This study provides preliminary estimates of Total Exchange Flow (TEF) along the Guadalquivir  Estuary (Spain) during low river flows. The analysis combines 3 years of observations with an analytical exchange flow scenario featuring a sectionally-homogeneous M2+M4 oscillating tidal flow and salinity. The results indicate that the largest net incoming water volume transport, on the order of 300 m³ s⁻¹, is attained at the lower part of the estuary, near where the largest salinity gradient and currents are observed. This value is about 12-fold the normal river flow from the head dam at Alcalá del Río. The covariance between M2 salinity and current seems to play a more important role in exchange flow in the Guadalquivir estuary than the effects due to tidal asymmetry. Overall, the inclusion of the M4 improves TEF estimates in ∼ 10 % in the Guadalquivir estuary. A sensitivity analysis shows that in other estuaries and semi-enclosed basins the effects of the M4 could be even larger.