Increased energy demand has led to plans for building many new dams in the western Amazon, mostly in the Andean region. Historical data and mechanistic scenarios are used to examine potential impacts above and below six of the largest dams planned for the region, including reductions in downstream sediment and nutrient supplies, changes in downstream flood pulse, changes in upstream and downstream fish yields, reservoir siltation, greenhouse gas emissions and mercury contamination. Together, these six dams are predicted to reduce the supply of sediments, phosphorus and nitrogen from the Andean region by 69, 67 and 57% and to the entire Amazon basin by 64, 51 and 23%, respectively. These large reductions in sediment and nutrient supplies will have major impacts on channel geomorphology, floodplain fertility and aquatic productivity. These effects will be greatest near the dams and extend to the lowland floodplains. Attenuation of the downstream flood pulse is expected to alter the survival, phenology and growth of floodplain vegetation and reduce fish yields below the dams. Reservoir filling times due to siltation are predicted to vary from 106-6240 years, affecting the storage performance of some dams. Total CO 2 equivalent carbon emission from 4 Andean dams was expected to average 10 Tg y . Mercury contamination in fish and local human populations is expected to increase both above and below the dams creating significant health risks. Reservoir fish yields will compensate some downstream losses, but increased mercury contamination could offset these benefits.

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Understanding hydrological processes occurring within a basin by looking at its outlet hydrograph can improve and foster comprehension of ungauged regions. In this context, we present an extensive examination of the roles that floodplains play on driving hydrograph shapes. Observations of many river hydrographs with large floodplain influence are carried out and indicate that a negative skewness of the hydrographs is present among many of them. Through a series of numerical experiments and analytical reasoning, we show how the relationship between flood wave celerity and discharge in such systems is responsible for determining the hydrograph shapes. The more water inundates the floodplains upstream of the observed point, the more negatively skewed is the observed hydrograph. A case study is performed in the Amazon River Basin, where major rivers with large floodplain attenuation (e.g., Purus, Madeira, and Juruá) are identified with higher negative skewness in the respective hydrographs. Finally, different wetland types could be distinguished by using this feature, e.g., wetlands maintained by endogenous processes, from wetlands governed by overbank flow (along river floodplains). A metric of hydrograph skewness was developed to quantify this effect, based on the time derivative of discharge. Together with the skewness concept, it may be used in other studies concerning the relevance of floodplain attenuation in large, ungauged rivers, where remote sensing data (e.g., satellite altimetry) can be very useful.

The Tocantins River, located at the northern region of Brazil with over 300 000 km2 of drainage area, is an important water body in terms of hydropower production. The occurrence of floods along the Tocantins River is a relatively frequent event that affects hydropower plant operations and several cities and their inhabitants. Motivated by recent flooding issues, a hydrological forecasting system was developed in order to assist the decision making of dam operation for flood control. The model uses merged rainfall information from ground‐based telemetric gauges and real‐time TRMM satellite rainfall estimates. Streamflow forecasts are obtained based on quantitative precipitation forecasts from two different sources, CPTEC Eta 15 km regional deterministic model and the Global Ensemble Forecasting System‐VII, maintained by the National Center for Environmental Prediction‐National Oceanic and Atmospheric Administration. We present here the forecasting system analysis of the 2011/2012 rainy season flood predictions with the use of ensemble forecasts, and comparison results of deterministic and ensemble forecasts for the major flood of 2012/2013.

This study presents an alternative model for hydrological tracking of surface waters designed to investigate multiyear storage and movement of water in large river basins. We coupled this model with a background solution from a large-scale hydrological-hydrodynamic model to study the fate of waters in the Amazon River basin (6 million km 2) considering the following: (i) the surface water travel times, (ii) the role of large floodable areas (17% of the basin), and (iii) the relations between water composition and lithological sources. Through numerical experiments, we investigated the spatial distribution of in-stream surface water travel time for the Amazon River Basin (relative to the outlet) and the role of floodplains in seasonal water storage. We estimate that surface water travel time is likely to be less than 90 days (median of 45 days), although about 20% of the flowing water passes through large floodplains. Our modeling exercises indicate that whole-basin monthly water net balance between river and floodplain can reach up to magnitudes of 10 5 m 3 /s; the water exchanged with floodplains during the annual floods can represent a large portion (up to 40%) of the basin discharge during low waters season. Finally, we took advantage of the tracking approach to study the hydrograph composition based on parent river lithology quantifying the role of major tributaries. Parallel to the development of global hydrology databases, new modeling approaches are also required to push advances in the discussion of renewal and travel times of surface waters in large river basins.

Atualmente é comum determinar a rede de drenagem, os divisores de água e a delimitação de bacias hidrográficas de forma automática, a partir de informações de relevo, representadas na forma de uma matriz de altitudes, os chamados modelos digitais de elevação (MDE). Uma etapa fundamental desta metodologia é a obtenção da hidrografia arbitrando um limite inferior de área de drenagem a partir do qual um pixel do MDE é considerado como parte da rede de drenagem. O valor deste limite, no entanto, pode variar em diferentes regiões, dependendo das características físicas locais e dos objetivos do mapeamento. Neste artigo são propostos alguns critérios que podem servir para definir a área de drenagem mínima necessária para a melhor representação das nascentes, ou do início da rede de drenagem, a partir do processamento de um MDE. Os critérios foram desenvolvidos a partir da análise de 28 bacias em diferentes regiões do Brasil, onde o valor da melhor área de drenagem limite foi estimado com base na comparação entre a rede de drenagem derivada do MDE e as nascentes identificadas em imagens de satélite de alta resolução. Nas mesmas regiões foram obtidos dados de características geológicas, declividade média, precipitação média anual e vazões. Como resultado foram identificadas relações gerais para o valor da área de drenagem mínima (ou limiar de área de drenagem) em função do tipo de geologia (porosa ou fissural), declividade média da bacia e relação Q90/Q50, de tal forma que os valores determinados podem servir como fundamento inicial para definição da área limite para início da rede de drenagem. Além disso, verificou-se que o uso de um único valor de área mínima para bacias com características físicas variadas pode ser inapropriado. Dessa forma foi sugerido uma metodologia para a delimitação da rede de drenagem para áreas compostas por regiões físicas distintas. Espera-se que este método seja utilizado como uma função de geoprocessamento para bacias heterogêneas.

Climate-driven alterations of hydro-meteorological conditions can change river flow regimes and potentially affect the migration behaviour of fishes and the productivity of important fisheries in the Amazon basin, such as those for the continental-scale migratory goliath catfishes (Brachyplatystoma, Pimelodidae). In this study, we investigated hydrologic responses to climate change using a hydrologic model forced with climate inputs, which integrate historical (2001-2010) observations and general circulation model (GCM) projections under the emission scenario Representative Concentration Pathway 8.5. We developed an empirical model to characterize future (2090-2099) climate-change impacts on goliath catfish spawning migrations as a function of river flow depth dynamics at the upstream elevational limit of spawning (250 m) in headwater basins of the Amazon. The model results revealed spatially variable impacts of climate change on the catfish spawning migrations. The Marañón, Ucayali, Juruá, Purus, and Madeira basins had a predicted increase in the annual mean (3-8%) and maximum (1.1-4.9%) spawning migration rate (i.e., the fraction of fish that migrate to the spawning grounds in a day), mainly due to the lengthened rising phase of flow-driven migratory events during wet seasons. The Caquetá-Japurá, Putumayo-Içá, Napo, and Blanco rivers had predicted decreases (3-7%) in the mean migration rate because of decreases in the length of the rising season of flow depth and the frequency of migratory events. The predicted timing of fish spawning migrations (quantified by the temporal centroid of migration rates) was delayed by