Holocene and Anthropocene Landscape Change: Arroyo Formation on Santa Cruz Island, California

Abstract

In this study, we untangle the relative importance of climatic, tectonic, and anthropogenic drivers as triggers of arroyo formation and geomorphic change for a small watershed on Santa Cruz Island, California. Within the Pozo watershed (6.47 km 2 ), historic arroyo incision occurred contemporaneously with arroyo incision across many of the world's dryland regions. Unlike many of these other sites, Pozo contains a datable record that allows quantification of sedimentation rates from the mid-to-late Holocene to the twentieth century. Basin-wide environmental changes were assessed using a combination of cosmogenic radionuclide inventories, midden and marine-shell deposits, relict soil properties, airborne and ground-based light detection and ranging (lidar) data, ranching artifacts, and historic records. Shortly after the introduction of sheep in 1853, localized sedimentation rates on the Pozo floodplain increased by two orders of magnitude from 0.4 mm/year to ∼25 mm/year. Accelerated sedimentation was followed by arroyo formation ca. 1878 and rapid expansion of the incipient gully network, the lateral extent of which has been largely maintained since 1929. Catchment-mean erosion rates from cosmogenic radionuclide measurements indicate that presettlement rates were less than 0.08 mm/year, whereas lidar-derived measurements of historic gully erosion produce estimates almost thirty times higher (∼2.2 mm/year). Topographic measurements since 2005 indicate that the active channel of the Pozo watershed is aggrading. We argue that accelerated sedimentation due to overgrazing, and an unusually large 1878 rainstorm event, set the stage for arroyo formation in the Pozo watershed between 1875 and 1886. We hypothesize that even in the absence of modern human disturbance, downcutting would have occurred due to intrinsic hillslope stability thresholds. Key Words: cosmogenic radionuclides, gully incision, lidar, overgrazing. En este estudio rescatamos la importancia relativa que tienen los factores climáticos, tectónicos y antropogénicos como fuerzas que inician la formación de arroyos y el cambio geomórfico en una pequeña cuenca hidrográfica de la Isla de Santa Cruz, en California. En la cuenca del Pozo (6.47 km2), el excavado histórico de arroyos ocurrió contemporáneamente con procesos similares que afectaron a muchas de las regionesáridas del mundo. A diferencia de lo que ocurre en muchos de estos otros lugares, el Pozo contiene un registro susceptible de datación que permite la cuantificación de tasas de sedimentación desde el Holoceno medio y tardío hasta el siglo XX. Los cambios ambientales ocurridos en toda la cuenca se calcularon utilizando una combinación de inventarios radionúclidos cosmogénicos, basureros y concherías marinas, propiedades de relictos pedológicos, datos sobre detección yámbito de luminosidad (LIDAR) desde el aire y a nivel del suelo, artefactos usados en los ranchos

How does grid-resolution modulate the topographic expression of geomorphic processes?

“…The island has a Mediterranean climate similar to that of Gabilan Mesa (Pinter and Vestal, 2005), supporting extensive grassland with occasional patches of pine forest and chaparral vegetation (Pinter and Vestal, 2005;Perroy et al, 2010Perroy et al, , 2012. Human activities led to overgrazing across the island at the turn of the 19th century, causing a period of gullying and rapid erosion, particularly evident in the southwest of the island (Pinter and Vestal, 2005;Perroy et al, 2012).…”

“…High tangential curvature at Santa Cruz Island corresponds to observations of extensive gullying and hillslope erosion (Pinter and Vestal, 2005;Perroy et al, 2012). In the Oregon Coast Range, features such as pit and mound topography produced by tree throw and other biotic activity are resolved in the lidar dataset (Roering et al, 2010;Marshall and Roering, 2014), which manifests itself as an increase in values of curvature.…”

“…However, more recent work has demonstrated the spatial variability of many topographic measurements in this landscape (Marshall and Roering, 2014;Grieve et al, 2016b), and as such it provides a more challenging test case for our analyses. Furthermore, these sites were selected as they each have high-resolution lidar data covering a large spatial area and have been the subject of many previous studies (Reneau and Dietrich, 1991;Roering et al, 1999Roering et al, , 2001Montgomery, 2001;Pinter and Vestal, 2005;Roering et al, 2007;Perron et al, 2009;Perroy et al, 2010Perroy et al, , 2012Marshall and Roering, 2014;Grieve et al, 2016a, b), which should provide a good basis for the evaluation of the results of this study in a wider geomorphic context.…”

“…Human activities led to overgrazing across the island at the turn of the 19th century, causing a period of gullying and rapid erosion, particularly evident in the southwest of the island (Pinter and Vestal, 2005;Perroy et al, 2012). The lidar data collected for this location have been extensively tested and ground truthed, ensuring that they are suitable for use in a geomorphic context (Perroy et al, 2010) and for performing topographic analysis at high spatial resolutions.…”

“…2c), the largest of the eight California Channel Islands located to the west of California, USA, is divided by a large east-west trending valley, which follows the Santa Cruz fault (Pinter et al, 2003;Muhs et al, 2014). Parallel to this valley are two large ridges -one to the north and one to the south -which exhibit regularly spaced parallel channels draining north to south (Pinter et al, 1998;Pinter and Vestal, 2005); this regular pattern is particularly evident in the northwest section of the study area. The Santa Cruz Fault has been demonstrated to have left-lateral strike slip motion, which deflects channels away from the perpendicular to the main valley in the center of the island (Pinter et al, 1998).…”