ABSTRACT
Extreme meteorological events can cause at least two types of catastrophic floods in northern Arizona and southern Utah. Streamflow floods occur in large drainage basins composed of Mesozoic sedimentary rocks or Tertiary volcanic rocks in southern Utah. A storm in the summer of 1909 triggered a streamflow flood in the Escalante River basin that initiated the erosion of a deep, rectangular-shaped arroyo into a previously unincised flood plain. Debris flows occur in small, high-relief basins developed in Paleozoic strata in the Grand Canyon National Park, Arizona. A winter frontal storm in December 1966 caused debris flows in the Lava-Chuar and Crystal Creek drainage basins that reached the Colorado River. The debris flows, triggered by slope failures during intense rainfall, caused bed scour, lateral migration of channels into older terraces, and deposition of levees of poorly sorted sediments.
Rectangular channels form in response to flooding regardless of the type of flood although channels affected by streamflow floods require more time to completely form. Channels change after the floods, by local bank collapse and channel widening, sedimentation on flood plains that form between the channel banks or walls, and reinvasion of riparian vegetation. The net effect of these changes is the creation of a low-water channel and flood-plain system within the arroyo walls or channel banks formed by the large flood.
