If you take a tour through any given landscape, via car or virtually through Google Earth, you are very likely to see a variety of different river types. At first glance, they may not appear so different (just a bunch of long tracks of flowing water), but if you look closer you will see that each river is, in a sense, unique, with some having a single channel while others may flow in multiple, interweaving channels. You’ll see that each river has a different pattern of sinuosity (i.e., the frequency and amplitude of ‘wiggles’), and each has their own variations of width and depth, differences in the material composing the channel bed and banks, and differences in the vegetation lining the channel. Figure 28 shows a few examples of different channel types.
The shape and size of a river depend on a multitude of factors that vary over time and space. A comprehensive discussion of these factors and the interactions between them is beyond the scope of this course, but it is useful to discuss how rivers are self-formed dynamic systems. To a large extent, water ‘designs’ the channels through which it flows and, in the process, acts as the primary factor sculpting the features that comprise a landscape. Understanding how river channels form and change over time is a very active research topic in the fields of hydrology and geomorphology. Recent breakthroughs in numerical modeling (including computational fluid dynamics models that can resolve the complex structures of turbulence and fluid flow as well as morphodynamic models that can simulate interactions between flow, sediment and vegetation) and increasing availability of high resolution topography data (aerial light detection and ranging (lidar) data, terrestrial lidar, and high resolution surveying and 3-D photography techniques) have greatly enhanced our ability to study the form and dynamics of river channels in great detail, over vast areas. In the broadest sense, river channel form is controlled by a) the amount of water (especially the size of ‘common’ floods that occur once every few years, as discussed below), b) the underlying geology (the type of rock and variability within the rock structure), c) the amount and type of sediment supplied to the channel (coarse material such as sand and gravel as well as fine material such as silt and clay), and d) the type of riparian vegetation along the channel.
While the variety of river types is best thought of as a continuum, rather than a bunch of discrete boxes, it is often useful in science to create a taxonomy to classify items for the purpose of description and communication. Figure 30 illustrates some of the most common characteristics by which rivers can be classified (see Brierley and Fryirs, 2005 or Montgomery and Buffington, 1997 for detailed discussions of channel classification). At the most basic level it is useful to classify rivers according to the number of channels they contain, from single-threaded to braided (with more than three interweaving channels that are frequently reorganized) to anastomosing (which typically have somewhat stable, vegetated islands between channel threads), to discontinuous streams that have un-channelized reaches). Wandering rivers are those that alternate between single-threaded and slightly braided reaches. Another useful metric, particularly for single-threaded channels is sinuosity, which is calculated as the length along the river divided by the straight-line distance along the river valley. Rivers can have sinuosity ranging from one up to three (i.e., the river length is three times longer than the valley). Bends in rivers are called meanders. Meanders can exhibit a variety of forms with some in nature being remarkably regular (see the Fall River in Rocky Mountain National Park in Google Earth) and others being irregular or tortuous (frequently folding back on itself).
While there is currently no generalizable equation or universal law describing what a river channel should look like, a vast array of field data and modeling has culminated in some useful generalities. Stream power, defined as the product of water density (about 1000 kg/m3), gravitational acceleration (9.8 m/s2), discharge (m3/s), and channel slope (m/m), is one useful predictor of channel form and dynamics because it quantifies the amount of ‘work’ that can be done by a stream, such as moving sediment on the bed or in the banks of the river (i.e., erosion or sediment transport). For example, braided rivers tend to have more stream power than single threaded meandering rivers because their channel slope tends to be higher as they often flow closer to mountains (on steeper topography).
River channels are self-formed. Typically they are only partially filled, the water level is well below the tops of the banks. Sometimes, they are overfilled and water spills out onto the floodplain. These simple observations lead to the fundamental question, ‘what sets the size of a river channel?’ Figure 30 conceptually illustrates the rationale supporting the empirical finding that an ‘effective discharge’, which occurs frequently enough and has sufficient power to do work, ultimately dictates the size of the channel. Specifically, the brown curve illustrates that the frequency distribution of discharge in a river is typically right (positively) skewed, meaning that relatively low discharges are quite common and increasingly higher discharges occur with diminishing frequency. There is some discharge below which sediment does not move on the river bed because there is insufficient power to move the sand or gravel, as indicated by the light orange line starting at some moderate discharge and increasing in a non-linear manner at progressively higher discharge. Multiplying the brown and light orange lines together yields the darker orange line, which has a peak at some relatively high discharge value. This ‘effective discharge’ tends to occur when the river is approximately full up to its naturally formed banks. Even very large floods, which greatly exceed the capacity of the channel, do not necessarily add a proportionate amount of power to the channel because much of the additional water (and therefore the energy to transport sediment) is dissipated on the adjacent floodplain.
As changes in climate alter precipitation patterns or as land and water management modulates the proportion of precipitation that becomes streamflow, the frequency curve in Figure 30 may change and thus change the effective discharge as well as the geometry of the channel. In this way, rivers are dynamic features in the landscape, growing bigger when more water is flowing through the landscape and smaller during extended drier periods.