san gabriel mountains
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Geology ◽  
2021 ◽  
Author(s):  
Erika L. Groh ◽  
Joel S. Scheingross

Waterfalls can form due to external perturbation of river base level, lithologic heterogeneity, and internal feedbacks (i.e., autogenic dynamics). While waterfalls formed by lithologic heterogeneity and external perturbation are well documented, there is a lack of criteria with which to identify autogenic waterfalls, thereby limiting the ability to assess the influence of autogenic waterfalls on landscape evolution. We propose that autogenic waterfalls evolve from bedrock bedforms known as cyclic steps and therefore form as a series of steps with spacing and height set primarily by channel slope. We identified 360 waterfalls split between a transient and steady-state portion of the San Gabriel Mountains in California, USA. Our results show that while waterfalls have different spatial distributions in the transient and steady-state landscapes, waterfalls in both landscapes tend to form at slopes >3%, coinciding with the onset of Froude supercritical flow, and the waterfall height to spacing ratio in both landscapes increases with slope, consistent with cyclic step theory and flume experiments. We suggest that in unglaciated mountain ranges with relatively uniform rock strength, individual waterfalls are predominately autogenic in origin, while the spatial distribution of waterfalls may be set by external perturbations.


Western Birds ◽  
2021 ◽  
Vol 52 (4) ◽  
pp. 322-339
Author(s):  
Ryan S. Terrill ◽  
Christine A. Dean ◽  
John Garrett ◽  
Daniel J. Maxwell ◽  
Lauren Hill ◽  
...  

Avian migration is a spectacular phenomenon, representing the annual movements of billions of birds globally. Because the greatest diversity and numbers of birds migrate at night, opportunities to observe active migration are rare. At a number of localities in North America, however, observers can quantify movements of many typically nocturnal migrants during daylight where they continue after dawn. Such locations have provided much information about species-specific phenology, status, and orientation during migration. Localities where morning flights of land birds can be observed are unevenly distributed, however, and are little reported along the Pacific coast. Here we describe a novel location for the observation of spectacular morning flights of nocturnal migrants during spring migration at Bear Divide, in the western San Gabriel Mountains, Los Angeles County, California. In two years of informal surveys at the site, we have recorded at least one morning with an estimated ~13,500 individual birds passing. Our preliminary analyses suggest that the peak of a species’ migration at Bear Divide is correlated with the latitude of a species’ breeding, being later in the spring as that latitude increases. Our data from Bear Divide provide an independent perspective on migration as quantified by local radar. Further work at this locality may help inform our knowledge of migration phenology and population trends.


2021 ◽  
Author(s):  
Erika L. Groh ◽  
Joel Scheingross

Supplemental text with methods, six figures, four tables, and MATLAB code.<br>


2021 ◽  
Author(s):  
Erika L. Groh ◽  
Joel Scheingross

Supplemental text with methods, six figures, four tables, and MATLAB code.<br>


2021 ◽  
Author(s):  
Rose Shillito ◽  
Markus Berli ◽  
Ian Floyd ◽  
Li Chen ◽  
Teamrat Ghezzehei

&lt;p&gt;Several factors are believed to contribute to post-wildfire flooding and debris flows. One contributing factor&amp;#8212;the occurrence of post-wildfire soil water repellency&amp;#8212;lacks a quantitative mechanism to incorporate the effects in physically-based runoff models. For this study, a physically-based model was developed linking the contact angle (degree of water repellency) to sorptivity. The model was verified in laboratory experiments using a silica sand proxy. The effects of water repellency on infiltration were illustrated. Further, the effect of water repellency on runoff was simulated using the AGWA-KINEROS2 watershed model with data from rainfall following the 2009 Station fire in the San Gabriel Mountains of southern California, USA. Results show water repellency has a quantifiable effect on runoff production, an effect enhanced by the dry soil moisture conditions common after wildfires.&lt;/p&gt;


2021 ◽  
Vol 21 (8) ◽  
pp. 6129-6153
Author(s):  
Fernando Chouza ◽  
Thierry Leblanc ◽  
Mark Brewer ◽  
Patrick Wang ◽  
Sabino Piazzolla ◽  
...  

Abstract. In this work, the impact of Los Angeles Basin pollution transport and stratospheric intrusions on the surface ozone levels observed in the San Gabriel Mountains is investigated based on a combination of surface and lidar measurements as well as WRF-Chem (Weather Research and Forecasting with Chemistry) and WACCM (Whole Atmosphere Community Climate Model) runs. The number of days with observed surface ozone levels exceeding the National Ambient Air Quality Standards exhibit a clear seasonal pattern, with a maximum during summer, when models suggest a minimum influence of stratospheric intrusions and the largest impact from Los Angeles Basin pollution transport. Additionally, measured and modeled surface ozone and PM10 were analyzed as a function of season, time of the day, and wind direction. Measurements and models are in good qualitative agreement, with maximum surface ozone observed for southwest and west winds. For the prevailing summer wind direction, slightly south of the ozone maximum and corresponding to south-southwest winds, lower ozone levels were observed. Back trajectories suggest that this is associated with transport from the central Los Angeles Basin, where titration limits the amount of surface ozone. A quantitative comparison of the lidar profiles with WRF-Chem and WACCM models revealed good agreement near the surface, with models showing an increasing positive bias as function of altitude, reaching 75 % at 15 km above sea level. Finally, three selected case studies covering the different mechanisms affecting the near-surface ozone concentration over the San Gabriel Mountains, namely stratospheric intrusions and pollution transport, are analyzed based on surface and ozone lidar measurements, as well as co-located ceilometer measurements and models.


2021 ◽  
Author(s):  
Katherine Scharer ◽  
Jenifer Leidelmeijer ◽  
Matthew Kirby ◽  
Nicole Bonuso ◽  
Devin McPhillips

&lt;div&gt; &lt;p&gt;In tectonically active regions, sedimentary records are overprinted by landscape response to climate, fire, and local earthquakes.&amp;#160; We explore this issue using a new paleoclimate record developed at the Pallett Creek paleoseismic site in southern California USA, a recently incised distal fan located along the San Andreas Fault at the base of a 35 km&lt;sup&gt;2&lt;/sup&gt; catchment in the San Gabriel Mountains.&amp;#160; To date, we have analyzed 6 m of section, spanning the last 1300 yr, for grain size, total organic material (TOM), carbon/nitrogen (C/N) ratios, magnetic susceptibility, and charcoal count. Existing C-14 dates (Scharer et al., 2011) inform rates of sediment deposition and charcoal accumulation (CHAR). Additional dating and macrofossil analysis is ongoing.&amp;#160; Sedimentological variability within the section is dominated by two general units. Unit 1 is characterized by high % clay, % silt, and % TOM, while Unit 2 is distinctly coarser with higher % sand and lower % TOM.&amp;#160; Pulses of high CHAR occur from 1150-1260 yr BP and during the Little Ice Age (100-500 yr BP) and are associated with high sedimentation rates (0.3-2 cm/yr), while only a few relatively weak fire episodes are recorded in the Medieval Climate Anomaly (700-1000 yr BP), despite similarly high sedimentation rates (0.6 cm/yr).&amp;#160; Ten earthquakes documented at the site (Sieh et al., 1989) occurred about every 135 years and impart no obvious short-term impact on sedimentation rates, perhaps reflecting the distance between the site and steeper portions of the drainage network (&gt;4 km) likely to produce mass wasting.&amp;#160; Overall, the landscape response of this large, integrated catchment appears to reflect a stronger influence of fire and climate than earthquakes. Future work will focus on the impact of the fire episodes on sediment delivery and resultant paleoearthquake ages.&lt;/p&gt; &lt;/div&gt;


2021 ◽  
Author(s):  
Mel O. Guirro ◽  
Rebecca A. Hodge ◽  
Fiona Clubb ◽  
Laura Turnbull

&lt;p&gt;Sediment transport in rivers depends on interactions between sediment supply, topography, and flow characteristics. Erosion in bedrock rivers controls topography and is paramount in landscape evolution models. The riverbed cover indicates sediment transport processes: alluvial cover indicates low transport capacity or high sediment supply, and bedrock cover demonstrates high transport capacity or low sediment supply. This study aims to evaluate controls on the spatial distributions of bedrock and alluvial covers, by analysing scaling geometric relations between bedrock and alluvial channels. A Principal Component Analysis (PCA) was conducted to evaluate correlations between river slope, depth, width, and sediment size. The two principal components were used to implement a clustering analysis in order to identify differences in alluvial and bedrock sections. Spatial distributions of mixed bedrock-alluvial sections were investigated from two datasets - Scottish Highlands (Whitbread 2015) and the San Gabriel Mountains in the USA (Dibiase 2011)-, representing different environmental conditions, such as erosion rates, lithology, tectonics, and climate. The rock strength of both areas is high, and therefore it is excluded as a factor that explains the difference between the areas. The results of the cluster analysis were different in each environment. The main sources of variation among river sections identified by PCA were slope and width for the San Gabriel Mountains, and drainage area and depth for the Scottish Highlands. The rivers in the Scottish Highlands formed clusters that differentiate bedrock and alluvial patches, showing a clear geometric distinction between channels. However, the river analysis from the San Gabriel Mountains showed no clusters. Bedrock rivers are typically described as narrower and steeper than alluvial rivers, as demonstrated by rivers in the Scottish Highlands (e.g. slope was around 0.1 m/m for bedrock sections and 0.01 m/m for alluvial sections). However, this may not be always the case: both bedrock and alluvial sections in San Gabriel Mountains presented similar slope around 0.1 m/m. The inability to demonstrate significant geometry differences in bedrock and alluvial sections in the San Gabriel Mountains may be due to the frequency and magnitude of sediment supply of that region, which are influenced by tectonics and climate. A major difference in the supply of sediment in rivers of the San Gabriel Mountains is the frequent occurrence of debris flow. Non-linear interactions between hydraulic and sediment processes may constantly modify the geometry of bedrock-alluvial channels, increasing the complexity of analysis at larger temporal and spatial scales. This study is part of the i-CONN project, which links connectivity in different scientific disciplines. A sediment connectivity assessment in different environments and scales may be useful to evaluate the controls on the spatial distribution of bedrock and alluvial rivers.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;Dibiase, R.A. 2011. Tectonic Geomorphology of the San Gabriel Mountains, CA. PhD Thesis. Arizona State University, Phoenix, 247pp.&lt;/p&gt;&lt;p&gt;Whitbread, K. 2015. Channel geometry data set for the northwest Scottish Highlands. British Geological Survey Open Report, OR/15/040. 12pp.&lt;/p&gt;


2021 ◽  
Vol 27 (1) ◽  
pp. 43-56
Author(s):  
Luke A. McGuire ◽  
Francis K. Rengers ◽  
Nina Oakley ◽  
Jason W. Kean ◽  
Dennis M. Staley ◽  
...  

ABSTRACT The extreme heat from wildfire alters soil properties and incinerates vegetation, leading to changes in infiltration capacity, ground cover, soil erodibility, and rainfall interception. These changes promote elevated rates of runoff and sediment transport that increase the likelihood of runoff-generated debris flows. Debris flows are most common in the year immediately following wildfire, but temporal changes in the likelihood and magnitude of debris flows following wildfire are not well constrained. In this study, we combine measurements of soil-hydraulic properties with vegetation survey data and numerical modeling to understand how debris-flow threats are likely to change in steep, burned watersheds during the first 3 years of recovery. We focus on documenting recovery following the 2016 Fish Fire in the San Gabriel Mountains, California, and demonstrate how a numerical model can be used to predict temporal changes in debris-flow properties and initiation thresholds. Numerical modeling suggests that the 15-minute intensity-duration (ID) threshold for debris flows in post-fire year 1 can vary from 15 to 30 mm/hr, depending on how rainfall is temporally distributed within a storm. Simulations further demonstrate that expected debris-flow volumes would be reduced by more than a factor of three following 1 year of recovery and that the 15-minute rainfall ID threshold would increase from 15 to 30 mm/hr to greater than 60 mm/hr by post-fire year 3. These results provide constraints on debris-flow thresholds within the San Gabriel Mountains and highlight the importance of considering local rainfall characteristics when using numerical models to assess debris-flow and flood potential.


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