What I know about geology and trout streams can be summarized in one word:  limestone.

Tom Rosenbauer, in his book, Prospecting for Trout (one of my favorites, by the way) has a good discussion of the difference between limestone and freestone streams.  He says...

There are other factors that make streams running through limestone richer than those running through quartzite, sandstone, or gneiss. Even streams that are not spring-fed but that run through limestone or other calcareous rock are much richer than those that run through mostly silicate rock. Penn's Creek in central Pennsylvania is a good example. It is hardly a model of stability — in fact, every time I have tried to fish this famous river, it has been chocolate brown and over its banks. Yet even though it does not have the rich, clear, weedy character of a spring creek, because it flows through limestone bedrock it is much richer in insect and crustacean life than similar-appearing, silicate-bedded streams.

Given smaller seasonal stability and runoff pattern, a "hard" water stream will be richer than one with "soft" water. Any rock composed primarily of calcium or magnesium carbonate will leach into water, thereby "hardening" the water and giving you a richer trout stream. The most important such rocks for our purposes are limestone, composed primarily of calcium carbonate, dolomite, or calcium magnesium carbonate, and marble, composed of metamorphosed limestone or dolomite. Many empirical studies have proven that trout grow faster and behave differently in hard water streams. A Pennsylvania study of three soft water and three hard water streams with similar drainage patterns showed that trout's growth rate was directly related to specific conductivity, which is a measure of dissolved calcium and magnesium salts in the water. Another study, in England, proved that limestone stream trout have a much lower seasonal variation in diet than those from soft water streams, and they grow faster as a result.

The USGS publishes a series of state-level geologic maps in KMZ form.  These are nicely designed so that if you click on the area of interest, it will give you a comprehensive description of the underlying bedrock formation.  If you're researching a wild trout stream, and you find it flows through a limestone structure, it's an added reason to check it out.  Note that some of these files are huge (PA in particular).  Even though the data will be local, it may take your system a long time to render them properly.  Be patient.

Source: USGS and Google Earth