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Segura, Catalina ¹ ; Pitlick, John ²

¹ University of Colorado
² University of Colorado

The spatial variability of sediment transport intensity at the reach scale is controlled both by the available shear stress, Ï„, and the grain size distribution of the channel bed. This variability creates patches in the bed with different disturbance regimes that in turn influence the spatial and temporal variability of benthic communities. Two-dimensional flow modeling was used to compute Ï„ for several discharge levels in three reaches of the Williams Fork River, CO. The distributions of Ï„ were evaluated and used, with detailed measurements of grain size distribution of the bed, to investigate the spatial variability of channel bed disturbance and to compute sediment transport intensity. The relationship between transport intensity and periphyton recovery was investigated by comparing the rate of Chl a accrual over time during two years with contrasting disturbance intensity (2004 and 2005), and by looking at the relationship between the spatial variation of disturbance intensity and benthic Chl a during the summer season of 2007.

It was found that, even though mean Ï„ values for a given flow size are highly variable among sites, the properties of the normalized distributions of shear stress are similar across sites (Fig. 1). Analysis of sediment transport intensity indicated that most of the bedload in the study sites is composed by intermediate grain sizes (16-45mm) carried through places in the bed that experience the highest range of shear stress. In addition, transport intensity influences the post-flood accrual pattern of periphyton in years of both low and high peak flows, the accrual of periphyton is faster when the preceding peak flow is smaller, and the accrual rate of periphyton is not uniform, with locations at higher disturbance characterized by slower accrual than locations at lower disturbance (Fig. 2).