THE MORPHOLOGICAL REPRESENTATION OF CHANNEL-FORMING FLOW IN ARROYOS

Three large arroyos, the Santa Cruz River, the San Simon River and the Rio Puerco, were studied to determine whether a channel-forming flow, or range of flows, exists in arroyos. These three arroyos were chosen for this study due to the availability of long-term discharge records and measured suspended-sediment discharge data. The hydrologic regime of each channel was examined by observing the seasonality of flow and sediment discharge variability, flashiness and attenuation rates using daily average discharge data. Discharge records collected every 15 minutes were also available for the Rio Puerco and Santa Cruz River, enabling a comparison of results. Magnitude-frequency analysis of both detailed and averaged discharge records were combined with a sediment rating curve to produce the effective discharge for each arroyo.

Significant arroyo morphologies were quantified by carrying out repeat cross-sections. These surveys highlighted the fact that arroyo systems are complex, with different channels within the main arroyo trench occupied by different magnitude flows. The surveyed morphologies were compared with the calculated effective discharge to enable process-form relationships to be established. The three arroyos evolved differently under different scenarios and yet all have similar morphological characteristics and behaviour patterns. The majority of arroyos in the southwestern USA have been aggrading for the past thirty or more years, forming a distinct low-flow (active) channel. Preliminary results from the arroyos studied indicate that this active channel is adjusted to a dominant flow, the exact value of which has yet to be determined.