[1] This study analyzes tide transformation in the Guadalquivir estuary (SW Spain). When fresh water discharges are less than 40 m 3 /s, the estuary is tidally-dominated (flood-dominated) and well mixed. Under such conditions, the estuary can be divided into three stretches, each characterized by a different tide propagation process. In the first stretch of 25 km, the dominant process is diffusion. In the next stretch, approximately over 35 km length, convergence and friction processes are in balance. At the head of the estuary, in the last stretch, the tidal motion is partially standing because of tidal reflection on the Alcalá del Río dam, located 110 km upstream from the estuary mouth. The reflection coefficient R varies with the frequency; for diurnal constituents its magnitude |R D | is 0.25; this value increases in the case of semi-diurnal (|R S | ≈ 0.40), and quarter-diurnal constituents (|R Q | ≈ 0.65), and reaches its minimum at the sixth-diurnal components (|R X | ≈ 0.10). The tidal reflection can generate residual currents that have consequences in the bed morphology. Furthermore, when the fresh water discharges are greater than 400 m 3 /s, the estuary is fluvially-dominated and the water level can be calculated as the linear superposition of tide and river contributions. However, superposition arguments do not hold for currents at any point in the estuary.

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[1] This paper presents an experimental analysis of the salinity distribution, the salt balance, and the variation of the saline intrusion in comparison to the freshwater discharge in the Guadalquivir estuary, which is a mesotidal system regulated and normally subjected to extremely low river flows. In such low-flow conditions, it is positive, well-mixed, and tidally dominated. The estuary is also characterized by a nonstationary, effective longitudinal dispersion coefficient, whose probability density becomes increasingly narrower and whose mean value is higher further upstream. The tidal-averaged salt flux is controlled by the following mechanisms (in order of importance): the nontidal transport, the Stokes transport, and the tidal pumping induced by the covariance between the current and salinity. These three factors account for more than 98% of the flux variation. In high river-flow conditions, the subtidal response and recovery of the estuary to changes in the river flow is analyzed. The increase in the tidal-averaged salinity during the first 2 weeks of the post-riverflood recovery in the middle and upper sections of the estuary is found to be linear in time. During that time, the celerity of the salt intrusion front was 4 cm/s. The 2 psu isohaline salt intrusion X 2 exhibits a complex dependence on the river flow Q d , including the effects of human interventions in the estuary. Three regimes are identified for the intrusion: X 2 = 57.02. 1

The temporal variability of hydrological variables in the Guadalquivir estuary was examined during three years through a real-time remote monitoring network (RTRM). The network was developed with the aim of studying the influence of hydrodynamical and hydrological features within the estuary on the functioning of the pelagic ecosystem. Completing this data-gathering network, monthly cruises were performed in order to measure biogeochemical variables that are indicative of the trophic status of the aquatic environment. The results showed that several sources of physical forcing, such as wind, tide-associated currents and river discharge were responsible for the spatio-temporal patterns of dissolved oxygen, salinity and turbidity in the estuary. The analysis was conducted under tidal and flood regime, which allowed us to identify river discharge as the main forcing agent of the hydrology inside the estuary. In particular, episodes of elevated turbidity detected by the network, together with episodes of low salinity and dissolved oxygen were closely related to the increase in water supply from a dam located upstream. The network installed provided accurate data that can be rapidly used for research or educational applications and by policy-makers or agencies in charge of the management of the coastal area.

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This paper presents a study on the tidal‐fluvial interaction in the highly regulated Guadalquivir River Estuary (SW Spain), which is occasionally subjected to high discharge episodes that affect navigational conditions and increase flood risks. The study specifically focuses on the processes and controlling mechanisms of the nonstationary response of water levels and currents to high discharges. Measurements show a 60 day postdischarge amplification of tidal current and elevation amplitudes and a clockwise rotation of the tidal ellipse in the upper layers. A decrease of amplitudes and an anticlockwise rotation predominate near the bed. Such episodes significantly increase the tidal wave celerity, and especially at high and low water. These features are due to the suspended sediment stratification triggered by the discharge event. The increase in stratification restricts frictional influence to bottom layers, partially decoupling the overlying flow from the bottom. A nonstationary harmonic decomposition method, intended for identifying which nonlinear terms in the governing hydrodynamic equations control overtide and compound tide generation, shows that quadratic bottom stress contributes the most during high discharge periods. The consequence in the subtidal balance is that, during peak discharge and in the upper stretches, friction is largely balanced by the water level gradient, although the density gradient term becomes comparable to the friction term soon after peak discharge. Advection is also important to the force balance in the lower estuary. For both parts, to correctly explain subtidal dynamics, it is necessary to account for the time variability of the friction coefficient due to flow‐sediment feedback.