The morphological representation of channel-forming flow in arroyos
This thesis presents a study of three similar, yet contrasting, arroyos: the Rio Puerco, New Mexico and the Santa Cruz and San Simon Rivers in Arizona. Arroyo systems have been subject to several periods of cut and fill within recent geologic time. The most recent incision phase occurred during the mid- to late-19th Century, enabling subsequent morphological changes to be observed. Observations indicate that these systems have developed distinct geomorphological reaches, which evolve at different rates within the constraints of the specific controlling variables, most importantly discharge, sediment load and artificial alteration. Thus, location-for-time substitution cannot be assumed. Within the continuum of morphological change, six stages were identified, enabling a general model of arroyo evolution to be developed. Precipitation has increased in the south-western USA. However, within the arroyos studied, some gauges have seen a concurrent increase in discharge, whereas others have experienced a decline. It was discovered that the different responses were explained by the interaction between discharge and morphology at different stages of evolution. The first four stages of arroyo evolution differ little from those proposed previously. However, this study has emphasised the necessity of recognising the event-driven nature of morphological change within the system during these stages. Since some change occurs during each flood, the arroyo morphology is controlled by a combination of the flow variability and channel-deforming events. As aggradation predominates, the arroyo develops a quasi-stable form, creating a feedback between discharge and morphology. High-magnitude flows are suppressed and aggradation is promoted, permitting continued stabilisation until a quasi-equilibrium form is attained, typified by a trapezoidal active channel and densely vegetated inner floodplain. These morphological changes alter the discharge regime sufficiently that lower-magnitude flows assume greater relative effectiveness. It is not until a quasi-equilibrium morphology becomes adjusted to the effective discharge, at which point the effective discharge becomes a truly channel-forming and maintaining flow.