Stream Ecology info here
Classifying a cold, flowing body of water according to its geology, chemistry, and biological life forms that live within it is often difficult, due in large part to the fact that there are so many different terrains over which they flow. Designating a moving body of water as a river, stream, or creek is based primarily on size and volume. Rivers are larger than streams, which are usually larger than creeks. Regardless of what we call them, each takes on the chemical characteristics of the dissolved portion of the ground (substrate) over which it flows, making each one unique compared to all others. A river is a linear gradient with respect to the amount of dissolved oxygen, pH (acidic or basic), particulates, temperature, flow rate, and a host of other physical and chemical characteristics, all of which vary from its source to its mouth. These characteristics, in combination, select for an incredibly varied series of niches. The life forms that live there have tolerance limits that fall within the local environment of each moving body of water.
Each day brings with it a new set of external conditions to influence those conditions. The riparian-terrestrial environment also contributes to the outcome of each moving body of water, confounding our ability to classify them even further. However, some features that are shared by most can be used to group cold, moving bodies of water into just a few general types, particularly when they are keyed to the presence or absence of salmonids and their food webs. The vast majority of rivers and streams begin as runoff from snow melt and rain in areas of high elevation, while another large sub-set begin their journeys to the sea in more gentle surroundings, such as springs that bubble up out of the ground. Increasingly, many are altered by damming up free flowing waters (tailwater fisheries) and the tailwaters below them have produced some of the most amazing trout habitat on earth.