We use a minimally invasive, shallow geophysical technique to image the structure of the criti cal zone from surface to bedrock (0-20 m) in two small drainages within the Boulder Creek Criti cal Zone Observatory (BcCZO). Shallow seismic refracti on (SSR) surveys provide a three-dimensional network of two-dimensional cross-secti ons (termed quasi-3D) of criti cal zone compressional wave velocity (V p ) structure within each catchment, yielding a spati al descripti on of the current criti cal zone structure. The two catchments, Betasso and Gordon Gulch, represent contrasti ng geomorphic histories within the Front Range: Betasso shows hillslope response to a late Cenozoic increase in fl uvial incision of Boulder Creek, while Gordon Gulch represents more steady erosion. The mean depth to fresh bedrock in both catchments is roughly 15 m. Unique subsurface features in each catchment refl ect acti ve geomorphic processes not suggested by similariti es in mean interface depths. Betasso contains thick disaggregated materials high in the drainage that are nearly absent near the outlet. This presumably refl ects the impact of base-level lowering, which we suggest has progressed roughly 500 to 1000 m up into the catchment. Aspect-driven diff erences in the subsurface within each catchment add complexity and overprint the broader geomorphic signals. Shallow seismic refracti on subsurface structure models will guide future investi gati ons of criti cal zone processes from landscape to hydrologic modeling and are invaluable as connecti ons between ti me-consuming point measurements of physical, chemical, and biological processes.Abbreviati ons: BcCZO, Boulder Creek Criti cal Zone Observatory; ERT, electrical resisti vity tomography; GPR, ground-penetrati ng radar; Ma, megaannum; quasi-3D, a three-dimensional grouping of two-dimensional surveys; SSR, shallow seismic refracti on.Shallow seismic refracti on allows minimally invasive, broad spatial investigations of the subsurface (Leopold et al., 2008b). We use this geophysical technique to pair surficial evidence of geomorphic processes with physical and chemical weathering signatures interpreted from SSR surveys.Th e interactions between weathering and transport processes sculpt terrestrial landscapes. Hillslope processes that include downslope movement of material by rain splash, frost creep, biological activity, and other gravity-driven mechanisms serve to redistribute material that is freed from the underlying bedrock by weathering processes. Th e hillslopes are in turn coupled to adjacent streams, which serve as their boundary conditions. Th is highly coupled geomorphic system therefore encompasses the majority of hydrological, geochemical, and biological activities that can all change in effi ciency and in dominance through time.Recently, the term critical zone (CZ) has been applied to the shallow terrestrial environment spanning from the lowest extent of groundwater to the top of the vegetation canopy (Brantley et al., 2007;Anderson et al., 2007) (Fig. 1). Th e defi nition...

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Information about the internal structure of rock glaciers is needed to understand their reaction to ongoing climate warming. Three different geophysical techniques—shallow seismic refraction, ground‐penetrating radar (GPR) and electrical resistivity tomography—were used to develop a detailed subsurface model of the Green Lake 5 rock glacier in the Colorado Front Range, USA. Below a thin zone of fine sediments and soils (0.7 – 1‐m thickness; 0 – 20 kΩm and 320 – 370 m s−1), a 1 – 3‐m thick zone with low p‐wave velocities (790 – 820 m s−1) and high electrical resistivity (20 – 100 kΩm) is interpreted as the ice‐free, blocky active layer with large void spaces. The data corroborate strong reflections of the GPR signals which travel at this depth at 0.11 m ns−1. A third layer that extends from depths of 1 – 3 m to about 5 m is characterised by lower electric resistivities (5 – 20 kΩm) and has lower electromagnetic wave velocities (0.65 m ns−1), representing unfrozen, finer and wetter sediments. At around 5‐m depth, the measured physical parameters change drastically (vp = 3200 – 3300 m s−1, 50 – 150 kΩm, vGPR = 0.15 m ns−1), showing an ice‐rich permafrost zone above the bedrock. This model of the internal structure was used to evaluate an existing hydrological flowpath model based on the hydrochemical properties of water outflow from the rock glacier. Copyright © 2011 John Wiley & Sons, Ltd.

The multipart Riffeltal rock glacier, located in a tributary valley of the Kaunertal, Tyrol, Austria is investigated to enlarge the knowledge about spatial and temporal development of rock glaciers in and at the margins of pro‐glacial areas and to get a better understanding of glacier–rock glacier interactions. The subject of interest consists of a complex system of two adjacent rock glacier tongues and various superposed lobes with differing ages, origin and root zones, and therefore diverse development. To determine the reasons for their diverging development, the internal structure and permafrost occurrence on and in the surrounding area of the rock glacier were studied by application of geomorphological mapping, geophysical methods and measurement of the basal temperature of the winter snow cover (BTS). Permafrost modelling was performed on the basis of BTS data and land surface parameters derived from a high resolution airborne laser scanning (ALS) digital elevation model (DEM). Additionally, the ALS data were used to measure vertical and horizontal changes of the rock glacier surface between 2006 and 2012. Glacier–rock glacier interactions during and since the Little Ice Age (LIA) are evident for the development of the studied rock glacier. A geomorphic map gives important information about the connection between glacial advance or retreat and permafrost or ground ice occurrence. The combination of all information helps in the analysis of diverse kinematic action of neighbouring rock glacier tongues. Copyright © 2014 John Wiley & Sons, Ltd.

The architecture of the critical zone includes the distribution, thickness, and contacts of various types of slope deposits and weathering products such as saprolite and weathered bedrock resting on solid bedrock. A quantitative analysis of architecture is necessary for many model-driven approaches used by pedologic, geomorphic, hydrologic or biologic studies. We have used electrical resistivity tomography, a well-established geophysical technique causing minimum surficial disturbance, to portray the subsurface electrical resistivity differences at three study sites (Green Lakes Valley; Gordon Gulch; Betasso) at the Boulder Creek Critical Zone Observatory (BcCZO). Possible limitations of the technique are discussed. Interpretation of the specific resistivity values using natural outcrops, pits, roadcuts and drilling data as ground truth information allows us to image the critical zone architecture of each site. Green Lakes Valley (3700 MASL), a glacially eroded alpine basin, shows a rather simple, split configuration with coarse blockfields and sediments, partly containing permafrost above bedrock. The critical zone in Gordon Gulch (2650 MASL), a montane basin with rolling hills, and Betasso (1925 MASL), a lower montane basin with v-shaped valleys, is more variable due to a complex Quaternary geomorphic history. Boundaries between overlying stratified slope deposits and saprolite were identified at mean depths of 3.0 AE 2.2 m and 4.1 AE 3.6 m in the respective sites. The boundary between saprolite and weathered bedrock is deeper in Betasso at 5.8 AE 3.7 m, compared with 4.3 AE 3.0 m in Gordon Gulch. In general, the data are consistent with results from seismic studies, but electrical resistivity tomography documents a 0.5-1.5 m shallower critical zone above the weathered bedrock on average. Additionally, we document high lateral variability, which results from the weathering and sedimentation history and seems to be a consistent aspect of critical zone architecture within the BcCZO.

Shallow seismic refraction (SSR) and ground penetrating radar (GPR) are noninvasive geophysical techniques that enhance studies of the shallow subsurface deposits which control many geomorphic and biogeochemical processes. These techniques permit measuring the thickness and material properties of these deposits in sensitive alpine areas without using extensive pits and trenches that can impact current biogeospheric processes or distort them for future research. Application of GPR and SSR along 1.5 km of geophysical lines shows that layers of fine to coarse, blocky deposits of periglacial origin underlie alpine slopes in the vicinity of Niwot Ridge, Colorado Front Range. Interpretation of geophysical and drilling data shows that depth to bedrock ranges from 4 to .10 m and is not simply related to local slope. Our measurements suggest that ice lenses form seasonally beneath solifluction lobes; ice was not present in adjacent areas. Ice lenses are associated with local ponded water and saturated sediments that result from topographic focusing and low-permeability layers beneath active periglacial features. Geophysical interpretations are mainly consistent with data derived from nearby drill cores and corroborate the utility of GPR in combination with SSR for collecting subsurface data required by different landscape models in sensitive alpine environments.