Mike,
Here are two journal articles that you might find interesting. The first is an article called "A classification of natural rivers", by Dave Rosgen. It explains a stream classification system that is used by most fishery biologists and hydrologists to classify streams into different types based on their overall morphology. It also supplies insight into the type of impacts that the stream may be susceptible to, or undergoing. It is a GREAT article, and I use this system almost daily. It can be downloaded for free at wildlandhydrology.com. Once at the home page, click on "references" on the upper right, then scroll down until you see "a classification of natural rivers", click on it, and a pdf file of the article will pop up.

The second article I recommend is "The river continuum concept" by Vannote et al. It is a short article, but explains a concept that is regularly used by all aquatic ecologists. It explain how (in very general terms) the biological components of stream systems change from the headwaters to when they become large rivers. It supplies some great concepts to have in your mind when observing rivers. The full reference is: "The river continuum concept", Vannote et al., 1980, Canadian Journal of Fisheries and Aquatic Sciences, volume 37, pages 130-137. -- Rick