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Landslides in the Thompson River valley, British Columbia, Canada, have historically impacted vital transportation infrastructure, the environment and natural resources, cultural heritage features, communities, public safety, and the economy. To better understand and manage geohazard risks in Canada’s primary national railway corridor, government agencies, universities, and railway industry partners are focusing research efforts on Ripley Landslide, 7 km south of Ashcroft. Electrical resistivity tomography (ERT) datasets collected in November 2013 (on land) and November 2014 (over water) were successfully combined and inverted into a pseudo-3D model that produced significantly deeper resistivity values than previously available in 2D profiles. The lithology, degree of saturation, porosity, presence of dissolved electrolytes, and temperature all influence electrical resistivity of earth materials in the landslide. Continuous (real-time) ERT monitoring began in November 2017 to characterize the long-term hydrological behavior of geological units in the landslide. Seventy-two electrodes were positioned in two arrays across the slide body and connected to a proactive infrastructure monitoring and evaluation (PRIME) system with internet access. PRIME resistivity results corroborate data from other geophysical techniques and hints at an unusual distribution pattern for surface moisture and groundwater in fractured bedrock and overlying clay-rich sediments containing vertical tension cracks and discrete sub-horizontal planar features interpreted as slide surfaces within pre-sheared zones. A greater understanding of the composition and internal structure of slope failures in the valley is gained at the site from terrain analysis and modeling of multi-dimensional geophysical datasets. This insight helps with the interpretation of multi-year monitoring datasets and will guide future efforts to record landslide activity in the valley, reducing stakeholder risks.

This paper reports on the landform assemblages at the northern confluence of the Late Wisconsinan Laurentide and Cordilleran ice sheets with montane and piedmont glaciers in the northern Rockies and southern Mackenzie Mountains. Recent observations in northeastern British Columbia refine our knowledge of the pattern and style of ice sheet retreat, glacial lake formation, and meltwater drainage. At the onset of deglaciation, confluent Laurentide and Cordilleran terminal ice margins lay between 59°N, 124°30′W and 60°N, 125°15′W. From this terminal limit, ice sheets retreated into north-central British Columbia and Yukon Territory, with remnant Cordilleran ice and montane glaciers confined to mountain valleys and the Liard Plateau. Distinctive end moraines are not associated with the retreat of Cordilleran ice in these areas. Laurentide ice retreated northeastward from uplands and the plateaus; then separated into lobes occupying the Fort Nelson and Petitot river valleys. Ice-retreat landforms include recessional end moraines (sometimes overridden and drumlinized), hill–hole pairs, crevasse-fill deposits, De Geer-like ribbed till ridges, hummocky moraines, kames, meltwater features, and glacial lake deposits that fall within the elevation range of glacial Lake Liard and glacial Lake Fort Nelson (ca. 840–380 m). Meltwater and sediment transport into glacial lakes Fort Nelson, Liard, Nahanni, and Mackenzie was sustained by remnant ice in the Liard River and Fort Nelson River drainage basins until the end of glaciation. Optical dating of sand from stabilized parabolic dunes on the Liard Plateau indicates that proglacial conditions, lake formation, and drainage began before 13.0 ± 0.5 ka (calendar years). The Petitot, Fort Nelson, and Liard rivers all occupy spillways incised into glacial deposits and bedrock by meltwater overflow from glacial lakes Peace and Hay.

To fully understand tsunami hazards along Canada’s Pacific coast we must consider all potential sources including local and region-wide earthquakes, volcanic activity, slope failure events and bolide impacts. Furthermore, as climate changes, future extreme meteorological conditions will exacerbate slope failures on steep and over-steepened slopes, and potentially augment the magnitude and frequency of earthquake-generated tsunamis. There have been numerous reviews of the geological records of palaeotsunamis along the Cascadia margin, but as yet there has been no attempt made to pull together all data related to palaeotsunamis along the Pacific coast of Canada. We summarise all geological data related to palaeotsunamis reported in the region, all relevant non-geological data, and other information concerning potential tsunamigenic sources. Evidence for between 10 and 22 palaeotsunamis (prior to European arrival) have been documented, with the Cascadia Subduction Zone Y event (AD1700) being the most widely recognised. Geological, archaeological and anthropological data all indicate a complex, multi-sourced, prehistory of seismic/tsunami activity that is poorly understood. A clustering of potential activity off the SE coast of Vancouver Island suggests that we should also attempt to better understand locally generated events along Canada’s Pacific coast.

A vital section of Canada’s national railway transportation corridor traverses a 7 km-long section of unstable terrain in the Thompson River valley, British Columbia. Landslides in this region have adversely impacted vital national railway infrastructure and operations, the environment, cultural heritage features, communities, public safety and economy since the late 19th Century. To help manage the potential risks associated with railway operations across this active slide zone, field investigations and monitoring of a very slow-moving Ripley Landslide are being undertaken by a consortium of research partners from government, academia and industry. Knowledge of the internal composition and structure of the landslide as interpreted through surficial geology mapping and geophysical surveys provide contextual baseline data for interpreting monitoring results; in addition to understanding mass-wasting processes in the Thompson River transportation corridor. Bathymetry, electrical resistivity tomography, frequency-domain electromagnetic terrain conductivity, ground penetrating radar, seismic refraction, multi-spectral surface wave analyses, and borehole logging of natural gamma, conductivity and magnetic susceptibility all suggest a moderately high relief bedrock sub-surface overlain by a >20 m thick package of clay, silt, till diamicton, gravel containing groundwater. Planar physical sub-surface features revealed in geophysical profiles and logs include tabular bedding and terrain unit contacts. Field observations and geophysical profiles also show curvilinear-rectilinear features interpreted as sub-horizontal translational failure planes in clay-rich beds beneath the rail ballast and lock-block retaining wall at depths between 5 and 15 m below the surface of the main landslide body. The landslide toe extends under the Thompson River where clay-rich sediments are confined to a >20 m deep bedrock basin. The upper clay beds are armoured from erosion by a lag deposit of modern fluvial boulders except along the west river bank where a deep trough has been carved by strong currents. High waterborne conductivity levels indicate discharge of groundwater through the boulder lag. Fluvial incision of the submerged toe slope at the south end of the landslide is observed

Open image in new window In this paper we present the first results from Coherent Points Analyses and Differential Stacking of RADARSAT-2 InSAR persistent scatterer interferograms covering a portion of the Thompson River valley, south of Ashcroft in British Columbia Canada. Surface displacements amounting to less than 5 cm/year are detected on landslides that are crossed by national railway infrastructure (train tracks and lock-block retaining walls). Our study shows that many landslides in the Thompson River valley have zones of displacement that are more active than others. For the portions of the North Slide, South Slide and Barnard Slide, zones of active displacement landslide can be resolved within the InSAR data acquired between 2013 and 2016. In contrast, both the Ripley Landslide and Red Hill Slide show marked variations in displacement rates related to seasonal changes in river stage and groundwater level, and compound translational-rotational sliding of coherent blocks of sediment. InSAR techniques effectively capture the surface movement detected by GPS stations, ground-based LiDAR, borehole piezometers and fibre optic installations at the Ripley Landslide test site. This successful application of Coherent Points Analysis and Differential Stacking of persistent scatterer interferograms suggests both techniques are suitable for monitoring unstable terrain in other remote settings where infrastructure, natural resources, the environment, local communities and public safety are at risk.

Deformation structures were observed in glaciofluvial sediments near Big Creek, central British Columbia. These sediments record a sequence of polyphase deformation resulting from the advance and retreat of the Late Wisconsinan (Fraser Glaciation) Cordilleran Ice Sheet. Deformation is attributed to ductile then brittle failure resulting from: (a) horizontal compression and loading as ice advanced over saturated sediments; followed by (b) lateral extension then (c) compression under frozen conditions during glacier overriding; and finally (d) vertical extension during unloading upon déglaciation. Most deformation (a-c, above) appears to have occurred during the advance phase of the Fraser Glaciation., Des structures de déformation ont été observées dans des sédiments fluvioglaciaires, près de Big Creek. Ces sédiments comprennent la séquence d'une déformation polyphasée résultant de l'avancée et du retrait de l'Inlandsis de la Cordillère (Glaciation de Fraser) au Wisconsinien supérieur. La déformation est attribuée à des fractures d'abord de type ductile, puis cassant résultant : (a) d'une compression et d'une charge horizontales à mesure que la glace avançait sur les sédiments saturés; suivie (b) d'une extension latérale, puis (c) d'une compression en milieu gelé pendant la phase de chevauchement glaciaire; et enfin (d) d'une extension verticale pendant la décharge au moment de la déglaciation. La plus grande partie de la déformation (a-c) s'est produite au cours de la phase d'avancée de la Glaciation de Fraser., Nahe bei Big Creek im Zentrum von British Columbia hat man Verformungsstrukturen in fluvioglazialen Sedimenten beobachtet. Diese Sedimente belegen eine Sequenz mehrphasiger Verformung, welche durch den VorstoB und Rùckzug der Kordilleren-Eisdecke im spaten Wisconsin (Fraser-Vereisung) verursacht wurde. Die Verformung fuhrt man auf zunàchst geschmeidiges, dann sprodes Nachgeben zuruck, veranlaRt durch: (a) horizontale Verdichtung und Anhâufung wàhrend das Eis uber die saturierten Sedimente vordrang, darauf folgend (b) eine latérale Ausdehnung und dann (c) Verdichtung in vereistem Milieu wàhrend der Gletscherûberschiebung, und schlie&lich (d) vertikale Ausdehnung wàhrend der durch die Enteisung bewirkten Stromungen. Der grôRte TeN der Verformung (siehe a-c oben) scheint wàhrend der VorstoRphase der Fraser-Vereisung geschehen zu sein.

The Thompson River, south of Ashcroft, British Columbia, Canada is a particularly unique area where complex glacial geology, active geomorphic processes and critical infrastructure (both major national rail lines—CPR and CN) intersect with and are affected by a long history of slope instability. Well documented landslides along a +10 km stretch of the valley have been impacting infrastructure as far back as the 19th century. The Ripley landslide is a small slow moving translational failure that is known to have been active since 1951. It poses a hazard to the onsite infrastructure since both the CN and CPR tracks run adjacent to each other along the entire breadth of the landslide. The economic repercussions of severing both railways here would be pronounced. In response to this threat, an extensive suite of monitoring technology is now being applied that includes: traditional applications including permanent monitoring using GPS stations and piezometers; subsurface investigations involving drilling and shallow seismic surveys; the adoption of novel technologies such as linear fibre optic sensing and vertical subsurface ShapeAccelArray (SAA) inclinometry, the installation of corner reflectors for satellite based (RADARSAT-2) interferometry and the deployment of ground-based SAR and LiDAR for ongoing quantitative assessment. Herein we summarize the collective efforts associated with this extensive array of instrumentation and monitoring studies being undertaken to better manage this and other landslide hazards in Canada and elsewhere.

Landslides in British Columbia are costly geological hazards that have challenged the major rail companies for over 120 years. Presented here are preliminary results and analyses of fiber Bragg grating and Brillouin optical time domain reflectometry monitoring of a deforming trackside lock-block retaining wall on the Ripley Slide in the Thompson River valley south of Ashcroft, British Columbia. Fiber optic strain data are evaluated in the context of results from global positioning system monitoring, field mapping and electrical resistivity tomographic survey across the landslide. This research aims to reduce the economic, environmental, health and public safety risks that landslides pose to the railway network operating in Canada and elsewhere.

Independent estimates of absolute hydraulic conductivity were obtained by a standard aquifer pumping test and a forced-gradient tracer test in a highly heterogeneous aquifer. An aquifer hydraulic test was conducted to evaluate the average hydraulic conductivity (K), and to establish steady-state flow for the tracer test. An average K of 48 m/day was interpreted from the draw-down data in a fully screened well. Type-curve matching and simulation with MODFLOW of the hydraulic response in partially screened wells indicates K of 10 to 15 m/day for the upper section and 71 to 73 m/day for the deeper section. Iodide and fluorescent dye tracers were injected at low rates in wells located approximately 8 m upgradient of the production well. Tracer breakthrough was monitored in the production well and at ten depth intervals within the fully screened monitoring well. Interpretation of tracer response in the production well reveals tracer transport is limited to a 3.9 m thick section of the 20 m thick aquifer, with a hydraulic conductivity of 248 m/day. However, the depth distribution of these permeable strata cannot be determined from the production well tracer response. When sampled at 1.5 m depth intervals in the monitoring well, breakthrough was observedmore » in only three intervals along the entire 18.2 m screened well. K estimates from tracer travel time within discrete high-permeability strata range from 31 to 317 m/day. Inclusion of permeameter K estimates for the lower permeability aquifer sands result in a range in relative K of 0.01 to 1.0. This field site has the highest absolute K estimate for a discrete stratum and the widest range in relative hydraulic conductivity among research field sites with K estimates for discrete strata. Within such a highly stratified aquifer, the use of an average K from an aquifer pumping test to predict solute transport results in great underestimation of transport distances for a given time period.« less

Light nonaqueous phase liquid (LNAPL) saturation and movement in the subsurface are controlled by soil capillary characteristics, permeability, and fluid properties. Where free-product occurs in monitoring wells, LNAPL saturations in the formation vary significantly as a function of the observed thickness in the monitoring well and soil type. Fine-grained soils generally exhibit lower LNAPL saturations than coarse-grained material for the same observed thickness in a monitoring well. MAGNAS3 [MAGNAS3. HydroGeoLogic, Inc., Herndon, VA, 1992; Huyakorn, P. S.; Panday, S.; Wu, Y. S. J. Contam. Hydrol. 1994, 16, 190−130], a three-dimensional, finite-element model that can simulate movement of three active phases (air, water, and LNAPL), was used to investigate LNAPL recovery in three different soil types and using several recovery designs to examine the effect of these factors on LNAPL recovery. The results of this analysis show that, because the relative mobility of LNAPL decreases with decreasing saturation ...

Late Wisconsinan Fraser Glaciation retreat-phase deposits and landforms in the east-central Taseko Lakes area, British Columbia, are used to demonstrate a four-phase model of deglaciation. During phase I, at the onset of ice retreat, the Cordilleran Ice Sheet occupied much of the study area, blocking southward drainage of Fraser River. Phase II was marked by the deglaciation of uplands and plateaux. Meltwater drainage patterns were controlled by stagnating glaciers confined to valleys. Phase III commenced as remnant ice in the Fraser Valley downwasted to between 850 and 760 m elevation. At this time, interlobate glacial lakes formed in hanging valleys east of Fraser River. Drainage of glacial lakes occurred subglacially, and was accompanied by disintegration of remnant ice and an increase in mass movements in valleys. These events were followed by decreased sedimentation rates, reflecting lower meltwater volumes and exhaustion of unstable glacial debris during phase IV. Postglaciation valley fill was subject to fluvial degradation and terracing as modern drainage patterns became established.

Use of the specific capacity of a pumping well to predict aquifer transmissivity is desirable due to the cost of pumping tests and the availability of specific capacity measurements. The geostatistical technique of cokriging is a method of incorporating the spatial variability of a correlated variable (e.g., specific capacity) in estimating a related undersampled variable (e.g., transmissivity). This study examines the reliability of cokriging transmissivity estimates using a data set of 215 pairs of transmissivity and specific capacity. Subsets of pairs of transmissivity and specific capacity were selected and cokriged to estimate transmissivity at the remaining well locations. The estimates of transmissivity were then compared to actual measurements of transmissivity. The same subsets of pairs were used to estimate transmissivity with loglinear regression of transmissivity on specific capacity and ordinary kriging of transmissivity alone. Comparison of these three methods indicates the number of wells with both transmissivity and specific capacity data necessary to obtain improvement in transmissivity estimates with cokriging over the simpler regression and kriging methods. The results show that significant improvement in the transmissivity estimate is obtained by cokriging with 50 or more pairs of transmissivity and specific capacity, and that loglinear regression is superior when less than 30 pairs are available. With between 30 and 50 pairs of available data measurements, cokriging does not reliably improve the estimate over loglinear regression.

The exaggeration of light nonaqueous phase liquids (LNAPL), typically hydrocarbon, by monitoring wells is a well-known problem that introduces significant errors in the estimation of recoverable hydrocarbon. Farr et al. (1990) and Lenhard and Parker (1990) show that significantly different volumes of hydrocarbon may produce the same thickness of hydrocarbon in a monitoring well, due to differences in the capillary characteristics between soil types. The purpose of our investigation was to evaluate the influence of local sediment variability on estimation of hydrocarbon volumes. Sediment samples from two sites underlain by a relatively homogeneous sandy deposit were collected within a small area. Capillary characteristic curves were determined for 10 samples from one site and 41 samples from the second site using a pressure plate. Grain-size analysis was performed on all samples for which pressure plate data were available. The results show significant variability, even for small sites. For example, a hydrocarbon volume of three cm3/cm2 could produce anywhere between 45 and 200 cm of hydrocarbon within an observation well. This suggests that use of an “average” soil sample to characterize hydrocarbon exaggeration, within even a very small site, can lead to substantial errors. It also suggests that maps of apparent hydrocarbon thickness can be extremely misleading, leading hydrologists to place remediation wells in areas of greatly exaggerated thicknesses produced by fine-grained materials. Comparison between characteristic curves calculated from grain-size analysis using the approach of Mishra et al. (1989) to those measured using the pressure plate shows poor correlation that introduces large errors into the estimated hydrocarbon volumes.

The distribution and mobility of light, nonaqueous phase hydrocarbons in the subsurface is poorly understood, particularly in relation to observations of hydrocarbons in monitoring wells. Recent work by Lenhard and Parker (1990) and Farr et al. (1990) resulted in an equilibrium model in which the degree of hydrocarbon saturation was shown to be a function of the apparent thickness of hydrocarbon in a monitoring well, the height above the oil/water interface, and the capillary pressure/saturation characteristics of the soil. The current investigation compared measured hydrocarbon saturations, hydrocarbon volumes, and hydrocarbon mobility to those predicted by the capillary characteristic model of Farr et al. (1990) and Lenhard and Parker (1990). Five monitoring wells were completed in boreholes drilled in a hydrocarbon-contaminated area of downtown San Diego, where apparent hydrocarbon thickness varied from zero to over 1.5 meters. Up to 2.5 meters of continuous core was obtained from each of the boreholes and split into 3 cm intervals. Alternating 3 cm samples were analyzed for total petroleum hydrocarbon, and the remaining samples were used for grain-size analysis and measurement of capillary characteristic curves. Slug-withdrawal tests were conducted on the hydrocarbon intervals, and constant-rate discharge tests were conducted on the water-saturated intervals of the monitoring wells. The results show typical hydrocarbon saturations of 5% to 20%, with one borehole showing a limited zone of up to 50% saturation. Hydrocarbon mobility (relative permeability) is greatly reduced due to the low saturations, as predicted by capillary theory. These observations are consistent with both the capillary model of hydrocarbon distribution and with the very low recoveries achieved using hydrocarbon skimming systems.

In the Gang Ranch area, British Columbia, interaction between the regional physiography and ice flow during the Late Wisconsinan Fraser Glaciation resulted in the formation of a proglacial lake confined to valleys of the Fraser River and its tributaries. Lithostratigraphic and geomorphic evidence suggests that ponding was initiated in the Big Bar Creek area where the Fraser River is confined to a deep canyon in the Camelsfoot Range. During ice advance, a proglacial lake system developed that progressively deepened and reached a minimum upper elevation of approximately 710 m asl prior to being overridden by ice. We propose that this system be formally named "Glacial Lake Camelsfoot." A composite stratotype, comprising lithostratigraphic units associated with Glacial Lake Camelsfoot, is described from eight reference sections along Fraser River, Churn, and Lone Cabin creeks in the Gang Ranch area. Additional geomorphic evidence indicates that at the Fraser Glaciation maximum, the Cordilleran Ice Sheet in the study area ranged from 600 to 2000 m in thickness.

Transmissivity is often estimated from specific capacity data because of the expense of conducting standard aquifer tests to obtain transmissivity and the relative availability of specific capacity data. Most often, analytic expressions relating specific capacity to transmissivity derived by Thomasson and others (1960), Theis (1963), or Brown (1963) are used in this analysis. This paper focuses on a test of these relations using a large (215 pairs) data set from a heterogeneous aquifer. The analytic solutions predicting transmissivity from specific capacity do not agree well with the measured transmissivities, apparently due to turbulent well loss within the production wells, which is not taken into account by any of the analytic solutions. Empirical relations are better than the theoretical relations. Log-log functions have greater correlation coefficients than linear functions. The best relation found for the data set chosen for this study has a correlation coefficient of 0.63, but the prediction interval was about 1.2 log cycles, indicating that the range of probable transmissivities corresponding to a single specific capacity was more than one order of magnitude. Tests with smaller subsets of data suggest that correlations based on data sets of 10 points or less are of limited value.

Patterns appearing on a thermal infrared image are primarily a function of the temperature of the earth's surface. Diurnal surface temperatures are affected by the thermal inertia and solar reflectance of the surface, by meteorologic variables, by evaporation rates, and by subsurface temperature effects such as the effect due to varying groundwater depth. Variations in surface material result in variations in both thermal inertia and solar reflectance, causing a change in both the magnitude of temperatures through the diurnal cycle and the amplitude of diurnal temperature variation. Variations in soil moisture result in similar effects and, in addition, result in a change in evaporative cooling rates. Variation in groundwater depth produces an effect on surface temperature that is discriminable from variations in solar reflectance and thermal inertia but is not discriminable from variations in evaporative cooling rates.

Increased demand for quantitative answers to ground-water problems, particularly associated with the use of numerical models, has increased the need to accurately determine the distribution of hydraulic parameters. Researchers have attempted to find correlations between electrical resistivity and the permeability of fresh-water aquifers since 1951. Several recent studies report either direct or inverse relations between apparent formation factor and aquifer permeability. The basis for these relations is a direct or inverse relation between porosity and permeability and, as matrix conduction effects are not taken into account, constant fluid conductivity is either implicitly or explicitly assumed. Laboratory experiments conducted on granular materials suggest that matrix conduction (surface conduction) effects are either as important as, or dominant over, porosity-permeability relations. Our experiments on granular materials show only weak relations between true formation factor and permeability. Relations between apparent formation factor and permeability are good only for constant fluid conductivity. Most importantly, the strongest relationship found was that between permeability and matrix conductivity. These data suggest either that (1) relations between permeability and apparent formation factor must be applied in very restricted geologic environments and only where fluid conductivity remains relatively constant, or (2) more fundamental relations between matrix conductivity and aquifer permeability should be applied.

The northern San Luis Valley is a high mountain valley encompassing about 4,000 sq km (1,500 sq mi) of south-central Colorado and is located 210 km (130 mi) southwest of Denver (Fig. 1). The valley is situated along the Rio Grande Rift, a zone of Cenozoic normal faulting, volcanic activity, and high heat flow (Chapin, 1971). The valley is bounded on the east and west by the Sangre de Cristo and San Juan Mountains, respectively, on the north by Poncha Pass, and is bounded in part on the south by a shallow ground and surface water divide north of the Rio Grande. PREVIOUS WORK Previous studies of the northern San Luis Basin can be grouped into two categories: (1) geologic studies of portions of the Sangre de Cristo and San Juan Mountains, such as those by Knepper (1974), Peel (1971), Bolyard (1956), Johnson (1969), Larsen and Cross (1956), Steven and others (1974), and Huntley (1976); and (2) general hydrologic studies of San Luis Valley, such as the work of Siebenthal (1910), Powell (1958), Emery and others (1969, 1971, 1972, 1973, 1975), and Huntley (1976). This report results from an attempt to integrate geologic and hydrologic data of the valley in order to evaluate mechanisms of groundwater recharge to the basin.

The northern San Luis Drainage Basin, which includes the northern closed basin of San Luis Valley and contributing watersheds of the, Sangre de Cristo and San Juan Mountains.(Fig. 1), is a high mountain basin encompassing about 8,400 sq km of south-central Colorado. The basin is located 210 km southwest of Denver, along the Rio Grande Rift Zone, a zone of Cenozoic normal faulting, volcanic activity, and high heat flow (Chapin, 1971). The basin is bounded on the east and west by the surface water divides of the Sangre de Cristo and San Juan Mountains, respectively, on the north by Poncha Pass, and on the south by a shallow ground- and surface-water divide north of the Rio Grande River. The aquifers of San Luis Valley have undergone rapid development since the discovery of artesian water in 1887. A definition of the sources of ground-water recharge to these aquifers is necessary for the rational management of both the developed and potential resources of the valley. More important perhaps is that the ground-water recharge system must be understood before the relations between aquifer development and flow in the Rio Grande can be defined and used to meet Colorado9s obligations to New Mexico under the 1938 Rio Grande Compact. Although several studies have discussed the hydrogeology of portions of the valley (Siebenthal, 1910; Powell, 1958), the ground-water chemistry of the valley (Klein, 1971; Glanzman, 1972; Emery and others, 1973), and the general hydrology of the valley (Emery and others, 1969, 1971, 1972, 1973), this is the first such study to focus specifically on the ground-water recharge system of the northern San Luis Basin.