The method also does not intrinsically provide a specific definition for riffles. Complications arise because residual depth is influenced by sediment deposition at low flow, which can fill pools and reduce length. A minimum width can be imposed with pools occupying at least half the channel width and narrower depressions classified as scour holes ( Lisle, 1986), and minimum-length criterion used to avoid subdividing channels into numerous unrealistically short segments ( Lofthouse and Robert, 2008). In either bathometric analyses or field surveys, pools are identified using residual-depth criteria, also called the control-point method, which assumes water is impounded by a downstream hydraulic control composed of sediment ( Lisle and Hilton, 1992). Pool and riffle features and definitions. Despite these common tendencies, there are several fundamental differences in basic characteristics and processes observed in PRS that have challenged researchers to develop comprehensive theories to explain formation and maintenance of these features.įig. PRS are generally wider and deeper than other morphologies for similar grain sizes, with distinct patterns of sediment sorting, shear stress and variation in roughness ( Montgomery and Buffington, 1997 Wohl and Merritt, 2008).
![riffle definition riffle definition](http://img.tfd.com/wn/17/665F3-riffle.png)
However, PRS are also common in less sinuous alluvial channels ( Leopold and Wolman, 1960 Montgomery et al., 1995) and in bedrock-controlled canyon settings ( Dolan et al., 1978 Wohl, 2007). In meandering systems, PRS are frequently linked to planform geometry with pools located at bends and riffles, or crossings, located in straighter sections between bends ( Keller and Melhorn, 1978 Richards, 1982 Knighton, 1998). Pools offer deeper water habitat during periods of low-flow or ice cover, riffles provide spawning gravels and primary food-production areas, and PRS encourage hyporheic exchange ( Gorman and Karr, 1978 Heede and Rinne, 1990 Hanrahan, 2007 Tonina and Buffington, 2007). Pool–riffle sequences (PRS) are bedform undulations developed in either supply- or transport-limited systems with gravel beds and gradients < 0.02 ( Leopold et al., 1964 Montgomery and Buffington, 1997 Knighton, 1998 Wohl, 2000 Robert, 2003).
![riffle definition riffle definition](https://www.medcontroller.de/wp-content/uploads/2013/09/Akutes-Nierenversagen-Einteilung-AKIN-RIFLE.jpg)
Thompson, in Reference Module in Earth Systems and Environmental Sciences, 2018 Pool–Riffle Sequence Characteristics