ABSTRACT Mount Diablo State Park exemplifies many other conservation areas where managers balance the dual missions of protecting natural resources while providing public access. Roads and trails that crisscross the park are etched into the geomorphic surface, capturing and redirecting storm runoff, and presenting both a challenge for soil conservation and a consequence of construction and maintenance. We used field mapping, remote sensing, and modeling to assess erosion along the roads and trails in Mount Diablo State Park, which encompasses the headwaters of several urbanized watersheds. The field mapping in 2011 determined that 56% of the assessed roads and trails required either repair or reconstruction to control erosion and that ~67% of the culverts in the park required either repair or replacement. Aerial photography and modeling showed that other erosion (unrelated to roads or trails) preferentially occurred during wet periods, in specific lithologies, and on convergent slopes. Although lithology and climate drive slope-forming geomorphic processes, we found that the road and trail system (1) expanded the stream network with a capillary-like system of rills, (2) catalyzed prolonged erosion, and (3) altered the timing and pattern of sediment yield. In addition to water-driven erosion during wet periods, road and trail surfaces were subject to mechanical and wind erosion during dry periods. Spatially, dry erosion and runoff both conformed with and crossed topographic gradients by following the road and trail network. Road- and trail-induced erosion occurred across a wider range of rock properties and slope geometries than is typical for other erosion. Hence, the roads and trails have expanded the spatial and temporal boundary conditions over which geomorphic processes operate and, due to continual soil disturbance, have accelerated erosion rates. Although road density is a commonly used metric to rank road-related impacts at watershed scales, it misses both spatial variability and the opportunity to identify specific road and trail segments for remediation. We developed a spatially explicit scoring scheme based on actual erosion and the potential for sedimentation of discrete waterbodies. The data were incorporated into the park’s road and trail management plan in 2016.

ABSTRACT Montecito, California, has a complicated Quaternary history of debris flows, the most recent being the Montecito debris flows of 9 January 2018, which were wildfire-debris flow–linked events that took 23 lives and damaged or destroyed several hundred homes. Relative flow chronology, based on boulder weathering, incision rates, and soil dates with limited numerical (radiocarbon and exposure) dating, is used to identify paths of prehistoric debris flows. Topography of debris flow fans on the piedmont is significantly affected by the south-side-up reverse Mission Ridge fault system. Examination of weathering rinds from Pleistocene debris flows confirms that the Rattlesnake Creek–Mission ridge debris flows are folded over the ridge, and that lateral propagation linked to uplift of marine terraces (uplift rate of ~0.5–1 m/k.y.) significantly altered debris flow paths. As communities continue to rebuild and live in these hazard-prone areas, disaster risk reduction measures must take into account both spatial and temporal components of vulnerability. This field guide includes four stops from Montecito to Santa Barbara. The first stop will be to observe debris flow stratigraphy over the past ~30 ka beneath an earthquake terrace and a prehistoric Chumash site on the beach near the Biltmore Hotel in Montecito. The second stop will be at San Ysidro Creek in San Ysidro Canyon, the site of the largest Montecito debris flow that occurred on 9 January 2018. We will discuss source area and processes of the debris flow, and take a short hike up the canyon to visit the debris flow basin and a ring net designed to reduce the future hazard. The final two stops will explore the debris flow chronology of Santa Barbara over the past ~100 ka. Figure 1 shows the location of the field-trip stops. There is no road log as field sites can be found with a search on a smartphone.

Abstract Land managers of military training lands must conserve the soil to ensure that training can continue on those lands. However, military maneuvers damage vegetation, break up soil crusts, loosen the surface soil, change soil-surface geometry, compact the soil, and often form ruts in which runoff is concentrated. The constrained flow in ruts can detach and transport far more sediment than can unchanneled, overland flows. Rills often form in ruts as a result. However, natural processes in the soil alter the impacts of maneuvers over time, and our objectives were to measure how soil freeze-thaw (FT) cycling changes compacted soil and the geometry of military-vehicle ruts and how these changes compare to those in natural rills. We established research sites at Yakima Training Center (YTC) in south central Washington and Ethan Allen Firing Range (EAFR) in northwestern Vermont and made field observations and measurements at these sites over two winters. The cross sections of tank ruts at YTC became smoother as soil from rut crests slid into the rut during thaw. Tank ruts at EAFR were shallower than those at YTC and smoothed over the winter, but rills also formed in the ruts over one winter. Scattered soil slumps occurred along the sides of deeper rills at EAFR during spring thaw, but the slumped sediment was removed by subsequent flows. FT at both sites reduced the mean penetration resistance and bulk density of the top 5 cm of soil in ruts. Below 5 cm, resistance and density were statistically greater in than out of ruts at YTC, especially where the soil contained 15% water by volume during maneuvers. Saturated hydraulic conductivity in and out of ruts at YTC was not statistically different when ruts were formed in soil that contained 5% water, was lower in 75% of straight ruts made in soil containing 15% water, and was lower yet in curved ruts. Surface-water runoff at YTC began sooner in ruts than on adjacent, unrutted soil, and runoff rates were 67% to 77% higher due to the persistence of subsurface soil compaction in ruts. Incipient rills formed in tank ruts at EAFR after one winter on the 7% and 18% slopes. In-rut rills up to 11-cm deep formed on the 21% and 31 % slopes. The adjacent, untrafficked soil on any of the slopes showed no new rills. These results can be used to parameterize soil-erosion models used by land managers of military-training lands in cold regions.