Academic literature on the topic 'River Guaíba'

Abstract. In this work, a first-order upwind and a high-order flux-limiter schemes for solving the advection–diffusion equation on unstructured grids were evaluated. The numerical schemes were implemented as a module of an unstructured two-dimensional depth-averaged circulation model for shallow lakes (IPH-UnTRIM2D), and they were applied to the Guaíba River in Brazil. Their performances were evaluated by comparing mass conservation balance errors for two scenarios of a passive tracer released into the Guaíba River. The circulation model showed good agreement with observed data collected at four water level stations along the Guaíba River, where correlation coefficients achieved values up to 0.93. In addition, volume conservation errors were lower than 1% of the total volume of the Guaíba River. For all scenarios, the higher order flux-limiter scheme has been shown to be less diffusive than a first-order upwind scheme. Accumulated conservation mass balance errors calculated for the flux limiter reached 8%, whereas for a first-order upwind scheme, they were close to 18% over a 15-day period. Although both schemes have presented mass conservation errors, these errors are assumed negligible compared with kinetic processes such as erosion, sedimentation or decay rates.

Benthic macroinvertebrate in four rivers, three in the Pelotas River basin (Divisa, Marco and Silveira rivers, in the headwaters of the Uruguai River) and one in the Taquari-Antas system (Antas River), a tributary in the Guaíba basin, in the state of Rio Grande do Sul, were identified. Two samples were collected in summer, autumn and spring, with one replicate in each river. The total of 28,961 specimens included members of Platyhelminthes, Annelida, Acarina, Insecta, Crustacea and Mollusca. The Silveira and Marco rivers showed significant differences in the indices of Shannon-Weaver (H’), Simpson’s Reciprocal (1/D), Margalef (DMg) and Equitability (E). The Silveira River showed the highest means of diversity and the EPT index (Ephemeroptera, Plecoptera, Trichoptera). Comparison among the diversity indices, considered individually, were insufficient to show differences in community structure, for the purpose of ecological characterization of the rivers. The EPT values characterized the Divisa River as having the highest abundance (73%), followed by the Marco (71%), Antas (48%) and Silveira (36%). These results suggest that the Silveira River is subject to moderate environmental stress, from human impact, although it showed the highest diversity of the major macrobenthic groups.

ABSTRACT. This work aimed at studying the bottom and sub-bottom geoacoustics of the Guaíba River, whose echo patterns are related to hydrodynamic patterns and sedimentary processes. A total of 324 km of seismic transects were surveyed using the Stratabox high-frequency (10 kHz) profiler with concomitant imaging using a side-scan sonar in the northern and central areas of the Guaíba system. The records were interpreted with the aid of complementary data (bathymetry and bottom samples), and maps containing the spatial arrangement of echo-characters were developed. The echo-characters were classified into seven different types according to the presence of sediments controlled by the hydrodynamics of the system or modified by human action. In general, the echoes related to the deposition of fine sediments occurred in deeper areas, the navigation channel or surrounding areas. The echoes in the shallower areas presented strong acoustic reflectivity at sites with lower deposition of fine sediments and predominance of sandy sediments. The occurrence and morphology of asymmetric subaqueous dunes indicated a predominant north-to-south flow and a river behavior.Keywords: shallow geophysics, sedimentology, hydrodynamics, geoprocessing.RESUMO. Este trabalho tem por objetivo o estudo geoacústico de fundo e subfundo do rio Guaíba, cujos padrões de ecos são relacionados aos padrões hidrodinâmicos e aos processos sedimentares atuantes. Foram levantados 324 km de perfis sísmicos em extensão, com o perfilador de alta frequência (10 kHz) Stratabox e concomitante imageamento com Sonar de Varredura Lateral nas áreas Norte e Central do Guaíba. Os registros foram interpretados com o auxílio de dados complementares (batimetria e amostras de fundo) e foram confeccionados mapas com a disposição espacial dos eco-caráteres. Os eco-caráteres foram classificados em sete diferentes tipos, de acordo com a presença de sedimentos controlados pela hidrodinâmica do sistema ou modificados pela ação humana. De uma forma geral, os ecos relacionados à deposição de sedimentos finos ocorrem nas áreas mais profundas, no canal de navegação ou adjacências. Os ecos nas áreas mais rasas apresentam forte refletividade acústica, em locais com menor deposição de sedimentos finos e predomínio de sedimentos arenosos. A ocorrência e morfologia de dunas subaquosas assimétricas indicam um fluxo preponderante no sentido Norte – Sul e um comportamento fluvial do sistema aquático.Palavras-chave: geofísica rasa, sedimentologia, hidrodinâmica, geoprocessamento.

Sediments preserve environmental information at the time of deposition which is a useful tool to evaluate the extent of the environmental change from its pristine condition. We evaluated historic environmental trends in the sedimentary record by analysing three sediment cores collected within the freshwater region of Patos Lagoon (southern Brazil). As, Ba, Cr, Cu, Ni, Pb, and Zn were analysed by inductively coupled plasma optical emission spectrometry. C/N ratios and δ13C values were also determined. Contamination indices such as Enrichment Factor, Geoaccumulation Index, Sediment Pollution Index and Pollution Load Index were calculated to evaluate the influence of anthropic activities. Multiple-elemental indices showed a general overview of the environmental conditions of the lagoon throughout time. Furthermore, single-element indices explained which elements triggered the observed changes in the environmental trends. Near Camaquã River mouth, we inferred fairly undisturbed conditions and did not seem to be affected by the mining activities upstream Camaquã River. The influence of these activities was observed upstream the lagoon, between Camaquã and Guaíba rivers, probably as a consequence of the hydrodynamics of the lagoon. Especially, As, Ba and Pb showed moderately polluted levels after 2009 AD. In Guaíba site, a progressive deterioration of the lagoon quality was observed after 1980 reaching the worst environmental degradation by 2000 AD. However, Enrichment Factor values only showed moderate pollution levels suggesting a considerable dilution process within the system given its large size. Similar δ13C signature was observed in the whole lagoon prior to the anthropogenic influence. However, such a condition changed differentially after the anthropogenic fingerprint. Recent sediments reflected a shift in the plant cover, and particularly in Guaíba, the influence of urban and industrial activities of Porto Alegre City were reflected in the sedimentary record. Overall, metals together with organic matter composition proxies allowed us to determine the anthropic-related changes in Patos Lagoon. APLICAÇÃO DE MULTI-INDICADORES NA ANÁLISE TEMPORAL DE IMPACTOS ANTRÓPICOS NA LAGOA DOS PATOS, SUL DO BRASILResumoOs sedimentos preservam a informação ambiental no momento da deposição, sendo uma ferramenta útil para avaliar a extensão da mudança ambiental ao longo do tempo. avaliamos as tendências ambientais históricas no registro sedimentar, analisando três testemunhos de sedimentos coletados na região de água doce da Lagoa dos Patos (sul do Brasil). Amostras sedimentares coletadas ao longos dos testemunhos foram submetidas a análises geoquímicas, tendo sido determinadas as concentrações de Ba, Cr, Cu, Ni, Pb e Zn, analisadas por espectrometria de emissão ótica com plasma indutivamente acoplado. A razão C/N e os valores de δ13C também foram determinados. Índices de contaminação como Fator de Enriquecimento, Índice de Geoacumulação, Índice de Poluição de Sedimentos e Índice de Carga Poluente foram calculados para avaliar a influência das atividades antrópicas. Os índices aplicados permitiram analisar o padrão evolutivo geral das condições ambientais da Lagoa dos Patos ao longo do tempo e mostraram quais os elementos que desencadearam as mudanças observadas nas tendências ambientais. Com base nos dados obtidos, foi possível inferir que próximo à foz do Rio Camaquã, o ambiente não está perturbado e parece não ter sido afetado pelas atividades de mineração realizadas a montante deste rio. A influência dessas atividades foi registrada numa zona mais externa da lagoa, entre os rios Camaquã e Guaíba, provavelmente em consequência da hidrodinâmica da lagoa. Nessa região foram encontrados sedimentos moderadamente poluídos sobretudo por As, Ba e Pb após 2009 AD. Na zona de Guaíba, observou-se uma deterioração progressiva da qualidade ambiental após 1980, tendo-se atingido a pior degradação ambiental em 2000 AD. No entanto, os valores do Fator de Enriquecimento evidenciaram apenas níveis moderados de poluição, sugerindo que o processo de diluição dentro do sistema é considerável, devido á grande extensão desta laguna. No período anterior à influência antropogênica, a assinatura isotópica de δ13C foi similar em toda a lagoa. No entanto, tal condição mudou diferencialmente após a intensificação das atividades antrópicas na área. Os sedimentos recentes mostraram a ocorrência de uma mudança na cobertura vegetal sobretudo em Guaíba. A influência das atividades urbanas e industriais da cidade de Porto Alegre refletiu-se no registro sedimentar. A análise das concentrações de metais em conjunto com os proxies que permitem identificar mudanças na composição da matéria orgânica permitiram identificar alterações causadas por ações antrópicas na Lagoa dos Patos.Palavras-chave: Metais Pesados. δC13. Impacto Antrópico. Aplicação de multi-proxies. Índices de poluição. Litoral.

The Sinos River basin is located Northeast of the state of Rio Grande do Sul (29º 20' to 30º 10' S and 50º 15' to 51º20'W), Southern Brazil, covering two geomorphologic provinces: the Southern plateau and central depression. It is part of the Guaíba basin and has an area of approximately 800 km², encompassing 32 municipalities. The objective of this study was to monitor water quality in the Sinos River, the largest river in this basin. Water samples were collected at four selected sites in the Sinos River, and the following parameters were analysed: pH, dissolved oxygen, biochemical oxygen demand (BOD5), turbidity, fecal coliforms, total dissolved solids, temperature, nitrate, nitrite, phosphorous, chromium, lead, aluminum, zinc, iron, and copper. The results were analysed based on Resolution No. 357/2005 of the Brazilian National Environmental Council (CONAMA) regarding regulatory limits for residues in water. A second analysis was performed based on a water quality index (WQI) used by the Sinos River Basin Management Committee (COMITESINOS). Poor water quality in the Sinos River presents a worrying scenario for the region, since this river is the main source of water supply for the urban core. Health conditions found in the Sinos River, mainly in its lower reaches, are worrying and a strong indicator of human activities on the basin.

Environmental monitoring studies in the Hydrographical Basin of Pardo River, RS, Brazil, in the time series from 2007 to 2009, showed no agreement between the water quality evaluation obtained through the Water Quality Index (WQI - NSF), which ranged from "regular" to "good", and the Environment National Council (CONAMA) Resolution 357/2005, which was bad ("class 4"), highlighting as critical variables for this classification the thermotolerant coliforms and the phosphate concentration, indicating process of water eutrophication, related to the large amount of nutrients and organic load from domestic sewage and excess fertilizers used in agriculture. AIM: This research aimed to calibrate the WQI - NSF for the Pardo River Basin, based on the CONAMA Resolution 357/2005. METHODS: Using the database (2007 a 2009) from the Laboratory of Limnology of the University of Santa Cruz do Sul (UNISC), RS, corresponding to the environmental monitoring studies realized in the Hydrographical Basin of Pardo River new weights of the relative importance of the variables used to calculate the WQI were attributed, by means of principal component analysis. RESULTS: There was a significant agreement between the results of the assessment of water quality using the CONAMA Resolution 357/2005 and the new WQI calibrated. CONCLUSION: The results certify the use of the calibrated WQI in environmental monitoring programs in lotic systems belonging to Guaíba Hydrographical Basin, RS.

<p>The Sinos River Basin (SRB) is located in the northeastern region of the state of Rio Grande do Sul (29º20' to 30º10'S and 50º15' to 51º20'W), southern Brazil, and covers two geomorphologic provinces: the southern plateau and the central depression. It is part of the Guaíba basin, has an area of approximately 800 km² and contains 32 counties. The basin provides drinking water for 1.6 million inhabitants in one of the most important industrial centres in Brazil. This study describes different water quality indices (WQI) used for the sub-basins of three important streams in the SRB: Pampa, Estância Velha/Portão and Schmidt streams. Physical, chemical and microbiological parameters assessed bimonthly using samples collected at each stream source were used to calculate the Horton Index (HI), the Dinius Index (DI) and the water quality index adopted by the US National Sanitation Foundation (NSF WQI) in the additive and multiplicative forms. These indices describe mean water quality levels at the streams sources. The results obtained for these 3 indexes showed a worrying scenario in which water quality has already been negatively affected at the sites where three important sub-basins in the Sinos River Basin begin to form.</p>

AIM: The Taquari River, a tributary of Guaíba Lake, is a major economic and environmental source for Rio Grande do Sul, a state located in the far south of Brazil; METHODS: This study evaluated the interference of sediment from this river in the reproduction and survival of 320 microcrustaceans (80 organisms per site) of the Daphnia magna species. Four sites between the middle course and the river mouth were monitored using eight samples from each site, between June/06 and April/10. Mortality higher than 20% suggested the acute action of the sample on the daphniids, while low reproduction indicated the presence of chronic toxicity. Sometimes an effect on survival was noted, but the number of individuals generated was always low. Semi-static chronic assays (21 day) were developed in 50 mL beakers using sediment and M4 culture medium at a ratio of 1:4 (v:v). Each beakers received a microcrustacean born from lots presenting LC50-24 hours = 0.98 mg K2Cr2O7 ± 0.04 mg totaling ten replicas per sample. The assays were developed under controlled light (16 hours light/day) and temperature (20 ºC ± 2 ºC) conditions; RESULTS: Duncan test showed a variation in reproduction (p < 0.05) with the presence of chronic toxicity. There was less reproductive activity in 91% of the samples, while for survival the reduction attained 9%. Spearman correlation identified the dependence between the rainfall index and reproduction in one of the samples and was absent when the rainfall index was compared to survival; CONCLUSIONS: Taquari River presented episodes of toxicity, and it is necessary to continue studies in the area and to inspect local sources of pollution, as already has been done. Chronic bioassays should be maintained when the purpose is to evaluate environmental quality, since reproduction proved more sensitive than survival.

AIM: The Sinos River, a tributary of Guaíba Lake, is 200 km long from the headwaters to the mouth and is influenced by several sources of pollution. A study to learn more about conditions in this river was performed in the middle and lower reaches. The study evaluated the effects of sediment contamination by xenobiotics on the survival and reproduction of Daphnia magna. METHODS: Eight sediment samples were collected per site from Dec/2007 to Aug/2009, as a substrate for 480 young cladocera (2-26 h old at the beginning of the trials) in chronic toxicity tests (21 days). For this purpose, D. magna individuals were exposed to sediment and M4 culture medium, at a proportion of 1:4 (v:v) using 50 mL beakers, kept at 20 ºC ± 2 ºC in 16h light:8h dark cycles. The test organisms came from lots with LC50-24h = 1.04 mgK2Cr2O7 ± 0.11 mg. Percentage survival and reproduction were considered to define acute and/or chronic responses. Duncan (p < 0.05) and T (p < 0.05) Tests, ANOVA and Spearman correlation of the biological data were used as statistical complementation. The Spearman correlation was also applied looking for dependencies between the rainfall measured at the sites and the biological parameters but there was no evidence of correlation. The presence of acute toxicity was diagnosed for the sample in which survival was less than 80%, and for chronic toxicity when the mean number of neonates was less than 20 daphniids. RESULTS: Reproductive delays were found coinciding with physical and chemical parameters, as well as the presence of metals at values that were not in accordance with the law. Statistical analysis suggested the predominance of point sources of contamination. An effect on survival was observed in 17% of the samples, and on reproduction in 87%. CONCLUSIONS: This study found that pollutants discharged into the Sinos River remain and can interfere in the equilibrium of the trophic network, since they increase mortality and diminish the production of offspring among the cladocera, representatives of the primary consumers in the food chain.

The fishes presented herein were collected in small streams, in channel of lagoons and in the main channel of the Guariba River, as well as in one tributary of the right bank of the Roosevelt River, both clear-water tributaries of Aripuanã River, Madeira River basin. Field work was carried out in November 2008, during the low water season. Sampling resulted in 3924 specimens belonging to 160 species distributed in 34 families, and seven orders. Eight species are recognized as new, two of which were recently described. This study represents the first fish survey for the region and will certainly provide valuable information for future studies and maintenance of the poorly known fish diversity of these two conservation areas.

O Antropoceno é marcado pelo aumento da demanda populacional e pelo desenvolvimento industrial. Essas alterações antrópicas têm tido como consequência mudanças ambientais e climáticas, principalmente nos últimos 200 anos. O Rio Guaíba, localizado no Estado brasileiro do Rio Grande do Sul, é considerado de extrema importância por diferentes atributos: (1) principal fonte de abastecimento de água potável para a população de Porto Alegre; (2) importante via de navegação, que liga a região central do Estado com a Lagoa dos Patos e, consequentemente, com o Oceano Atlântico; e (3) abriga o setor industrial da capital Porto Alegre. Este trabalho apresenta como objetivo principal a realização de um estudo de reconstrução ambiental em três testemunhos coletados ao longo do Rio Guaíba. Para atingir este objetivo, determinaram-se níveis de elementos traço (As, Cr, Cu, Ni, P, Pb, Sc e Zn), em conjunto com a geocronologia recente, obtida por meio das atividades dos radionuclídeos 210Pb e 137Cs, utilizando o modelo CRS (Constant Rate of Supply). Foram calculados os índices de geoacumulação: Fator de Enriquecimento, Pollution Load Index e Sediment Pollution Index. As colunas sedimentares G1 e G2 demonstraram resultados que expressam as consequências da mineração de areia que ocorre no rio Jacuí, principal fonte de sedimento para o Rio Guaíba. A partir de aproximadamente 1998, houve uma maior fiscalização das dragas operantes neste rio, e a dinâmica deposicional desses testemunhos foi novamente alterada. A reconstrução realizada no testemunho G3 demonstrou-se diferente das demais. Alterações no perfil granulométrico foram explicadas por eventos climáticos (El Niño e La Niña); e o aumento na concentração de elementos traço, principalmente Cr, Pb, P e Zn, está ligado a possíveis fontes antropogênicas. Neste estudo de reconstrução ambiental, a utilização do modelo CRS, de proxies de metais e da granulometria de finos mostrou-se adequada, principalmente no caso das consequências relacionadas à mineração de areia no rio Jacuí.
The new epoch Anthropocene is marked by the increase of population and industrial development. These anthropic alterations have had as consequences environmental and climatic changes, especially in the last two hundred years. The Guaíba River, located at the Brazilian State of Rio Grande do Sul, is extremely important for different attributes: (1) it is the main source of potable water to the capital Porto Alegre; (2) it is an important navigation route, that connects the State central region with the Patos Lagoon and the Atlantic Ocean; and (3) it harbors the main industries of Porto Alegre. The principal objective of this study was to obtain environmental reconstructions for three sediment cores from Guaíba River. To accomplish this goal, we determined the concentrations of trace elements (As, Cr, Cu, Ni, P, Pb, Sc and Zn) and proceeded the analysis of recent geochronology, which was acquired through 210Pb and 137Cs activities with the CRS model. Furthermore, we calculated geoaccumulation indices: Enrichment Factor, Pollution Load Index and Sediment Pollution Index. The sediment cores G1 and G2 presented results that expressed the consequences of the sand mining that occurs at Jacuí River. Approximately after 1998, there has been better control of the dredges that operate on the river and the depositional dynamics were changed again. The G3 reconstruction had different conclusions compared with the others. Grain size alterations were explained by climatic events, like El Niño and La Niña; and the concentration increase of trace elements, mainly Cr, Pb, P and Zn, was connected with possible anthropogenic sources. In this study of environmental reconstruction, the utilization of the CRS model and of metals concentrations and grain size as proxies proved to be adequate, especially in the case of the sand mining at Jacuí River and its consequences.

Devido à proximidade de grandes cidades, ecossistemas aquáticos são alvos de despejos industriais, esgotos domésticos e resíduos sólidos. Modelos matemáticos são largamente utilizados para entender os padrões de fluxo e o transporte de substâncias nestes ecossistemas. Neste trabalho, desenvolveu-se um modelo hidrodinâmico e de transporte de poluentes bidimensional para corpos d’água rasos que utiliza grades não estruturadas. O modelo foi desenvolvido em dois módulos: hidrodinâmico e de transporte de massa. O módulo hidrodinâmico utiliza as equações completas de Saint Venant para quantificação da circulação em corpos d’águas. No módulo hidrodinâmico aqui desenvolvido é feito um aperfeiçoamento de um esquema semi-implícito de diferenças finitas utilizado no modelo IPH-ECO para um esquema semi-implícito de volumes finitos. O módulo de transporte de massa, por sua vez, representa o transporte de um escalar conservativo sob influência dos processos físicos do meio. Dois esquemas foram empregados e comparados para a solução numérica da equação de transporte: um esquema de diferenças centrais e um esquema com limitador de fluxo. O modelo foi aplicado em um estudo de caso simplificado formado por uma lagoa circular e no rio Guaíba. O módulo hidrodinâmico foi avaliado através de testes de conservação de volume feitos comparando com os resultados encontrados nno modelo IPHECO na aplicação realizada na lagoa circular e na solução analítica no rio Guaíba. O módulo hidrodinâmico do modelo de grades não estruturadas (IPH-UnTRIM2D) mostrou-se mais conservativo que o modelo IPH-ECO em todos os cenários simulados na lagoa circular. A solução hidrodinâmica e de transporte do modelo de grades não estruturadas no estudo de caso idealizado no rio Guaíba representou meandros, sulcos, estreitamentos e alargamentos de canais com comprometimento com uma situação real. Quanto à simulação de transporte de escalares e poluentes, foi comprovada a eficiência do esquema com limitador de fluxo sobre os esquemas de diferenças centrais. Os testes de conservação de massa para diferentes cenários no rio Guaíba indicaram erros pouco significativos em ambos os esquemas quando comparados com a ordem de grandeza do ecossistema.
Aquatics ecossistems around large towns are always exposed waste domestic and industrial and solid residues. Mathematical models are widely used to understand flow patterns and transport of mass in these ecosystems. In this work a model was developed on unstructured two-dimensional complex dynamic model for shallow lakes. The model was divided in two modules: (a) hydrodynamic module, describing quantitative flows and water level; (b) a mass transport module. The hydrodynamic module solves Saint Venant equations to quantify water circulation in a water body. Futhermore, the hydrodynamic module was improved to semiimplicit finite volume scheme. The mass transport module represents transport of a conservative scalar under the physical processes influence. Two schemes were used and compared to the transport equation numerical solution : a central difference scheme and a flux limiter scheme. The model was applied in the circular lake and river Guaíba. The hydrodynamic module was validated by evaluating tests of volume conservation made comparing with results of the model IPH-ECO and analytical solution. It proved to be more conservative than the model IPH-ECO in all scenarios simulated in the circular lake. The hydrodynamic and transport solution in river Guaíba study case represented bends, grooves, and channels widening and narrowing. In the transport simulation, the flux limiter scheme was more efficient than central differences scheme. Futhermore, mass conservation tests for different scenarios in Guaíba indicated minor errors in both schemes when compared with ecosystem magnitude.

Thesis (M.S.)--Florida State University, 2005.
Advisor: Dr. Robert Deyle, Florida State University, College of Social Sciences, Dept. of Urban and Regional Planning. Title and description from dissertation home page (viewed June 13, 2005). Document formatted into pages; contains ix, 70 pages. Includes bibliographical references.

The nature of the relationships between physical and biological processes in the ocean is subtle and complex. Not only do the physical phenomena create a structure, such as a shallow, mixed layer or a front, within which biological processes may proceed, but they also influence the rates of biological processes in many indirect ways. In the ocean, physical phenomena control the distribution of nutrients necessary for phytoplankton photosynthesis. Places with higher kinetic energy have higher concentrations of planktonic organisms, and that makes the whole food web richer (Mann and Lazier 1996). For example, in the midriff region of the Sea of Cortés (Tiburón and Ángel de la Guarda; fig. 1.2), tidal currents are strong, and intense mixing occurs, creating a situation similar to constant upwelling. Thus, primary productivity is high, and this area supports large numbers of sea birds and marine mammals (Maluf 1983). The Gulf of California is a dynamic marginal sea of the Pacific Ocean and has been described as an area of great fertility since the time of early explorers. Gilbert and Allen (1943) described it as fabulously rich in marine life, with waters fairly teeming with multitudes of fish, and to maintain these large numbers, there must be correspondingly huge crops of their ultimate food, the phytoplankton. Topographically the gulf is divided into a series of basins and trenches, deepening to the south and separated from each other by transverse ridges (Shepard 1950; fig. 3.1). Input of nutrients into the gulf from rivers is low and has only local coastal effects. The Baja peninsula has only one, very small river, near 27°N; rivers in mainland Mexico and the Colorado River have dams that divert most of the water for agricultural and urban use. The gulf has three main natural fertilization mechanisms: wind-induced upwelling, tidal mixing, and thermohaline circulation. Upwelling occurs off the eastern coast with northwesterly winds (winter conditions from December through May) and off the Baja California coast with southeasterly winds (summer conditions from July through October), with June and November as transition periods (Álvarez-Borrego and Lara- Lara 1991).

The Sea of Cortés (el Mar de Cortés), also known as the Gulf of California, is the body of water that separates the California peninsula from the mainland of Mexico. It extends in a northwest-southeast axis for 1070 km, varying in width from 100 to 200 km. The gulf was formerly much longer, but sediments carried by the Colorado River created a delta and dammed off its upper end, forming what is now the Imperial Valley. The western side of the gulf is dotted with islands, the longest of which is Ángel de la Guarda, measuring 67 km long, up to 16 km wide, and 1315 m high (see app. 1.1 for a list of island names and measurements). Most of the islands are geological remnants of the peninsula's separation from the mainland, a continuing process that started 4 or more million years ago. One central gulf island, Tortuga, is an emerged volcano, whereas San Marcos Island to its west is largely gypsum, possibly precipitated from an ancient lake. The largest island in the gulf is Tiburón, with an area of approximately 1000 km2. It is barely separated from the mainland to the east and has a curiously mixed biota of peninsular and mainland species. One tiny island, San Pedro Nolasco, is only 13 km off shore in San Pedro Bay, Sonora, but has an unusual flora that includes a high percentage of endemics. The isolation of organisms that colonized or were established previously on the Sea of Cortés islands provided an opportunity for genetic and ecological change. In one plastic and rapidly evolving plant family, the Cactaceae, about one-half of the 120 species found on the islands are endemic. Similarly, populations isolated by climate on peninsular mountains are well differentiated. Because of the topographical diversity of the area and its effect on the disruption and integration of populations, the Sea of Cortes and its islands have been called a natural laboratory for the investigation of speciation.