Alluvial architecture in headwater streams with special emphasis on step-pool topography

Abstract

Alluvial mountain streams exhibit a range of channel forms: pool±riffle, plane bed, step±pool and cascades. Previous work suggested that these forms exist within discrete, and progressively steeper slope classes. Measurements conducted at over 100 sites in west-central and central Idaho confirm that slope steepens progressively as one moves from pool±riffle, to plane bed, to step±pool, and finally to cascades. Median slope for pool±riffle topography is 0Á0060, for plane beds 0Á013, for step± pools 0Á044, and for cascades 0Á068. There is substantial overlap in the slopes associated with these channel forms. Pool± riffle topography was found at slopes between 0Á0010 and 0Á015, plane beds between 0Á0010 and 0Á035, step±pools between 0Á015 and 0Á134, and cascades between 0Á050 and 0Á12.Step±pools are particularly striking features in headwater streams. They are characterized by alternating steep and gentle channel segments. The steep segments (step risers) are transverse accumulations of boulder and cobbles, while the gentle segments (pools) contain finer material.Step wavelength is best correlated to step height which is in turn best correlated to the median particle size found on step risers. This result differs from past studies that have reported channel slope to be the dominant control on step wavelength. The presumed geometry and Froude number associated with the features under formative conditions are consistent with the existence field for antidunes and by extension with the hypothesis that step±pools are formed by antidunes.

Prediction of mountain stream morphology

“…Montgomery and Buffington's original data set indicated specific ranges of gradient, shear stress and relative roughness for each channel type: cascade channels are most common at gradients greater than 0.065, step-pool channels at 0.03 -0.065, plane-bed channels at 0.015 -0.03, and pool-riffle channels at gradients less than 0.015. Expanding the original data set to other geographic regions indicates greater variability in gradient range for each channel type, although the progression from cascade channels at the highest gradients to pool-riffle channels at the lowest gradients appears to be consistent [Chin, 1989;Grant et al, 1990;Chartrand and Whiting, 2000].…”

Biogeomorphological effects of leaf accumulations in stepped-bed channels: Exploratory study, Moravskoslezské Beskydy Mountains, Czech Republic

“…One exception is the work of Borák (2012) from steep headwater streams. We also compared the geometric characteristics of foliaged step-pools with alluvial (Chin, 1999;Chartrand and Whiting, 2000;Lenzi, 2001;Nickolotsky and Pawlowsky, 2007;Recking et al, 2012;Frandofer and Lehotský, 2013) and bedrock (Duckson and Duckson, 2001) step-pool morphologies that occurred in areas similar to our study location, with small watersheds and steep channel gradients. Borák (2012) investigated foliaged step-pool systems in the flysch-based high-gradient Mazák Stream (A = 0.9 km 2 ) in the Western Carpathians, where the mean bankfull width was 1.08 m and the channel gradient was 0.24 m/m.…”

“…This is consistent with our findings for foliaged step-pools with both boulders and SW. Significantly higher values with H/L/S ≥ 3 were found in a large dataset of step-pool reaches in Idaho, USA (Chartrand and Whiting, 2000) and a value of 2.5 was obtained for the steppool channel Cold Creek in California (USA) (Chin, 1999). In both cases, much higher step heights were observed (0.5-0.6 m).…”

Determinants of modelling choices for 1-D free-surface flow and morphodynamics in hydrology and hydraulics: a review

“…Progressively lower inundation ratios were investigated (Smith et al, 2007) until the real cases of emergent obstacles received attention (Bayazit, 1976;Abrahams and Parsons, 1994;Bathurst, 2006;Meile, 2007;Mü-gler et al, 2010), including for non-submerged vegetation Nepf, 1999;Järvelä, 2005;. For site-specific friction laws, the default f -Re relation is sometimes complemented by f -Fr trends (Grant, 1997;Gimenez et al, 2004;Tatard et al, 2008) or f -z relations Chin, 1999;Chartrand and Whiting, 2000;Church and Zimmermann, 2007) in steep bed morphologies, where Fr is the Froude number (Froude, 1868).…”