The self-organization of step-pools in mountain streams

Spontaneous, autogenic self-organization has been described in numerous geomorphic systems, but it has not been investigated in detail with respect to coarse bedforms in general or step-pools in particular. In this paper, we review the spatial organization of step-pool systems and present example evidence of step-pool development as an autogenic self-organization process. We then outline the mathematical language for defining spatially divergent self-organization and test these ideas using two unique field examples from Oregon (Andrews Experimental Forest) and California (Baxter Creek), where step-pools developed from planar beds in artificially manipulated channels. Results show that step-pool development is consistent with a spatially divergent self-organization phenomenon. Entropy increases as initially undifferentiated planar channels diverge into steps and pools, then declines when a series of steps and pools of consistent size and spacing is established, signifying stability in the system. The self-organization process is accompanied by increasing flow resistance and decreasing slope (through increasing the “vertical sinuosity” of the step-pool profile and creation of low- or negative gradient pool areas), suggesting a minimization of stream power. The self-adjustment of the step-pool bed profile over time represents another manifestation of a general process that results in rhythmic patterns on the surface of Earth.