Great River watersheds, as the Mississippi River in North America, integrate climate and environmental changes (climate fluctuations, precipitations, streamflow, sediment loads) at near continent scale, as well as anthropogenic changes in physical environment (land uses, river management...) in their hydrologic response, which makes sometimes difficult the identification of linkages between hydrological and climate variability. The main objectives of this work is to determine and quantify the relationships between hydrological variability and climate fluctuations (regionalized precipitations, climate indices) of the Mississippi River and its main tributaries, using spectral approaches adapted to (the study of non-stationary processes (continuous wavelet transform, wavelet coherence). Hydrological variability of the Mississippi River and its main tributaries is structured by several scales of variability, from annual to inter-annual (2-4y, 3-6y, 5-8y), decadal (8-16y, 12-16y) and multi-decadal scales (22y, 22-26y). These modes of streamflow variability are very similar to those observed in regionalized precipitations (mean coherency is estimated from 77% to 89% according to the sub-watershed), and operates at same time-scales variability of the main climate fluctuations affecting this region (ENSO, PDO, AMO, NAO, NAM et PNA), preliminary identified and synthesized using an similar methodology. Streamflow variability of the Mississippi River watershed appears influenced by several teleconnections (mean coherency of 63% to 66% with all climate indices), which operate at different spatial and temporal scales and change across time. Furthermore and not surprisingly, the hydrological variability of the Mississippi River and its main tributaries appears to be closely linked to a major shift in the climate system - as well as many other hydrosystems around the world - observed at global scale around 1970. This change would result in an increase in both streamflow mean and variance, as highlighted by changes in the spectral content of climate and hydrological parameters. In this way, a so-called "hydroclimatic" index was proposed in order to resume all those characteristics of the climate system that would imprint the typical scales of variability detected in the hydrological processes analyzed according to each sub-watershed. Finally, even if the majority of hydrological parameters appears strongly affected by climate parameters, others factors such as changes in physical environment (land use, river management...) could also significantly influence hydrological parameters (e.g. low and high streamflow). We could detect such human-induced changes in the variability of suspended sediment loads and show that it involved a decrease in suspended sediment loads up to 2,25.108 metric t.y-1 between 1950 and 1975 using a spectral modelling approach. However, the influence of these physical environmental changes in hydrology would be associated to trends or to very localized changes in space and time, rather than associated to the existence of oscillations in hydrological parameters as we could detect them. We then conclude that, despite the potential strong influence of environmental changes, climate fluctuations remain the main factor involved in the observed hydrological changes.
