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A field study was conducted on two mountain streams in the Cascade Mountains of Washington State on the morphological characterization of cluster microforms. Morphological characterization of clusters is presented in terms of: (i) cluster shape; (ii) cluster geometric properties; and (iii) the spatial arrangement of clusters in the horizontal plane. Clusters were differentiated from other microtopography features such as reticulate structures and transverse ribs, and identified clusters were categorized by shape as being of pebble, line, comet, heap or ring type. The complex spatial arrangement of clusters at the sites was characterized by using a two-dimensional correlation function, which allowed for measurement of the average cluster-spacing properties. For the rivers examined, pebble-shaped clusters were the most frequently observed cluster shape. Cluster geometric properties were found to be controlled by particles of the largest size fraction in the bed and the projected frontal width of the cluster – with cluster length being linearly related to cluster width for cluster width-to-height ratios <3·5. Results of the cluster-spacing analysis suggest that cluster spacing increases with cluster size and decreases with local slope. Application of this principle to the available spacing data shows that cluster spacing λ scales with the ratio of S/ d0 such that λ S/ d0 = constant, where S is the local slope and d0 the diameter of the largest particle in the cluster. [ABSTRACT FROM AUTHOR]