Introduction to River Types

Classifying a cold, flowing body of water according to its geology, chemistry, and the biological life forms that live within it is often difficult, due in large part to the fact that there are so many kinds of moving waters. The term river, stream, or creek is somewhat optional here (Figure 3), and I use them, as most others have, to denote the hierarchy of size and volume of a particular aquatic system. Rivers are generally 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 substrate (rocks and such) over which it flows, making each one unique (Figure 4). A river is actually a linear gradient (Figure 5) of water which can be characterized by the amount of dissolved oxygen, its pH (acidic or basic), the degree to which particulates are suspended in it, its temperature, its 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 a multitude of life forms with tolerance limits (Figure 6) that fall within the local conditions of each moving body of water. 

In addition, each day brings with it a new set of physical conditions. The riparian environment (river banks) also contributes to the kind of river that evolves in that geographic region, further confounding our ability to classify them into neatly defined categories. 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 (i.e., salmon, trout and charr; see Appendix I) and their food webs (Figures 7, 8). The vast majority of rivers and streams begin their journeys as runoff from snow melt and rain in areas of high elevation, while another large sub-set begin their trek to the sea in more gentle surroundings, such as springs that bubble up out of the ground. Increasingly, many rivers are altered by dams that create tailwaters (tailwater fisheries), and this activity has, surprisingly, produced some of the most amazing trout habitat on earth. 

Salmonids are native only to the northern hemisphere, having evolved from a common ancestor some 20,000 years ago, at the end of the last interglacial cycle of cooling. As the vast sheets of glaciers retreated, salmonids rapidly radiated out into the newly formed habitats of North America, Europe, Russia, and parts of Asia. They found their way into a wide variety of aquatic habitats; lakes (lentic), and freestone, spring, and limestone creeks (lotic). As populations became geographically-isolated, new species evolved. In modern times, we have put them in places where they could not gain access because of geography, and most have thrived. This is especially the case in the southern hemisphere, where no salmonids existed prior to the intervention of fish stocking. Today, almost all of the Earth’s cold, aquatic ecosystems have some form of salmonid living in them. This suggests strongly that salmonids are highly adaptable creatures and will take advantage of most food sources when offered the opportunity to do so. What follows are a few detailed descriptions of the ecology of rivers, streams, and creeks that allow these beautiful fishes to thrive.