The only slightly obscure thing I know about river terminology is marking twain, since I live in St. Louis and Mark Twain is kind of a big deal here in Missouri. It means checking how deep or shallow the river is by throwing a string over the side that has a weight on the end of it and you take measurements. Other terms such as fork, branch, rivulet, creek, etc could probably be found by a simple search. Look under Tributary on wikipedia, scroll down for a bunch of links and stuff.