Bed load transport measurements were made in a small urban stream in Decatur, GA, from which thresholds for motion were calculated using methodologies from the published literature. These methodologies are discussed in terms of their limitations and assumptions. Mobility frequencies were calculated for single grains of each grain size fraction to illustrate the transition from size selective transport to equal mobility. In general, urban streams behave differently than many gravel rivers in non-urban settings because of differences in the availability and character of sediment sources and altered flow hydrographs. This comparison allows for discussion about the way sediment is transported in urban streams versus typical gravel-bed, armored channels.

To determine the relationship between the abundance and density of benthic invertebrates, and the critical shear stress of individual grain sizes, a reach along Smith Creek, was divided into ten 2m x 2m quadrants. Within each quadrant, five randomly selected clasts for each grain size ranging from 2.26 to 25.6 cm were cleaned for benthic invertebrates. Wolman pebble counts for each quadrant were also conducted and used to determine the critical Shields stress per grain size fraction from the model given by Wiberg and Smith (1987) that explicitly accounts for particle hiding/sheltering effects in mixed-bed rivers. Particle entrainment values were then compared with estimated bankfull Shields stress values to determine sediment transport potential during bankfull flow. Invertebrate abundance was strongly positively correlated with critical Shields stress up to the 18.0 cm grain size, indicating a preference for certain grain sizes; while density was positively correlated with all grain sizes present.