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<p> Fluvial suspended sediment is a master variable affecting a wide range of fluvial and coastal environmental processes, and dominating the terrestrial mass flux to the oceans. Although it has long been recognized that relationships between suspended sediment concentration and discharge are not stationary in small, mountainous rivers over time scales from hours to decades, most studies continue to assume stationarity. This collection of studies directly addresses the issue of non-stationarity in the suspended sediment &ndash;discharge relationship of the Salinas River, central California, and examines the progression of abandoned channel fill sequences in the Eel River Estuary of northern California. </p><p> Preceding these studies is a methodological analysis of the pretreatment of fluvial and marsh sediments for particle size analysis. Pretreatment of sediment with hydrogen peroxide to remove organic constituents and aid deflocculation is a common component of particle size analyses of terrestrial and marine sediments. The first chapter presents the quantitatively determined effect of a range of treatment levels on particle size distribution among four sediment types representing a range of mineral/organic particle size distributions, organic content and particle characterization (charcoal or detrital plant material). </p><p> The following three chapters examine the effects of antecedent basin conditions on the suspended sediment &ndash; discharge relationship in the Salinas River. In chapter two, forty-five years of suspended sediment data from the lower Salinas and 80 years of hydrologic data were used to construct hydrologic descriptors of basin preconditioning and test the effects of these preconditions on suspended sediment behavior. Fine (diameter (<i>D</i>) &lt; 63 &mu;m) and sand sized (<i>D</i> > 63 &mu;m) sediment were found to respond differently to antecedent hydrologic conditions. Fine sediment was most sensitive to flushing flows of moderate discharge (10 &ndash; 20x mean discharge (<i>Q_mean</i>) that led to lower subsequent fine sediment concentrations, while sand concentrations were generally decreased by periods of drought and longer elapsed time since a wide range of discharges acting as maintenance flows. </p><p> Chapter three examines the interannual to decadal scale persistence of suspended sediment &ndash; discharge relationship states in the lower Salinas River, assesses the role of antecedent hydrologic conditions in controlling these patterns, and addresses their relationship to El Ni&ntilde;o Southern Oscillation (ENSO) climatic states. The decadal scale variability in suspended sediment behavior was influenced by interannual to decadal scale fluctuations in hydrologic characteristics, including: elapsed time since small (&sim; 0.1x Qmean), and moderate (&sim; 10x Qmean) threshold discharge values, the number of preceding days that low/no flow occurred, and annual water yield. El Ni&ntilde;o climatic activity was found to have little effect on decadal-scale fluctuations in the fine suspended sediment &ndash; discharge relationship due to low or no effect on the frequency of moderate to low discharge magnitudes, annual precipitation, and water yield. However, sand concentrations generally increased in El Ni&ntilde;o years due to the increased frequency of moderate to high magnitude discharge events, which generally increase sand supply. </p><p> Chapter four brings to bear the decadal scale persistence of suspended sediment - discharge behavior, the effects of antecedent hydrologic conditions, and ENSO influences on the estimation of inter-decadal scale sediment flux from the Salinas River. The longer sampling records employed in this study and incorporation of decadal scale behavior or antecedent hydrologic conditions resulted in average annual load estimates of 2.1 or 2.4 Mt, in comparison to earlier estimates of &sim; 3.3 Mt by previous researchers. El Ni&ntilde;o years dominated the sediment budget by producing on average ten times more sediment than non-El Ni&ntilde;o years. </p><p> Chapter five proposes a modification of the current generic model for abandoned channel fill stratigraphy produced in unidirectional flow river reaches to incorporate seasonal tidal deposition. This work was based on evidence from two consecutive abandoned channel fill sequences in Ropers Slough of the lower Eel River Estuary. Planform geomorphic characteristics derived from these images were used in conjunction with sub-cm resolution stratigraphic analyses to describe the depositional environment processes and their resultant sedimentary deposits. The abandoned channel fill sequences appeared to differ due to the topographic steering of bed sediment transport and deposition previously identified in rivers experiencing only unidirectional flow, while also expressing the seasonal dichotomy of fluvial and tidal deposits.</p>

<p> Wastewater generated in the Tucson metropolitan region is conveyed to and treated at the Roger Road Wastewater Reclamation Facility (WRF) and Ina Road WRF. From 2005 to 2012, approximately 15,000 acre-feet per year of effluent was returned to the City of Tucson for additional filtration and reuse in the reclaimed water system. The remaining 48,000+ acre-feet per year of treated effluent was discharged to the Santa Cruz River, where a variable portion of the effluent infiltrates the streambed. The effluent that infiltrates the streambed contributes to recharge credits for participants invested in the Managed Underground Storage Facilities.</p><p> In the effluent-dependent river, physical, chemical, and biological processes work in combination to develop a clogging layer near the streambed surface, which reduces infiltration. Previous studies have shown that large storm events have the ability to scour away the clogging layer and are the most significant processes contributing to establishing infiltration rates. Without the occurrence of large storm events, other variables such as effluent discharge and effluent concentrations affect infiltration to a lesser degree.</p><p> Effluent discharge, biochemical oxygen demand, and total suspended solids are monitored and recorded daily at the outfalls of the WRFs. The parameters were investigated individually and in combination using statistical analyses to determine their correlations with streambed infiltration in the Santa Cruz River. The dry spring-early summer seasons from 2005 to 2012 were analyzed. A water balance was constructed for non-stormflow days during each time period. Evapotranspiration was calculated using riparian vegetation surveys and detailed delineations of aerial photography of the surface water and streamside herbaceous vegetation. Infiltration was derived as the residual of the water balance. </p><p> At the daily time scale, correlations among variables were unobtainable due to the extremely variable characteristics of infiltration. The seasonal time scale analyses demonstrated an inverse relationship between both the effluent concentrations of biochemical oxygen demand and total suspended solids with infiltration and a direct correlation between effluent discharge and infiltration under extreme conditions. Under normal conditions, the distribution of discharge between Roger Road WRF and Ina Road WRF had a critical effect on infiltration as a result of the different deposition and erosive regimes through the Santa Cruz River.</p>

It is well established that a river's natural flow regime is a key determinant of ecological integrity and that dam regulated-flow releases can in some cases be detrimental to biotic communities (e.g. Poff and Zimmerman, 2010). Regional flow classes, groups of rivers that share similar natural flow regimes (called 'river types' by Poff and Zimmerman (2010)) and to which regional fish communities are adapted, have been proposed (e.g. Poff, 1996; Poff and Zimmerman, 2010; McManamay et al., 2012a) as units of analysis to identify significant damming related flow alteration. Specifically, the natural range of flow behaviour within regional classes can be used to identify the most anomalous flow features in rivers regulated by dams. Through ordination analysis, 5 flow regime clusters were identified based on 70 ecologically important flow indices. These clusters are distinct classes of regional flow regimes for 96 unregulated rivers in proximity to 16 regulated sites chosen as NSERC HydroNet sites in Eastern and Western Canada. The distinguishing characteristics of natural flow regimes within each flow class are explored through visualization in principal component space. The 16 regulated HydroNet sites were assigned to these flow classes through discriminate function analysis based on shared geographic location and watershed characteristics. Anomalous flow features in the regulated rivers are identified and characterized in terms of type and strength by identification of flow indices that are significantly different from observed natural variability in the relevant regional class. We also discuss the potential biological implications of the dominant flow anomalies in this set of rivers regulated by hydro dams. / Selon le paradigme du 'Natural Flow Regime', le régime hydrologique naturel (ou régime de débits) d'une rivière est un déterminant clé de son intégrité écologique. L'altération significative de ce régime par un barrage hydroélectrique peut en effet négativement affecter les communautés biotiques d'un écosystème lotique (Poff and Zimmerman, 2010). Récemment, des auteurs influents (e.g. Poff, 1996; McManamay et al., 2012a) ont ainsi proposé qu'afin d'identifier les altérations du régime naturel les plus significatives causés par la régulation par barrage, on utilise un point de référence régional (les 'river type') (Poff and Zimmerman, 2010) constitué de la variabilité présente dans un sous-ensemble de rivières non-régulées coulant dans la région du site régulé, rivières qui partagent naturellement certaines caractéristiques de régime hydrologique (et auxquelles la faune aquatique régionale est adaptée). A cet effet, une analyse d'ordination a été effectuée sur les régimes naturels de débit de 96 rivières non régulées situées à proximité des 16 sites sur rivières régulées par barrage choisis pour étude dans le projet HydroNet, et ce dans deux régions distinctes, l'une dans l'ouest et la seconde dans l'est canadien. Cette ordination, effectuée à partir de 70 métriques de débit reconnus pour leur pertinence écologique, distingue 5 sous-ensembles régionaux de régimes naturels assez distincts (5 'flow classes', sensu McManamay et al., (2012a)) réparties à travers les régions à l'étude. Les caractéristiques contrastantes des régimes de chacune de ces 5 sous-ensembles sont présentées et discutées. Par la suite, une analyse par fonction discriminante permet d'assigner les 16 sites régulés à l'étude à une ou l'autre de ces 5 classes, comme point de comparaison naturel. Cette analyse nous permet par la suite de repérer et de décrire les divers aspects des régimes de débits aux sites régulés qui sont clairement 'hors normes' par rapport à ce qui est observé dans le sous-ensemble régional de référence. Nous discutons brièvement par la suite de certains effets possibles de ces anomalies particulières sur les populations de poisson dans ces écosystèmes.

The exchanges of mass and momentum between coastal current and its embayment influence the transport and distribution of pollutant and sediment in the current. In this study, a numerical model of forced oscillating flow was developed to study the exchange between the recirculating flow in a side basin and the main-channel flow. The numerical simulations were carried out based on the shallow water equations, using a staggered grid for minimal intervention. Time integration of the shallow water equations was accomplished using the fourth-order Runge-Kutta scheme. A fifth-order finite volume approximation known as weighted essentially non-oscillatory (WENO) scheme was employed for the spatial interpolation. To visualize the mass exchanges and determine the mass exchange coefficient, tracer mass were introduced into the square basin. The flow in the main channel is forced to oscillate. The mass exchange rate as a fraction of the total mass within the basin is shown to be a first-order reaction process. A mass exchange coefficient is then determined for the first-order process. The momentum exchange as a resistance to the main flow is obtained by integrating the momentum flux at the interface along the opening of the side basin to the main flow.Different flow patterns in the basin are identified relating to the amplitude and the period of the forced oscillations. The regular recirculating flow are dependent on the oscillation amplitude but not on the forcing period. At selected periods of forcing however, the flow is excited leading to self-sustained oscillations in the basin. Two kinds of excitations of different feedback mechanism have been identified. The excitation of the first kind occurs when the forced oscillation frequency matches the fundamental and harmonic mode frequencies of the gravity free-surface waves in the basin. The excitation of the second kind is due to vorticity-wave synchronization with the forced oscillation frequency. The excitation of the free-surface waves in the basin does not promote the mass exchange. On the contrary and in contradiction to intuition, it suppresses the circulation inside the basin and thereby leading to a substantial decrease in mass exchange rate between the basin and the main flow. The momentum exchange on the other hand is enormously enhanced by the excitation of the free-surface waves in the basin. / Le processus d'échange de masse et la quantité de mouvement entre le courant côtier et son embouchure influence le transport et la distribution de polluants et de de sédiments dans le courant. Dans cette étude, un modèle numérique de flux principal oscillant force a été développé pour étudier l'échange entre le flux de recirculation dans un sous-bassin d'extrémité et son flux principal. Les simulations numériques ont été élaborées basées sur les équations de flux en eau peu profonde dans un repère orthonormé. L'intégration du temps des équations de flux en eau peu profonde a été accomplie en utilisant la programmation du quatrième degré de Runge-Kutta. Un volume d'éléments finis de cinquième degré connu sous le nom de régime pondéré essentiellement non oscillatoire (en anglais WENO) a été utilisé pour l'interpolation spatiale.Pour visualiser les échanges de masse et déterminer le coefficient d'échange de masse, un traceur a été introduit dans le bassin de forme carré. L'écoulement dans le flux principal est forcé d'osciller. Le taux d'échange de masse en fonction de la masse totale dans le bassin est présenté en tant que procédé de réaction de premier ordre. Un coefficient d'échange de masse est ensuite déterminé pour ce procédé de premier ordre. L'échange de moment en tant que résistance au flux principal est obtenu par intégration du flux de la quantité de mouvement au niveau de l'interface sur l'ouverture du sous-bassin d'extrémité.Différents caractéristiques d'écoulement dans le bassin sont identifiés étant reliés à l'amplitude et à la période de forces oscillatoires. Les flux de recirculation réguliers sont dépendants de la force d'amplitude mais non à la période de la force oscillatoire. Cependant, au niveau de certaines périodes de force sélectionnées, l'écoulement est excité afin d'aboutir à des oscillations durables dans le bassin. Deux types d'excitation de différents mécanismes de feed-backs ont été identifiés. L'excitation du premier type survient quand la fréquence de l'oscillation forcée égale les fréquences des modes fondamental et harmonique associés à la force oscillatoire. L'excitation du second type est due à la synchronisation du tourbillon d'onde avec la fréquence de l'onde oscillatoire. L'excitation des ondes à surface libre dans le bassin ne garantit pas l'échange de masse. Dans le cas contraire et dans la contradiction par rapport à l'intuition, ceci supprime la circulation dans la le bassin et ainsi conduit à une diminution importante dans le taux d'échange de masse entre le bassin et l'écoulement principal. De l'autre côté, l'échange de la quantité de mouvement est énormément amélioré par l'excitation des ondes à surface libre dans le bassin.

It is important for watershed models to realistically simulate tile drainage flow and water table dynamics. Therefore, a new model, SWATDRAIN, was developed in this study by incorporating the DRAINMOD model into the Soil and Water Assessment Tool (SWAT) to better simulate surface and subsurface flow in tile-drained watersheds and also to improve the prediction of water table depth. This was accomplished by fully integrating the DRAINMOD model, which has been tested and widely used to simulate the performance of drainage and water table control systems on a continuous basis at field scale, into the SWAT model. The SWATDRAIN model was evaluated for a fully tile-drained watershed in eastern Ontario, Canada. The measured tile drainage outflow and water table depth data for the Green Belt watershed were used to evaluate the capability of the new model to simulate water balance for this fully tile drained agricultural watershed. Together with hydrographs, the Nash-Sutcliffe efficiency (NSE), percent bias (PBIAS) and coefficient of determination (R2) statistics were used in evaluating the accuracy of SWATDRAIN to predict tile flow and water table depth in light of the measured values. Simulations were carried out over the period of 1991 to 1993; 1991 and 1992 data served as model calibration and 1993 data were used to validate the process. Model accuracy statistics for the monthly and daily water table depth over the validation period were, respectively, 0.86 and 0.70 for R2, 0.11 and 2.90 for PBIAS, and 0.80 and 0.67 for the NSE. Model accuracy statistics for events, monthly and daily tile drainage over the validation period were, respectively, 0.86, 0.88 and 0.70 for R2, 11.7, 17.26 and 23.85 for PBIAS, and 0.84, 0.86 and 0.62 for the NSE. The SWATDRAIN model was also applied to a partially tile-drained watershed in southern Ontario. Simulations were carried out from 1975 to 1983; data from 1975 to 1978 were used for model calibration and data from 1980 to 1983 were used for validation. The new model was able to adequately simulate the hydrologic response at the outlet of the watershed. Comparing the observed monthly and daily tile drainage with the model's output over the validation period returned R2 values of 0.75 and 0.62, PBIAS of 13.96 and 17.99 and modeling efficiency of 0.71 and 0.62. In this study, the effects of a drainage water management operational strategy on hydrology were simulated using SWATDRAIN in the Green Belt watershed in Ontario. The effects of drainage water management on subsurface drainage and surface runoff were predicted for a period of four years from 2004 to 2007. Implementing the controlled drainage strategy from June 15 to August 15 during the cropping season and also from November 1 to May 1 in the non-growing season resulted in a reduction of the average annual drain flow by 18%, while it increased the surface runoff in the order of 30%. The results showed that the surface runoff increase mostly happened during the snowmelt period in April and also it was slightly increased during the month of November. However, higher amount of surface runoff in flat watersheds during the snowmelt period may not cause a serious problem. / Une bonne modélisation de bassin versant se doit de simuler correctement l'écoulement par drains souterrains ainsi que la dynamique des fluctuations de la nappe phréatique. Un nouveau modèle, SWATDRAIN, fut créé afin de mieux simuler à la fois l'écoulement en surface et souterrain, et le niveau de la nappe phréatique dans les bassins versants équipés d'un réseau de drainage souterrain. Ce modèle représente une intégration complète du modèle hydrologique DRAINMOD, bien éprouvé et largement utilisé pour simuler la performance de systèmes de drainage et de contrôle de la nappe phréatique en mode continue à l'échelle du champ, dans SWAT, un outil de prédiction des effets à l'échelle du bassin versant du mode de gestion agricole sur la quantité et qualité des eaux, ainsi que leur teneur en sédiments, éléments nutritifs et pesticides. La performance du modèle SWATDRAIN fut évaluée pour un bassin versant agricole de l'est ontarien, entièrement soumis au drainage souterrain. Les données d'écoulement souterrain et de profondeur de la nappe phréatique mesurées dans le bassin versant Green Belt servirent à évaluer la capacité du nouveau modèle à simuler avec précision l'équilibre hydrique de ce bassin versant. Conjointement avec la comparaison d'hydrographes, les statistiques du coefficient d'efficacité Nash-Sutcliffe (NSE), du pourcentage de biais (PBIAS) et de la valeur de R2 servirent à évaluer la précision de SWATDRAIN dans sa prédiction de l'écoulement souterrain et de la profondeur de la nappe phréatique. Les simulations furent entreprises pour une période s'étendant de 1991 à 1993, avec les années 1991 et 1992 servant à la calibration du modèle, et l'année 1993 à sa validation.. La précision du modèle en phase de validation pour les moyennes mensuelle et quotidiennes de profondeur de la nappe phréatique fut confirmée par des valeurs respectives de 0.86 et 0.70 pour R2, 0.11 et 2.90 pour le PBIAS, et 0.80 et 0.67 pour le NSE. La précision du modèle en phase de validation pour les moyennes mensuelle et quotidiennes d'écoulement souterrain fut confirmée par des valeurs respectives de 0.86 et 0.70 pour R2, 11.7 et 23.85 pour le PBIAS, et 0.84 et 0.62 pour le NSE. Le modèle SWATDRAIN fut également mis à l'œuvre dans un bassin versant agricole sud-ontarien, partiellement soumis au drainage souterrain. Les simulations furent entreprises pour une période s'étendant de 1975 à 1983, avec les années 1975 and 1978 servant à la calibration du modèle, et les années 1980 à 1983 à sa validation. SWATDRAIN simula adéquatement la réaction hydrologique à l'exutoire du bassin. La précision du modèle en phase de validation pour les moyennes mensuelles et quotidiennes d'écoulement souterrain fut confirmée par des valeurs respectives de 0.75 et 0.62 pour R2, 13.96 et 17.99 pour le PBIAS, et 0.71 et 0.62 pour le NSE. Dans une étude additionnelle, les effets potentiels d'une stratégie de gestion des eaux d'écoulement souterraines sur les paramètres hydrologiques du bassin versant ontarien Green Belt furent simulés avec la version de SWATDRAIN validée pour ce bassin versant. Les effets d'une gestion des eaux de drainage sur l'écoulement souterrain et de surface furent prédits pour les quatre années de 2004 à 2007. Mettant en œuvre d'une stratégie de drainage contrôlé durant la saison de culture (15 juin au 15 août), et hors-saison (1 novembre au 1 mai) donna lieu à une réduction de l'écoulement souterrain annuel moyen de 18%, tout en augmentant l'écoulement en surface annuel moyen de près de 30%. Ces résultats indiquèrent que la majorité de l'écoulement en surface eut lieu durant la fonte des neiges d'avril et que celle-ci était quelque peu élevée durant le mois de novembre. Cependant, un niveau élevé d'écoulement en surface dans un bassin relativement plat lors de la fonte ne risque pas de causer de problèmes majeurs.

This research was carried out in order to develop better methods for improving the accuracy of flood estimation. The first part of the thesis deals with the estimation of floods at ungauged sites. At the beginning, a comparative study was done to determine the best approach for delineating homogeneous regions. Three approaches were considered in this study: (1) the scaling method, which is based on the scaling behaviour of the flood series with the basin area; (2) the region of influence (ROI); and (3) the canonical correlation analysis (CCA). Analyses of hydrologic data from Quebec have indicated that the annual maximum series (AMS) exhibited a simple scaling behaviour. Results of this numerical application have indicated that the flood quantile estimates obtained from the scaling approach were more accurate and more robust than those given by the ROI or CCA methods. Then, a regional index-flood method for ungauged sites based on the partial duration series (PDS) method and the scaling approach was developed. A new approach for selecting a suitable flood threshold in the context of regionalization was introduced.The second part proposed a new method for flood estimation in the context of nonstationarity. This method was based on the detrending of the nonstationary flood series and the application of the GEV distribution to the transformed "stationary" series using the L-moments parameter estimation method (hereafter, called the LM-NS). The LM-NS method was assessed through a comparative study with the maximum likelihood (ML) method. The results have indicated that the LM-NS method could provide more accurate flood estimates than the ML method for most of the cases studied. The GEV and Gumbel distributions were then used for modeling the AMS in Quebec for the stationary case and for the three selected nonstationarity cases, comprising in all eight competing models. The performance of these eight models was assessed based on two different criteria: (1) the descriptive ability; and (2) the predictive ability. Results of the statistical analysis of flood data for 32 watersheds in Quebec have indicated a significant decreasing trend in flood magnitudes in Quebec for eight stations (25%). Furthermore, it was found that the proposed LM-NS method was a flexible and accurate method for estimating parameters of the GEV model in the context of nonstationarity. Finally, this study has indicated the importance of taking into consideration the nonstationary behaviour of floods to improve the accuracy of flood estimation, and it has provided necessary tools for identifying the possible effects of climate change on flood flow conditions in Quebec. / La présente étude avait été entreprise pour élaborer des nouvelles méthodes qui sont capable d'améliorer la précision de l'estimation des crues. La première partie de cette étude concerne l'estimation des crues en des sites non-jaugés en utilisant l'analyse fréquentielle régionale des crues. Premièrement, une étude comparative a été réalisée pour déterminer la meilleure méthode de regroupement des régions homogènes dans le contexte de l'estimation régionale des crues. Trois méthodes ont été considérées dans cette étude. (1) la méthode d'invariance d'échelle qui est basée sur le comportement invariable des crues en fonction du changement de la taille du bassin versant; (2) la région d'influence (ROI); et (3) l'analyse de la corrélation canonique (CCA). L'analyse des données de crues au Québec (Canada) a indiqué que les séries annuelles maximales des crues possèdent un comportement d'invariance d'échelle simple. Les résultats de cette application ont indiqué que l'estimation des crues par la méthode d'invariance d'échelle est plus précise que celle par la méthode ROI ou CCA. Ensuite, on a élaboré la méthode d'estimation régionale de crue-index pour les sites non-jaugés en se basant sur la méthode de durée partielle (PDS) et la méthode d'invariance d'échelle. Une nouvelle méthode de déterminer le niveau de crue de référence pour une région homogène a été proposée. La deuxième partie de cette étude consiste à proposer une nouvelle méthode pour l'estimation des crues dans un contexte de non-stationnarité. Cette méthode été basée sur la transformation de la série des crues non-stationnaire en celle stationnaire et sur l'application de la loi GEV à cette série stationnaire en utilisant la méthode d'estimation de paramètres L-moments (ci-après, appelée LM-NS). La comparaison de la performance de la méthode LM-NS proposée avec la méthode de maximum de vraisemblance (ML) a été effectuée. Les résultats de cette comparaison ont indiqué que la méthode LM-NS proposée était plus précise que la méthode de ML dans la majorité des cas considérés. Les lois de GEV et Gumbel ont été choisies pour la modélisation des séries de données annuelles maximales de crues au Québec pour le cas stationnaire et pour les trois cas de non-stationnarité considérés; donc au total huit modèles ont été choisies pour cette comparaison. La performance de ces huit modèles a été évaluée en fonction de deux critères différents : (1) la capacité descriptive; et (2) la capacité prédictive. Le résultat de l'analyse statistique de ces données de crues pour les 32 bassins versants au Québec a démontré une tendance significative de réduction des intensités de crues pour huit bassins (25%). De plus, on a trouvé que la méthode LM-NS proposée était une méthode flexible et précise pour l'estimation des paramètres du modèle GEV dans le contexte de non-stationnarité. Finalement, la présente étude a démontré l'importance de la considération de la non-stationnarité dans l'estimation des crues et cette étude avait fourni un outil nécessaire pour identifier les impacts possibles du changement climatique sur les caractéristiques de crues au Québec.

<p> This study investigates the potential for hydrothermal alteration and circulation in lava domes using combined analytical, remote sensing and numerical modeling approaches. This has been accomplished in three parts: <i>1) </i> A comprehensive field, geochemical and remote sensing investigation was undertaken of the hydrothermal system in the Santiaguito lava dome complex in Guatemala. The Santiaguito domes were found to contain mainly hydrous silica alteration, which is unlikely to weaken dome rock, but the summit of Santa Maria was found to contain pervasive argillic alteration (clay minerals), which do pose more of a collapse-related hazard. These results were confirmed by hot spring geochemistry which indicated that water in the domes was responsible for some rock dissolution but had a residence time too short to allow for secondary mineralization. <i>2)</i> A finite element numerical modeling approach was developed which was designed to simulate the percolation of meteoric water in two dome geometries (crater-confined and 'perched'), and the results were compared to the surface expression of hydrothermal systems on existing lava domes. In both cases, we concluded that simulated domes which lacked a high-temperature (magmatic) heat source could not develop a convecting hydrothermal system and were dominated by gravitational water flow. In these low-temperature simulations, warm springs (warmer high fluid fluxes) were produced at the base of the dome talus and cool springs were dispersed lower down the slope/substrate; fumaroles (high vapor fluxes) were confined to the dome summits. Comparison with existing dome cross sections indicates that the simulations were accurate in predicting fumarole locations and somewhat accurate at predicting spring locations, suggesting that springs may be subject to permeability contrasts created by more complicated structural features than were simulated in this study. <i>3)</i> The results of the numerical modeling were used to calculate alteration potential in the simulated domes, indicating the most likely areas where alteration processes might either reduce the strength of a dome or reduce permeability that could contribute to internal pressurization. Rock alteration potential in low-temperature lava domes was found to be controlled by material permeability and the presence or absence of a sustained heat source driving hydrothermal circulation. High RAI values were preserved longer in low-permeability domes, but were more strongly developed in domes with higher permeabilities. Potential for mineral dissolution was highest at the base of the dome core, while the potential for mineral precipitation is highest at the dome core-talus interface. If precipitated minerals are impermeable, the dome core/talus interface would be a likely location for accumulation of gases and initiation of gas-pressurization-related collapse; if alteration is depositing weak (i.e. clay) minerals in this area, the dome core/talus interface might be a candidate for collapses occurring as the result of alteration processes. </p><p> The results of this study are all geared toward answering two broad questions: <i> Where are hydrothermal alteration processes likely to occur or be focused within lava domes?</i> and <i>What effect could these processes have on dome stability?</i> In the specific case of the Santiaguito dome complex, the combination of a quickly-recharged, low-temperature hydrothermal system in the inactive domes actually indicated a low possibility of collapse related to alteration minerals. This result was reinforced by the results of the numerical modeling, which indicated that domes are unlikely to develop sustained hydrothermal convection without the presence of a significant (magmatic) heat source and&mdash;in the case of Santiaguito&mdash;are likely to produce more hydrous silica alteration minerals when they also lack a source of acidic gases. Models of alteration potential do detail, however, that both shallow and deep dome collapses are still a possibility with a low-temperature hydrothermal system, given either a) a source of acidic gases to drive the formation of clay minerals (which are most likely to be deposited at the core/talus interface of a dome, or b) enough deposition of silica minerals in pore spaces to lower permeability in dome rock and promote internal gas pressurization. The results of this study are not limited to lava domes, as the volcanic edifices on which they rest are composed of the same materials that comprise lava domes and are therefore susceptible to the same hydrothermal processes. Further simulations of both lava domes and their associated edifices, including mineral species models, could help constrain under what conditions a lava dome or volcano is likely to develop areas of weak mineral precipitates (such as clay minerals) which could provide sites for collapse, or develop an impermeable cap of silicate minerals which could trap rising vapor and contribute to the pressurization of the edifice in question (which can in turn lead to collapse).</p>