ORIGINS OF EPISODICITY IN ARROYO FORMATION AND DEVELOPMENT: EVIDENCE FROM THE COLORADO HIGH PLAINS

Arroyos are notoriously episodic. These trench-like dryland channels can form in a single day, and yet they can also remain essentially stable for decades at a time. Field examples in the High Plains of southeastern Colorado, together with mathematical models, provide insight into the unique combination of climate, physics, and material properties that give rise to the arroyo cycle. These data point toward the following as necessary and sufficient conditions for arroyo morphology and dynamics. First, pronounced seasonality in rainfall intensity, with dominance by summer monsoonal/convective storms, promotes episodicity in flow. In addition to increasing the intrinsic variability in sediment-transport rates, this flow episodicity enhances channel resistance by allowing for growth of a tough vegetation “armor layer” between flash floods. Second, the presence of silt- and clay-rich sediment beneath a valley floor has two important effects: the high erodibility of fine-grained sediment enhances the erosional contrast between bare and vegetation-armored soil, while the associated cohesion supports steep channel walls that constrict flow and contribute to maintaining high shear stresses. These factors help explain the common pattern of headward propagation accompanied by downstream aggradation. The high shear stresses immediately below a headcut, reflecting channel constriction and plunge-pool effects, promote rapid sediment entrainment. Downstream channel widening, which reflects progressive undercutting and failure of sidewalls in the wake of a passing headcut, leads to downstream reduction in shear stress and to sediment aggradation. Thus, the observed “self-healing” behavior of arroyos can be seen as a natural consequence of process and materials. This leads to a view of arroyos as inherently transient features that reflect strong nonlinearities in the geomorphic response to episodic rainfall. On the other hand, “transient” doesn't necessarily mean “anomalous.” Rather, on the basis of scaling arguments and simple experiments with a drainage basin evolution model, we argue that arroyo growth and healing can contribute to maintaining a dynamic equilibrium in which each branch of the valley network adjusts to a given frequency-magnitude spectrum of rainfall and runoff.