothing using this layer until you zoom in low enough...  below about 20 miles seems to be the magic distance.  The image below was captured at about 15 miles "eye alt", which you can see in the bottom right hand corner of the Google Earth screen.  At that point, any water bodies where WATERS has WQS information will be rendered in a green.  Here you see a section of Idaho.  Click on the water body and a "balloon" will pop up.  Click on the link, where the yellow arrow is pointing, "More about the Water Program Feature", and a web page will open within Google Earth (scroll down).

The web page that pops open wil then provide a listing of "designated uses".  For this stream you get a list of 8 uses, two of which are relevant to us.  "Cold Water Aquatic Life" and, even more encouraging for the wild trout angler, "Salmonid Spawning", which is defined as "Waters which provide or could provide a habitat for active self-propagating populations of salmonid fishes."  Isn't the EPA wonderful?

Much of the data in WATERS duplicates other datasets you'll find on this website.  But it's as close as we have to a consistent, national dataset of information about trout habitat, and is very useful in the states where we don't provide other coverage.