Kevin M. Schmidt

Pool Spacing in Forest Channels

Field surveys of stream channels in forested mountain drainage basins in southeast Alaska and Washington reveal that pool spacing depends on large woody debris (LWD) loading and channel type, slope, and width. Mean pool spacing in pool-riffle, plane-bed, and forced pool-riffle channels systematically decreases from greater than 13 channel widths per pool to less than 1 channel width with increasing LWD loading, whereas pool spacing in generally steeper, step-pool channels is independent of LWD loading. Although plane-bed and pool-riffle channels occur at similar low LWD loading, they exhibit typical pool spacings of greater than 9 and 2–4 channel widths, respectively. Forced pool-riffle channels have high LWD loading, typical pool spacing of <2 channel widths, and slopes that overlap the ranges of free-formed pool-riffle and plane-bed channel types. While a forced pool-riffle morphology may mask either of these low-LWD-loading morphologies, channel slope provides an indicator of probable morphologic response to wood loss in forced pool-riffle reaches. At all study sites, less than 40% of the LWD pieces force the formation of a pool. We also find that channel width strongly influences pool spacing in forest streams with similar debris loading and that reaches flowing through previously clear-cut forests have lower LWD loading and hence fewer pools than reaches in pristine forests.

Limits to Relief

Comparison of slope profiles in areas exhibiting widespread bedrock landsliding with the use of a model for the maximum size of stable hillslopes established that mountain-scale material strength can limit topographic relief. Conventional laboratory values for intact rock greatly exceeded integrative rock strength properties that were back-calculated from the upper limit to hillslope relief and gradient in the northern Cascade Range and Santa Cruz Mountains. Back-calculated strength values, however, were indistinguishable from those obtained through field and conventional laboratory measurements on the weakest members of each rock formation, as well as on glacial sediments along the Cascade front. These results contrast with the conventional assumption that relief is incision-limited and indicate that the relief of mountain ranges can reflect landscape-scale material strength, as well as the interaction of tectonic and climatic processes.

Forest clearing and regional landsliding

The influence of forest clearing on landsliding is central to longstanding concern over the effects of timber harvesting on slope stability. Here we document a strong topographic control on shallow landsliding by combining unique ground-based landslide surveys in an intensively monitored study area with digital terrain modeling using high-resolution laser altimetry and a coarser resolution regional study of 3224 landslides. As predicted by our digital terrain‐based model, landslides occur disproportionately in steep, convergent topography. In terrain predicted to be at low risk of slope failure, a random model performs equally well to our mechanism-based model. Our monitoring shows that storms with 24 hr rainfall recurrence intervals of less than 4 yr triggered landslides in the decade after forest clearing and that conventional monitoring programs can substantially underestimate the effects of forest clearing. Our regional analysis further substantiates that forest clearing dramatically accelerates shallow landsliding in steep terrain typical of the Pacific Northwest.

Controls on alluvial fan long-profiles

Water and debris flows exiting confined valleys have a tendency to deposit sediment on steep fans. On alluvial fans where water transport of gravel predominates, channel slopes tend to decrease downfan from ~0.10–0.04 to ~0.01 across wide ranges of climate and tectonism. Some have argued that this pattern reflects grain-size fining downfan such that higher threshold slopes are required just to entrain coarser particles in the waters of the upper fan, whereas lower slopes are required to entrain finer grains downfan (threshold hypothesis). An older hypothesis is that slope is adjusted to transport the supplied sediment load, which decreases downfan as deposition occurs (transport hypothesis). We have begun to test these hypotheses for alluvial fan long-profiles using detailed hydraulic and particle-size data in sediment transport models. On four alluvial fans in the western U.S., we find that channel hydraulic radiiare largely 0.5–0.9 m at fan heads, decreasing to 0.1–0.2 m at distal margins. We find that median gravel diameter does not change systematically along the upper 60%–80% of active fan channels as slope declines, so downstream gravel fining cannot explain most of the observed channel slope reduction. However, as slope declines, channel-bed sand cover increases systematically downfan from areal fractions of <20% above fan heads to distal fan values in excess of 70%. As a result, entrainment thresholds for bed material might decrease systematically downfan, leading to lower slopes. However, current models of this effect alone tend to underpredict downfan slope changes. This is likely due to off-channel gravel deposition. Calculations that match observed fan long-profiles require an exponential decline in gravel transport rate, so that on some fans approximately half of the load must be deposited off channel every ~0.20–1.4 km downfan. This leads us to hypothesize that some alluvial fan long-profiles are statements about the rate of overbank deposition of coarse particles downfan, a process for which there is currently no mechanistic theory.

Coseismic deformation during the 1989 Loma Prieta earthquake and range-front thrusting along the southwestern margin of the Santa Clara Valley, California

Damage patterns caused by the 1989 Loma Prieta earthquake along the southwestern margin of the Santa Clara Valley, California, form three zones that coincide with mapped and inferred traces of range-front thrust faults northeast of the San Andreas fault. Damage in these zones was largely contractional, consistent with past displacement associated with these faults. The damage zones coincide with gravity and aeromagnetic anomalies; modeling of the anomalies defines a southwest-dipping thrust fault that places the Franciscan Complex over Cenozoic sedimentary rocks to minimum depths of 2 km. Diffuse Loma Prieta earthquake aftershocks encompass the downward projection of this modeled thrust to depths of 9 km. Our results indicate that in this region the potential for concentrated damage arising from either primary deformation along the thrust faults themselves or by sympathetic motion triggered by earthquakes on the San Andreas fault may be higher than previously recognized.

Researchers consider U.S. Southwest's response to warmer, drier conditions

In 2000, the popular press frequently referred to reports that the southwestern United States might experience a shift from relatively wet to dry conditions during the next couple of decades (see http://topex-www.jpl.nasa.gov/discover/PDO.html). These predictions stemmed from observations that the Pacific Decadal Oscillation (PDO) appeared to abruptly change from a “positive” to a “negative” phase in 1999 (Figure 1). During the mid-twentieth century, a similar negative phase of the PDO was accompanied by prolonged dry conditions in the southwest. By extrapolation, some climatologists predicted future drought in the southwest. Such a change would heavily affect land use planning in the region, because national demographics have stressed the regions resources over the past century From 1990 to 2000, for instance, the population of Nevada and Arizona increased by almost 2.3 million people (http://www.census.gov/population/www/cen2000/respop.html). To discuss potential scenarios of landscape and ecosystem response to 25 years of hot and dry climate, scientists from diverse disciplines gathered at the University of Arizona in April 2001. The objectives of this workshop were to address evidence supporting predictions of warmer and drier climate and the possible landscape responses (http://geology.wr.usgs.gov/sw-workshop/).

Value of a Dual-Polarized Gap-Filling Radar in Support of Southern California Post-Fire Debris-Flow Warnings

AbstractA portable truck-mounted C-band Doppler weather radar was deployed to observe rainfall over the Station Fire burn area near Los Angeles, California, during the winter of 2009/10 to assist with debris-flow warning decisions. The deployments were a component of a joint NOAA–U.S. Geological Survey (USGS) research effort to improve definition of the rainfall conditions that trigger debris flows from steep topography within recent wildfire burn areas. A procedure was implemented to blend various dual-polarized estimators of precipitation (for radar observations taken below the freezing level) using threshold values for differential reflectivity and specific differential phase shift that improves the accuracy of the rainfall estimates over a specific burn area sited with terrestrial tipping-bucket rain gauges. The portable radar outperformed local Weather Surveillance Radar-1988 Doppler (WSR-88D) National Weather Service network radars in detecting rainfall capable of initiating post-fire runoff-generat...

Deformation from the 1989 Loma Prieta earthquake near the southwest margin of the Santa Clara Valley, California

Damage to pavement and near-surface utility pipes caused by the 17 October 1989 Loma Prieta earthquake provides evidence for ground deformation in a 663 km2 area near the southwest margin of the Santa Clara Valley, California (USA). A total of 1427 damage sites, collected from more than 30 sources, are concentrated in four zones, three of which are near previously mapped faults. In one of these zones, the channel lining of Los Gatos Creek, a 2-km-long concrete strip trending perpendicular to regional geologic structure, was broken by thrusts that were concentrated in two belts, each several tens of meters wide, separated by more than 300 m of relatively undeformed concrete. To gain additional measurement of any permanent ground deformation that accompanied this damage, we compiled and conducted post-earthquake surveys along two 5 km lines of horizontal control and a 15 km level line. Measurements of horizontal distortion indicate ∼0.1 m shortening in a northeast-southwest direction across the valley margin, similar to the amount measured in the channel lining. Evaluation of precise leveling by the National Geodetic Survey showed a downwarp with an amplitude of >0.1 m over a span of >12 km that resembled regional geodetic models of coseismic deformation. Although the leveling indicates broad, regional warping, abrupt discontinuities characteristic of faulting characterize both the broad-scale distribution of damage and the local deformation of the channel lining. Reverse movement, largely along preexisting faults and probably enhanced significantly by warping combined with enhanced ground shaking, produced the documented coseismic ground deformation.

Comparison of 1989 Loma Prieta earthquake damage to mapped lineaments along the range front of the Santa Cruz Mountains, California

Damage to public infrastructure at or below the ground surface (streets, curbs, and water and gas lines) in southwestern Santa Clara Valley, California, associated with the 1989 Loma Prieta earthquake, is used to support the assertion that the series of photointerpreted lineaments are tectonic in origin and related to long-term reverse faulting along the range front of the Santa Cruz Mountains. We quantitatively analyze whether the photointerpreted lineaments are spatially correlated with earthquake-induced damage by examining whether the damage was located preferentially closer to the mapped lineaments than to a spatially random set of points. The analysis confirms that damage related to the Loma Prieta earthquake is located preferentially close to mapped lineaments. This result supports the assertion that the lineaments have a tectonic origin related to range-front faulting along the Santa Cruz Mountains, and that their presence may be related to primary fault rupture, spatially focused shaking damage, or slip triggered by strong motion induced by nearby faults.

Optimizing the decomposition of time series using evolutionary algorithms: soil moisture analytics

Soil moisture plays a crucial part in earth science, with impact on agriculture, ecology, hydrology, landslides, and water resources. Extremes in soil moisture, which we denote as peaks and valleys, caused by heavy rainfalls and subsequent dry weather, are very important when predicting future soil moisture or even landslides. Existing methods, like moving averages, have limitations when it comes to smoothing time series data while preserving peaks and valleys. In this work, we propose a novel method, HyperSTL, for extrema-preserving smoothing of soil moisture time series. The method optimizes an existing time series decomposition technique, Seasonal Decomposition of Time Series by Loess (STL). HyperSTL optimizes STLs control parameters, which we call hyperparameters, using an objective function over the decomposed components. We demonstrate in experiments with nine soil moisture datasets that using HyperSTL generally results in improved predictions compared to using other smoothing methods.