Implications of climate change in the twenty-first century for simulated magnitude and frequency of bed-material transport in tributaries of the Saint-Lawrence River

More frequent extreme flood events are likely to occur in many areas in the twenty-first century due to climate change. The impacts of these changes on sediment transport are examined at the event scale using a 1D morphodynamic model (SEDROUT4-M) for three tributaries of the Saint-Lawrence River (Québec, Canada) using daily discharge series generated with a hydrological model (HSAMI) from three global climate models (GCMs). For all tributaries, larger flood events occur in all future scenarios, leading to increases in bed-material transport rates, number of transport events and number of days in the year where sediment transport occurs. The effective and half-load discharges increase under all GCM simulations. Differences in flood timing within the tributaries, with a shift of peak annual discharge from the spring towards the winter, compared to the hydrograph of the Saint-Lawrence River, generate higher sediment transport rates because of increased water surface slope and stream power. Previous research had shown that channel erosion is expected under all GCMs' discharge scenarios. This study shows that, despite lower bed elevations, flood risk is likely to increase as a result of higher flood magnitude, even with falling base level in the Saint-Lawrence River

Numerical modelling of climate change impacts on Saint-Lawrence River tributaries

The impacts of climate-induced changes in discharge and base level in three tributaries of the Saint-Lawrence River, Québec, Canada, are modelled for the period 2010–2099 using a one-dimensional morphodynamic model. Changes in channel stability and bed-material delivery to the Saint-Lawrence River over this period are simulated for all combinations of seven tributary hydrological regimes (present-day and those predicted using three global climate models and two greenhouse gas emission scenarios) and three scenarios of how the base level provided by the Saint-Lawrence River will alter (no change, gradual fall, step fall). Even with no change in base level the projected discharge scenarios lead to an increase in average bed material delivery for most combinations of river and global climate model, although the magnitude of simulated change depends on the choice of global climate model and the trend over time seems related to whether the river is currently aggrading, degrading or in equilibrium. The choice of greenhouse gas emission scenario makes much less difference than the choice of global climate model. As expected, a fall in base level leads to degradation in the rivers currently aggrading or in equilibrium, and amplifies the effects of climate change on sediment delivery to the Saint-Lawrence River. These differences highlight the importance of investigating several rivers using several climate models in order to determine trends in climate change impacts

A modified morphodynamic model for investigating the response of rivers to short-term climate change

Near-future climate change will affect the discharge and base level of rivers and thus cause channel changes. The nature and pace of such changes can be simulated using morphodynamic models. As part of an investigation of how the changing hydrology of the St-Lawrence River, Quebec, Canada, will affect its tributaries we have made additions and modifications to a one-dimensional morphodynamic model developed for gravel-bed rivers (SEDROUT). The changes allow simulation of sand-bed rivers, variable discharge, downstream water level fluctuations, and flow and sediment routing in channels with islands. A revised formulation for calculating the grain size distributions of the surface and subsurface material is presented to allow for alternating sedimentation and erosion. We test the enhanced model using small-scale simulations and present-day conditions in four tributaries of the St-Lawrence River. The model is calibrated and validated for the tributaries and the capability to simulate river morphology over a 100-year period is tested. Good validation agreement on water level, cross-sectional mean velocity, and sediment transport rate is obtained for the four tributaries of the St-Lawrence River. With these modifications, modelling a very wide range of river morphodynamic problems is now possible

Seasonal variations of dissolved nitrogen and DOC:DON ratios in an intermittent Mediterranean stream

22 páginas, 6 figuras, 4 tablas.Seasonal variations of dissolved inorganic nitrogen (DIN) (NO3–N and NH4–N) and dissolved organic nitrogen (DON) were determined in Fuirosos, an intermittent stream draining an unpolluted Mediterranean forested catchment (10.5 km2) in Catalonia (Spain). The influence of flow on streamwater concentrations and seasonal differences in quality and origin of dissolved organic matter, inferred from dissolved organic carbon to nitrogen ratios (DOC:DON ratios), were examined. During baseflow conditions, nitrate and ammonium had opposite behaviour, probably controlled by biological processes such as vegetation uptake and mineralization activity. DON concentrations did not have a seasonal trend. During storms, nitrate and DON increased by several times but discharge was not a good predictor of nutrient concentrations. DOC:DON ratios in streamwater were around 26, except during the months following drought when DOC:DON ratios ranged between 42 and 20 during baseflow and stormflow conditions, respectively. Annual N export during 2000–2001 was 70 kg km 1 year 1, of which 75% was delivered during stormflow. The relative contribution of nitrogen forms to the total annual export was 57, 35 and 8% as NO3–N, DON and NH4–N, respectively.This study was supported by a Formacio´n de Personal Investigador (FPI) grant and funds provided by the Comisio´ n Interministerial de Ciencia y Tecnologı´a (CICT, reference REN2001-3327).Peer reviewe